In Press

Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes/issues, but are citable by Digital Object Identifier (DOI).
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Original Paper
Impact of perturbation schemes on the ensemble prediction in a coupled Lorenz model
Qian Zou, Quanjia Zhong, Jiangyu Mao, Ruiqiang Ding, Deyu Lu, Jianping Li, Xuan LI
, Available online   , Manuscript accepted  01 June 2022, doi: 10.1007/s00376-022-1376-z
Abstract:
Based on a simple coupled Lorenz model, we investigate how to consider a suitable initial perturbation scheme for ensemble forecasting in a multiscale system involving slow dynamics and fast dynamics. Four initial perturbation approaches are used in the ensemble forecasting experiments: random perturbation (RP), the bred vector (BV), the ensemble transform Kalman filter (ETKF) and the nonlinear local Lyapunov vector (NLLV) methods. Results show that, regardless of the method used, the ensemble averages behave indistinguishably from the control forecasts during the first few time steps. Due to different error growth in different time-scale systems, the ensemble averages perform better than the control forecast after a very short period of lead time in a fast subsystem, but after a relatively long period of time in a slow subsystem. As a result of coupled dynamic processes, whether adding perturbations to fast variables or to slow variables can contribute to an improvement in the forecasting skill for fast variables and slow variables. When it comes to the initial perturbation approaches, the NLLVs show higher forecasting skill than BVs or RPs overall. NLLVs and ETKFs had nearly equivalent prediction skill, and NLLVs won by a narrow margin. In particular, when adding perturbations to slow variables, independent perturbations (NLLVs and ETKFs) perform much better in the ensemble prediction. These results are simply implied in a real coupled air–sea model. For the prediction of oceanic variables, independent perturbations (NLLVs) and adding perturbations to oceanic variables will be expected to perform better in the ensemble prediction.
Kinetic energy budgets during the rapid intensification of Typhoon Rammasun (2014)
Xin Quan, Xiaofan Li
, Available online   , Manuscript accepted  30 May 2022, doi: 10.1007/s00376-022-2060-z
Abstract:
In this study, Typhoon Rammasun (2014) was simulated using the Weather Research and Forecasting model to examine the kinetic energy during rapid intensification (RI). Budget analyses revealed that in the inner area of the typhoon, the conversion from symmetric divergent kinetic energy associated with the collocation of strong cyclonic circulations and inward flows under typhoon rotation led to an increase in the symmetric rotational kinetic energy in the lower troposphere. The increase in the symmetric rotational kinetic energy in the mid and upper troposphere resulted from the transport of symmetric rotational kinetic energy from the lower troposphere. In the outer area, both typhoon and Earth’s rotation play equally important roles in the conversion from symmetric divergent kinetic energy to symmetric rotational kinetic energy in the lower troposphere. The decrease in the symmetric rotational kinetic energy in the upper troposphere was caused by the conversion to asymmetric rotational kinetic energy through the collocation of symmetric tangential rotational winds and the radial advection of asymmetric tangential rotational winds by radial environmental winds.
On Key Dynamical Processes Supporting the 21.7 Zhengzhou Record-Breaking Hourly Rainfall in China
Peng Wei, Xin Xu, Ming Xue, ChenYue Zhang, Yuan Wang, Kun Zhao, Ang Zhou, ShuShi Zhang, Kefeng Zhu
, Available online   , Manuscript accepted  24 May 2022, doi: 10.1007/s00376-022-2061-y
Abstract:
An extremely heavy rainfall event occurred in Zhengzhou, China on 20 July 2021, and produced an hourly rainfall rate of 201.9 mm that broke station record of mainland China. Based on radar observations and a convection-permitting simulation using the WRF-ARW model, this paper investigates the multiscale processes, especially those at mesoscale, that support the extreme observed hourly rainfall. Results show that the extreme rainfall took place in an environment similar to that of warm-sector heavy rainfall, with abundant warm moist air transported from the ocean by abnormally northward-displaced western Pacific subtropical high and Typhoon In-Fa. However, rather than through back building and echo training of convective cells often found in warm-sector heavy rainfall events, this extreme hourly rainfall was caused by a single, quasi-stationary storm in Zhengzhou. Scale separation analysis reveals that the extreme-rain-producing storm was supported and maintained by the dynamical lifting of low-level converging flows from the north, south and east of the storm. The low-level northerly flow originated from a mesoscale barrier jet on the eastern slope of the Taihang Mountain due to terrain blocking of large-scale easterly flows, which reached an overall balance with the southerly winds in association with a low-level meso-β-scale vortex located to the west of Zhengzhou. The large-scale easterly inflows that fed the deep convection via transport of thermodynamically unstable air into the storm acted to prevent the eastward propagation of the weak shallow cold pool. As a result, the convective storm was nearly stationary over Zhengzhou, resulting in the record-breaking hourly precipitation
Estimation of Lightning-Generated NOX in China continental area based on Cloud-to-Ground Lightning Location Data
qi Li, Fengxia Guo, xiaoyu Ju, ze Liu, mingjun Gan, kun Zhang, binbin Cai
, Available online   , Manuscript accepted  19 May 2022, doi: 10.1007/s00376-022-1329-6
Abstract:
Abstract: Lightning-generated nitrogen oxides (LNOX) have a major influence on the atmosphere and global climate change, it is therefore of great importance to obtain a more accurate estimation of the LNOX. This paper attempts to provide a reference for the accurate estimation of the total LNOX in China continental area based on cloud-to-ground lightning (CG) location data from 2014 to 2018. The energy of each CG event was based on the number of return strokes per CG, the peak current of each return stroke and the assumed CG breakdown voltage. The energy of intracloud lightning (IC) was based on the estimated frequencies of IC and the assumed energy of each IC. Combining the energy of lightning and the number of nitric oxide (NO) molecules produced by unit energy (ρno), the total LNOX production in China continental area was determined. The LNOX in China continental area estimated in this paper are 0.157–0.321 109 kg per year (Tg(N) yr-1), which is on the high end of other scholars’ work. Negative cloud-to-ground lightning (NCG) flashes produce almost the most total moles of NOx, while positive cloud-to-ground lightning (PCG) flashes produce the least total moles of NOx. The breakdown voltage of PCG is greater than that of IC or NCG, while the latter has a greater output of LNOX.
Circulation patterns linked to the positive sub-tropical Indian Ocean dipole
Chibuike Ibebuchi
, Available online   , Manuscript accepted  18 May 2022, doi: 10.1007/s00376-022-2017-2
Abstract:
The positive phase of the subtropical Indian Ocean dipole (SIOD) is one of the climatic modes in the south Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types (CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral summer rain-bearing CTs were obtained. The CT that featured (i) enhanced cyclonic activity in the southwest Indian Ocean; (ii) positive widespread rainfall anomaly in the southwest Indian Ocean; (iii) and low-level convergence of moisture fluxes from the tropical South Atlantic Ocean, tropical Indian Ocean, and the southwest Indian Ocean, at the south-central landmasses of Africa, was found to be related to the positive SIOD climatic mode. The relationship also implies that positive SIOD can be expected to increase the amplitude and frequency of occurrence of the aforementioned CT. The linkage between the CT and austral summer homogeneous regions of rainfall anomalies in Africa south of the equator, showed that it is the principal CT that is related to the inter-annual rainfall variability of the south-central regions of Africa, where the SIOD is already known to significantly influence. Hence, through the large-scale patterns of atmospheric circulation associated with the CT, the SIOD can influence the spatial distribution and intensity of rainfall in the preferred landmasses through enhanced moisture convergence.
Macro- and Micro-physical Characteristics of Different Parts of Mixed Convective-stratiform Clouds and Differences in Their Responses to Seeding
Dejun LI, Chuanfeng ZHAO, Peiren LI, Cao Liu, Dianli GONG, Siyao LIU, Zhengteng YUAN, Yingying CHEN
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-2003-8
Abstract:
This study investigates the cloud macro- and micro-physical characteristics in the convective and stratiform regions and their different responses to the seeding for mixed convective-stratiform clouds that occurred in Shandong province on 21 May 2018, based on the observations from the aircraft, the Suomi National Polar-Orbiting Partnership (NPP) satellite, and the high-resolution Himawari-8 (H8) satellite. The aircraft observations show that convection was deeper and radar echoes were significantly enhanced with higher tops in response to seeding in the convective region. This is linked with the conversion of supercooled liquid droplets to ice crystals with released latent heat, resulting in strengthened updrafts, enhanced radar echoes, higher cloud tops, and more and larger precipitation particles. In contrast, in the stratiform cloud region, after the Silver Iodide (AgI) seeding, the radar echoes become significantly weaker at heights close to the seeding layer, with the echo tops lowered by 1.4–1.7 km. In addition, a hollow structure appears at the height of 6.2–7.8 km with a depth of about 1.6 km and a diameter of about 5.5 km, and features such as icing seeding tracks appear. These suggest that the transformation between droplets and ice particles was accelerated by the seeding in the stratiform part. The NPP and H8 satellites also show that convective activity was stronger in the convective region after seeding; while in the stratiform region, a cloud seeding track with a width of 1–3 km appears 10 km downstream of the seeding layer 15 minutes after the AgI seeding, which moves along the wind direction as width increases.
Coordinated Influence of Indian Ocean Sea Surface Temperature and Arctic Sea Ice on Anomalous Northeast China Cold Vortex Activities with Different Paths During Late Summer
Yitong LIN, Yihe Fang, Chunyu ZHAO, Zhi Gong, Siqi YANG, Yiqiu YU
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-1415-9
Abstract:
The Northeast China cold vortexes (NCCVs) during late summer (from July to August) are identified and classified into three types in terms of the movement path by using machine learning. The relationships of the NCCVs intensity with atmospheric circulations in late summer as well as the sea surface temperature (SST) and Arctic sea ice concentration (SIC) in the preceding months are analyzed. The sensitivity tests by the Community Atmosphere Model version 5.3 (CAM5.3) are used to verify the statistical results. The results show that the coordination pattern of East Asia-Pacific (EAP) and Lake Baikal high pressure forced by SST anomalies in the North Indian Ocean dipole mode (NIOD) in preceding April and SIC anomalies in Nansen Basin in preceding June results in the intensity anomaly of the first type of NCCV. While the pattern of the Urals and Okhotsk Sea high presssure with Lake Baikal low pressure during late summer, which is forced by SST anomalies in the South Indian Ocean dipole mode (SIOD) in preceding June and SIC anomalies in the Barents Sea in preceding April, causes the intensity anomaly of the second type. The third type is not a typical type and is not detailedly analyzed. The sensitivity test jointly forced by the SST and SIC in the preceding period can well reproduce the observations, while the results separately forced by the SST and SIC are poor, indicating that the NCCVs during late summer should be influenced by the coordinated influence of SST and SIC in preceding months.
On the Influences of Urbanization on the Extreme Rainfall over Zhengzhou on 20 July 2021: A Convection-Permitting Ensemble Modeling Study
Yali LUO, Jiahua ZHANG, Miao YU, Xudong LIANG, Rudi XIA, Yanyu GAO, Xiaoyu GAO, Jinfang YIN
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-2048-8
Abstract:
This study investigates the influences of urban land cover on the extreme rainfall event over the Zhengzhou city in central China on 20 July 2021 using the Weather Research and Forecasting model at a convection-permitting scale [1-km resolution in the innermost domain (d3)]. Two ensembles of simulation (CTRL, NURB), each consisting of 11 members with a multi-layer urban canopy model and various combinations of physics schemes, were conducted using different land cover scenarios: (i) the real urban land cover, (ii) all cities in d3 being replaced with natural land cover. The results suggest that CTRL reasonably reproduces the spatiotemporal evolution of rainstorms and the 24-h rainfall accumulation over the key region, although the maximum hourly rainfall is underestimated and displaced to the west or southwest by most members. The ensemble mean 24-h rainfall accumulation over the key region of heavy rainfall is reduced by 13%, and the maximum hourly rainfall simulated by each member is reduced by 15–70 mm in CTRL relative to NURB. The reduction in the simulated rainfall by urbanization is closely associated with numerous cities/towns to the south, southeast, and east of Zhengzhou. Their heating effects jointly lead to formation of anomalous upward motions in and above the planetary boundary layer (PBL), which exaggerates the PBL drying effect due to reduced evapotranspiration and also enhances the wind stilling effect due to increased surface friction in urban areas. As a result, the lateral inflows of moisture and high-θe (equivalent potential temperature) air from south and east to Zhengzhou are reduced.
Multi-scale Incremental Analysis Update Scheme and Its Application to Typhoon Mangkhut Prediction
Yan GAO, Jiali FENG, Xin XIA, Jian SUN, Yulong MA, Dongmei CHEN, Qilin WAN
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-1425-7
Abstract:
In the traditional incremental analysis update (IAU) process, all analysis increments are treated as constant forcing in a model’s prognostic equations over a certain time window. This approach effectively reduces high-frequency oscillations introduced by data assimilation. However, as different scales of increments have unique evolutionary speeds and life histories in a numerical model, the traditional IAU scheme cannot fully meet the requirements of short-term forecasting for the damping of high-frequency noise and may even cause systematic drifts. Therefore, a multi-scale IAU scheme is proposed in this paper. Analysis increments were divided into different scale parts using a spatial filtering technique. For each scale increment, the optimal relaxation time in the IAU scheme was determined by the skill of the forecasting results. Finally, different scales of analysis increments were added to the model integration during their optimal relaxation time. The multi-scale IAU scheme can effectively reduce the noise and further improve the balance between large-scale and small-scale increments in the model initialization stage. To evaluate its performance, several numerical experiments were conducted to simulate the path and intensity of Typhoon Mangkhut (2018) and showed that: (1) the multi-scale IAU scheme had an obvious effect on noise control at the initial stage of data assimilation; (2) the optimal relaxation time for large-scale and small-scale increments was estimated as 6 h and 3 h, respectively; (3) the forecast performance of the multi-scale IAU scheme in the prediction of Typhoon Mangkhut (2018) was better than that of the traditional IAU scheme. The results demonstrate the superiority of the multi-scale IAU scheme.
Assimilation of Ocean Surface Wind Data by the HY-2B Satellite in GRAPES: Impacts on Analyses and Forecasts
Jincheng WANG, Xingwei JIANG, Xueshun SHEN, Youguang ZHANG, Xiaomin WAN, Wei HAN, Dan WANG
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-1349-2
Abstract:
The ocean surface wind (OSW) data retrieved from microwave scatterometers have high spatial accuracy and represent the only wind data assimilated by global numerical models on the ocean surface, thus playing an important role in improving the forecast skills of global medium-range weather prediction models. To improve the forecast skills of the Global/Regional Assimilation and Prediction System Global Forecast System (GRAPES_GFS), the HY-2B OSW data is assimilated into the GRAPES_GFS four-dimensional variational assimilation (4DVAR) system. Then, the impacts of the HY-2B OSW data assimilation on the analyses and forecasts of GRAPES_GFS are analyzed based on one-month assimilation cycle experiments. The results show that after assimilating the HY-2B OSW data, the analysis errors of the wind fields in the lower-middle troposphere (1000–600 hPa) of the tropics and the southern hemisphere (SH) are significantly reduced by an average rate of about 5%. The impacts of the HY-2B OSW data assimilation on the analysis fields of wind, geopotential height, and temperature are not solely limited to the boundary layer but also extend throughout the entire troposphere after about two days of cycling assimilation. Furthermore, assimilating the HY-2B OSW data can significantly improve the forecast skill of wind, geopotential height, and temperature in the troposphere of the tropics and SH.
Relationships between Cloud Droplet Spectral Relative Dispersion and Entrainment Rate and their Impacting Factors
Shi LUO, Chunsong LU, Yangang LIU, Yaohui LI, Wenhua GAO, Yujun QIU, Xiaoqi XU, Junjun LI, Lei ZHU, Yuan WANG, Junjie WU, Xinlin YANG
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-1419-5
Abstract:
Cloud microphysical properties are significantly affected by entrainment and mixing processes. However, it is unclear how the entrainment rate affects the relative dispersion of cloud droplet size distribution. Previously, the relationship between relative dispersion and entrainment rate was found to be positive or negative. To reconcile the contrasting relationships, the Explicit Mixing Parcel Model is used to determine the underlying mechanisms. When evaporation is dominated by small droplets, and the entrained environmental air is further saturated during mixing, the relationship is negative. However, when the evaporation of big droplets is dominant, the relationship is positive. Whether or not the cloud condensation nuclei are considered in the entrained environmental air is a key factor as condensation on the entrained condensation nuclei is the main source of small droplets. However, if cloud condensation nuclei are not entrained, the relationship is positive. If cloud condensation nuclei are entrained, the relationship is dependent on many other factors. High values of vertical velocity, relative humidity of environmental air, and liquid water content, and low values of droplet number concentration, are more likely to cause the negative relationship since new saturation is easier to achieve by evaporation of small droplets. Further, the signs of the relationship are not strongly affected by the turbulence dissipation rate, but the higher dissipation rate causes the positive relationship to be more significant for a larger entrainment rate. A conceptual model is proposed to reconcile the contrasting relationships. This work enhances the understanding of relative dispersion and lays a foundation for the quantification of entrainment-mixing mechanisms.
Comparative analysis of generalized Omega equation and generalized vertical motion equation
Baofeng Jiao, Lingkun Ran, Na Li, Ren Cai, Tao Qu, Yushu Zhou
, Available online   , Manuscript accepted  04 May 2022, doi: 10.1007/s00376-022-1435-5
Abstract:
Research on vertical motion is an extraordinarily challenging effort in mesoscale systems. With fewer assumptions, a new form of generalized vertical motion equation and a homologous generalized Omega equation are derived respectively in the Cartesian coordinate system (nonhydrostatic equilibrium) and the isobaric coordinate system (hydrostatic equilibrium). The terms on the right-hand side of the equations, which is typically called the Q vector, are composed of three parts, namely, the dynamic factor, thermodynamic factor, and mass factor. Based on the equations, a heavy rain event in southern Xinjiang from July 18th to 19th, 2021 was selected to analyze the characteristics of the diagnostic variable in the generalized vertical motion equation ( ) and the diagnostic variable in the generalized Omega equation ( ) using high-resolution model data. The results show that the horizontal distribution of the vector divergence at 5.5 km is roughly similar to that of the at 500 hPa, both of which have a good relationship with the composite radar reflectivity, vertical movement, and hourly accumulated precipitation. The vector divergence is more effective in indicating weak precipitation. In the vertical section, regions with alternating positive and negative high values that match the precipitation are mainly concentrated in the middle levels for both forms of Q vectors. The temporal evolution of vertically integrated vector divergence and vector divergence is generally similar to each other. Both of them perform better than the classical quasigeostrophic Q vector and nongeostrophic Q vector in indicating the development of the precipitation system.
Effects of Plant Community Type on Soil Methane Flux in Semiarid Loess Hilly Region, Central Gansu Province, China
Chuanjie YANG, Guang LI, Lijuan YAN, Weiwei MA, Jiangqi WU, Yan TAN, Shuainan LIU, Shikang ZHANG
, Available online   , Manuscript accepted  25 April 2022, doi: 10.1007/s00376-022-1169-4
Abstract:
Methane (CH4) is an important greenhouse gas second only to CO2 in terms of its greenhouse effect. Vegetation plays an important role in controlling soil CH4 fluxes, but the spatial variability of soil CH4 fluxes during vegetation restoration in Loess Hilly Region (LHR) is not fully understood. The effects of different plant community types [Medicago sativa grassland (MS); Xanthoceras sorbifolium forestland (XS); Caragana korshinskii bushland (CK); Hippophae rhamnoides shrubland (HR); and Stipa bungeana grassland (SB)] on soil CH4 flux in LHR were studied via the static chamber technique. The results showed that the five plant community types were sinks of soil CH4 in LHR, the plant community type significantly affected the soil CH4 flux, and the average CH4 uptake from high to low was in SB, HR, CK, MS, and XS. During the whole study period, the soil CH4 flux showed similar interannual variation. The maximum absorption of soil CH4 appeared in the growing season, while the minimum appeared in winter. Soil CH4 uptake was positively correlated with soil temperature and soil moisture. Soil temperature and moisture are important controlling factors for the temporal variability of soil CH4 flux. In LHR, the Stipa bungeana grassland is the more suitable plant community type for reducing soil CH4 emissions. In the process of vegetation restoration in LHR, the soil CH4 absorption potential of different plant community types should be considered, ecological benefits should be taken into account, and vegetation more suitable for mitigating the greenhouse effect should be selected.
Decadal Methane Emission Trend Inferred from Proxy GOSAT XCH4 Retrievals: Impacts of Transport Model Spatial Resolution
Sihong ZHU, Liang FENG, Yi LIU, Jing WANG, Dongxu YANG
, Available online   , Manuscript accepted  25 April 2022, doi: 10.1007/s00376-022-1434-6
Abstract:
In recent studies, proxy XCH4 retrievals from the Japanese Greenhouse gases Observing SATellite (GOSAT) have been used to constrain top-down estimation of CH4 emissions. Still, the resulting interannual variations often show significant discrepancies over some of the most important CH4 source regions, such as China and Tropical South America, by causes yet to be determined. This study compares monthly CH4 flux estimates from two parallel assimilations of GOSAT XCH4 retrievals from 2010 to 2019 based on the same Ensemble Kalman Filter (EnKF) framework but with the global chemistry transport model (GEOS-Chem v12.5) being run at two different spatial resolutions of 4° × 5° (R4, lon × lat) and 2° × 2.5° (R2, lon × lat) to investigate the effects of resolution-related model errors on the derived long-term global and regional CH4 emission trends. We found that the mean annual global methane emission for the 2010s is 573.04 Tg yr–1 for the inversion using the R4 model, which becomes about 4.4 Tg yr–1 less (568.63 Tg yr–1) when a finer R2 model is used, though both are well within the ensemble range of the 22 top-down results (2008–17) included in the current Global Carbon Project (from 550 Tg yr–1 to 594 Tg yr–1). Compared to the R2 model, the inversion based on the R4 tends to overestimate tropical emissions (by 13.3 Tg yr–1), which is accompanied by a general underestimation (by 8.9 Tg yr–1) in the extratropics. Such a dipole reflects differences in tropical–mid-latitude air exchange in relation to the model’s convective and advective schemes at different resolutions. The two inversions show a rather consistent long-term CH4 emission trend at the global scale and over most of the continents, suggesting that the observed rapid increase in atmospheric methane can largely be attributed to the emission growth from North Africa (1.79 Tg yr–2 for R4 and 1.29 Tg yr–2 for R2) and South America Temperate (1.08 Tg yr–2 for R4 and 1.21 Tg yr–2 for R2) during the first half of the 2010s, and from Eurasia Boreal (1.46 Tg yr–2 for R4 and 1.63 Tg yr–2 for R2) and Tropical South America (1.72 Tg yr–2 for R4 and 1.43 Tg yr–2 for R2) over 2015–19. In the meantime, emissions in Europe have shown a consistent decrease over the past decade. However, the growth rates by the two parallel inversions show significant discrepancies over Eurasia Temperate, South America Temperate, and South Africa, which are also the places where recent GOSAT inversions usually disagree with one other.
Assimilation of All-sky Geostationary Satellite Infrared Radiances for Convection-Permitting Initialization and Prediction of Hurricane Joaquin (2015)
Lei Zhu, Zhiyong Meng, Yonghui Weng, Fuqing Zhang
, Available online   , Manuscript accepted  24 April 2022, doi: 10.1007/s00376-022-2015-4
Abstract:
Intensity forecast is one of the biggest challenges in tropical cyclone (TC) forecasts. This work examines the impact of assimilating high-resolution all-sky infrared radiance observations from geostationary satellite GOES-13 on the convection-permitting initialization and prediction of Hurricane Joaquin (2015) with an ensemble Kalman filter (EnKF) based on the Weather Research and Forecasting (WRF) model. Given that almost all operational global and regional models struggled in Hurricane Joaquin’s intensity, this study examines the potential in improving Joaquin’s prediction when assimilating all-sky infrared radiances from GOES-13’s water vapor channel. It is demonstrated that, after a few 3-hour cycles assimilating all-sky radiance, the WRF model was able to forecast reasonably well Joaquin’s intensity including the rapid intensification. The improvement was largely due to a more realistic initial hurricane structure with a stronger, warmer and more compact inner-core. Ensemble forecasts were further used to explore the important physical mechanisms driving the hurricane’s rapid intensification (RI). Results showed that the RI forecasts were greatly impacted by the initial inner-core vortex structure.
Reexamination of the Relationship between Tropical Cyclone Size and Intensity over the Western North Pacific
Kexin CHEN, Guanghua CHEN, Donglei SHI
, Available online   , Manuscript accepted  11 April 2022, doi: 10.1007/s00376-022-1450-6
Abstract:
This study reexamines the correlation between the size and intensity of tropical cyclones (TCs) over the western North Pacific from the perspective of individual TCs, rather than the previous large-sample framework mixing up all TC records. Statistics show that the positive size-intensity correlation based on individual TCs is relatively high. However, this correlation is obscured by mixing large samples. The weakened correlation based on all TC records is primarily due to the diversity in the size change relative to the same intensity change among TCs, which can be quantitatively measured by the linear regression coefficient (RC) of size against intensity. To further explore the factors that cause the variability in RCs that weakens the size-intensity correlation when considering all TC records, the TCs from 2001 to 2020 are classified into two groups according to their RC magnitudes, within which the high-RC TCs have a larger size expansion than the low-RC TCs given the same intensity change. Two key mechanisms responsible for the RC differences are proposed. First, the high-RC TCs are generally located at higher latitudes than the low-RC TCs, resulting in higher planetary vorticity and thus higher planetary angular momentum import at low levels. Second, the high-RC TCs are susceptible to stronger environmental vertical wind shear, leading to more prolific outer convection than the low-RC TCs. The positive feedback between outer diabatic heating and boundary layer inflow favors the inward import of absolute angular momentum in the outer region, thereby contributing to a larger size expansion in the high-RC TCs.
Evolution of Meteorological Conditions during a Heavy Air Pollution Event under the Influence of Shallow Foehn in Urumqi, China
Xia LI, Keming ZHAO, Shiyuan ZHONG, Xiaojing YU, Zhimin FENG, Yuting ZHONG, Ayitken MAULEN, Shuting LI
, Available online   , Manuscript accepted  11 April 2022, doi: 10.1007/s00376-022-1422-x
Abstract:
Urumqi, located on the northern slope of the Tianshan Mountains in northwestern China, is one of the most polluted cities in the world. Of particular importance is the influence of terrain-induced shallow foehn, known locally as elevated southeasterly gale (ESEG). It usually modulates atmospheric boundary layer structure and wind field patterns and produces favorable meteorological conditions conducive to hazardous air pollution. During 2013–17, Urumqi had an average of 50 d yr–1 of heavy pollution (daily average PM2.5 concentration >150 μg m–3), of which 41 days were in winter. The majority (71.4%) of heavy pollution processes were associated with the shallow foehn. Based on microwave radiometer, wind profiler, and surface observations, the surface meteorological fields and boundary layer evolution during the worst pollution episode in Urumqi during 16–23 February 2013 are investigated. The results illustrate the significant role of shallow foehn in the building, strengthening, and collapsing of temperature inversions. There were four wind field patterns corresponding to four different phases during the whole pollution event. The most serious pollution phase featured shallow foehn activity in the south of Urumqi city and the appearance of an intense inversion layer below 600 m. Intense convergence caused by foehn and mountain–valley winds was sustained during most of the phase, resulting in pollutants sinking downward to the lower boundary layer and accumulating around urban area. The key indicators of such events identified in this study are highly correlated to particulate matter concentrations and could be used to predict heavy pollution episodes in the feature.
Sub-seasonal Prediction of the South China Sea Summer Monsoon Onset in the NCEP Climate Forecast System Version 2
Weiwei WANG, Song YANG, Tuantuan ZHANG, Qingquan LI, Wei WEI
, Available online   , Manuscript accepted  29 March 2022, doi: 10.1007/s00376-022-1403-0
Abstract:
This study depicts the sub-seasonal prediction of the South China Sea summer monsoon onset (SCSSMO) and investigates the associated oceanic and atmospheric processes, utilizing the hindcasts of the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2). Typically, the SCSSMO is accompanied by an eastward retreat of the western North Pacific subtropical high (WNPSH), development of the cross-equatorial flow, and an increase in the east-west sea surface temperature (SST) gradient. These features are favorable for the onset of westerlies and strengthening of convection and precipitation over the South China Sea (SCS). A more vigorous SCSSMO process shows a higher predictability, and vice versa. The NCEP CFSv2 can successfully predict the onset date and evolution of the monsoon about 4 pentads (20 days) in advance (within 1–2 pentads) for more forceful (less vigorous) SCSSMO processes. On the other hand, the climatological SCSSMO that occurs around the 27th pentad can be accurately predicted in one pentad, and the predicted SCSSMO occurs 1–2 pentads earlier than the observed with a weaker intensity at longer leadtimes. Warm SST biases appear over the western equatorial Pacific preceding the SCSSMO. These biases induce a weaker-than-observed WNPSH as a Gill-type response, leading to weakened low-level easterlies over the SCS and hence an earlier and less vigorous SCSSMO. In addition, after the SCSSMO, remarkable warm biases over the eastern Indian Ocean and the SCS and cold biases over the WNP induce weaker-than-observed westerlies over the SCS, thus also contributing to the less vigorous SCSSMO.
The Microphysical Characteristics of Wintertime Cold Clouds in North China
Xuexu WU, Minghuai WANG, Delong ZHAO, Daniel ROSENFELD, Yannian ZHU, Yuanmou DU, Wei ZHOU, Ping TIAN, Jiujiang SHENG, Fei WANG, Deping DING
, Available online   , Manuscript accepted  28 March 2022, doi: 10.1007/s00376-022-1274-4
Abstract:
The microphysical characteristics of wintertime cold clouds in North China were investigated from 22 aircraft observation flights from 2014 to 2017, 2020, and 2021. The clouds were generated by mesoscale weather systems with little orographic component. Over the mixed-phase temperature range (–40℃ to 0℃), the average fraction of liquid, mixed-phase, and ice cloud was 4.9%, 23.3%, and 71.8%, respectively, and the probability distribution of ice mass fraction was a half-U-shape, suggesting that ice cloud was the primary cloud type. The wintertime mixed-phase clouds in North China were characterized by large cloud droplet number concentration, small liquid water content (LWC), and small effective diameter of cloud droplets. The main reason for larger cloud droplet number concentration and smaller effective diameter of cloud droplets was the heavy pollution in winter in North China, while for smaller LWC was the lower temperature during flights and the difference in air mass type. With the temperature increasing, cloud droplet number concentration, LWC, and the size of ice particles increased, but ice number concentration and effective diameter of cloud droplets decreased, similar to other mid-latitude regions, indicating the similarity in the temperature dependence of cloud properties of mixed-phase clouds. The variation of the cloud properties and ice habit at different temperatures indicated the operation of the aggregation and riming processes, which were commonly present in the wintertime mixed-phase clouds. This study fills a gap in the aircraft observation of wintertime cold clouds in North China.
An Extreme Drought over South China in 2020/21 Concurrent with an Unprecedented Warm Northwest Pacific and La Niña
Weijie FENG, Marco Y.-T. LEUNG, Dongxiao WANG, Wen ZHOU, Oscar Y. W. ZHANG
, Available online   , Manuscript accepted  28 March 2022, doi: 10.1007/s00376-022-1456-0
Abstract:
An extreme drought appeared in South China from October 2020 to March 2021. During that time, sea surface temperatures exhibited an unprecedented warm center over the northwest Pacific (NWP) and a cold center over the tropical eastern Pacific (La Niña). This study demonstrates the combined effects of an exceptionally warm NWP and a moderate La Niña are closely linked to the anomalous drought in South China. The sea surface temperature anomaly in these two regions induced a steeper horizontal geopotential height gradient over South China. As a result, anomalous northeasterly winds prevailed over South China, altering water vapor transport and moisture convergence. A simplified atmospheric general circulation model also verifies the influence of the NWP warm anomaly on South China precipitation. This study points out that the sea surface temperature variation in the NWP was important to the occurrence of extreme drought in South China from October 2020 to March 2021.
Simulating Aerosol Optical Depth and Direct Radiative Effects over the Tibetan Plateau with a High-Resolution CAS FGOALS-f3 Model
Min ZHAO, Tie DAI, Hao WANG, Qing BAO, Yimin LIU, Hua ZHANG, Guangyu SHI
, Available online   , Manuscript accepted  20 March 2022, doi: 10.1007/s00376-022-1424-8
Abstract:
Current global climate models cannot resolve the complex topography over the Tibetan Plateau (TP) due to their coarse resolution. This study investigates the impacts of horizontal resolution on simulating aerosol and its direct radiative effect (DRE) over the TP by applying two horizontal resolutions of about 100 km and 25 km to the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere Land System (CAS FGOALS-f3) over a 10-year period. Compared to the AErosol RObotic NETwork observations, a high-resolution model (HRM) can better reproduce the spatial distribution and seasonal cycles of aerosol optical depth (AOD) compared to a low-resolution model (LRM). The HRM bias and RMSE of AOD decreased by 0.08 and 0.12, and the correlation coefficient increased by 0.22 compared to the LRM. An LRM is not sufficient to reproduce the aerosol variations associated with fine-scale topographic forcing, such as in the eastern marginal region of the TP. The difference between hydrophilic aerosols in an HRM and LRM is caused by the divergence of the simulated relative humidity (RH). More reasonable distributions and variations of RH are conducive to simulating hydrophilic aerosols. An increase of the 10-m wind speed in winter by an HRM leads to increased dust emissions. The simulated aerosol DREs at the top of the atmosphere (TOA) and at the surface by the HRM are –0.76 W m–2 and –8.72 W m–2 over the TP, respectively. Both resolution models can capture the key feature that dust TOA DRE transitions from positive in spring to negative in the other seasons.
Fengyun-4 Geostationary Satellite-Based Solar Energy Nowcasting System and Its Application in North China
Chunlin HUANG, Hongrong SHI, Ling GAO, Mengqi LIU, Qixiang CHEN, Disong FU, Shu WANG, Yuan YUAN, Xiang′ao XIA
, Available online   , Manuscript accepted  17 March 2022, doi: 10.1007/s00376-022-1464-0
Abstract:
Surface solar irradiance (SSI) nowcasting (0–3 h) is an effective way to overcome the intermittency of solar energy and to ensure the safe operation of grid-connected solar power plants. In this study, an SSI estimate and nowcasting system was established using the near-infrared channel of Fengyun-4A (FY-4A) geostationary satellite. The system is composed of two key components: The first is a hybrid SSI estimation method combining a physical clear-sky model and an empirical cloudy-sky model. The second component is the SSI nowcasting model, the core of which is the derivation of the cloud motion vector (CMV) using the block-matching method. The goal of simultaneous estimation and nowcasting of global horizontal irradiance (GHI) and direct normal irradiance (DNI) is fulfilled. The system was evaluated under different sky conditions using SSI measurements at Xianghe, a radiation station in the North China Plain. The results show that the accuracy of GHI estimation is higher than that of DNI estimation, with a normalized root-mean-square error (nRMSE) of 22.4% relative to 45.4%. The nRMSE of forecasting GHI and DNI at 30–180 min ahead varied within 25.1%–30.8% and 48.1%–53.4%, respectively. The discrepancy of SSI estimation depends on cloud occurrence frequency and shows a seasonal pattern, being lower in spring–summer and higher in autumn–winter. The FY-4A has great potential in supporting SSI nowcasting, which promotes the development of photovoltaic energy and the reduction of carbon emissions in China. The system can be improved further if calibration of the empirical method is improved.
How Frequently Will the Persistent Heavy Rainfall Over the Middle and Lower Yangtze River Basin in Summer 2020 Happen Under Global Warming?
Zi-An GE, Lin CHEN, Tim LI, Lu WANG
, Available online   , Manuscript accepted  15 March 2022, doi: 10.1007/s00376-022-1351-8
Abstract:
The middle and lower Yangtze River basin (MLYRB) suffered persistent heavy rainfall in summer 2020, with nearly continuous rainfall for about six consecutive weeks. How the likelihood of persistent heavy rainfall resembling that which occurred over the MLYRB in summer 2020 (hereafter 2020PHR-like event) would change under global warming is investigated. An index that reflects maximum accumulated precipitation during a consecutive five-week period in summer (Rx35day) is introduced. This accumulated precipitation index in summer 2020 is 60% stronger than the climatology, and a statistical analysis further shows that the 2020 event is a 1-in-70-year event. The model projection results derived from the 50-member ensemble of CanESM2 and the multimodel ensemble (MME) of the CMIP5 and CMIP6 models show that the occurrence probability of the 2020PHR-like event will dramatically increase under global warming. Based on the Kolmogorov–Smirnoff test, one-third of the CMIP5 and CMIP6 models that have reasonable performance in reproducing the 2020PHR-like event in their historical simulations are selected for the future projection study. The CMIP5 and CMIP6 MME results show that the occurrence probability of the 2020PHR-like event under the present-day climate will be double under lower-emission scenarios (CMIP5 RCP4.5, CMIP6 SSP1-2.6, and SSP2-4.5) and 3–5 times greater under higher-emission scenarios (3.0 times for CMIP5 RCP8.5, 2.9 times for CMIP6 SSP3-7.0, and 4.8 times for CMIP6 SSP5-8.5). The inter-model spread of the probability change is small, lending confidence to the projection results. The results provide a scientific reference for mitigation of and adaptation to future climate change.
Transport Patterns and Potential Sources of Atmospheric Pollution during the XXIV Olympic Winter Games Period
Yuting ZHANG, Xiaole PAN, Yu TIAN, Hang LIU, Xueshun CHEN, Baozhu GE, Zhe WANG, Xiao TANG, Shandong LEI, Weijie YAO, Yuanzhe REN, Yongli TIAN, Jie LI, Pingqing FU, Jinyuan XIN, Yele SUN, Junji CAO, Zifa WANG
, Available online   , Manuscript accepted  03 March 2022, doi: 10.1007/s00376-022-1463-1
Abstract:
The attainment of suitable ambient air quality standards is a matter of great concern for successfully hosting the XXIV Olympic Winter Games (OWG). Transport patterns and potential sources of pollutants in Zhangjiakou (ZJK) were investigated using pollutant monitoring datasets and a dispersion model. The PM2.5 concentration during February in ZJK has increased slightly (28%) from 2018 to 2021, mostly owing to the shift of main potential source regions of west-central Inner Mongolia and Mongolian areas (2015–18) to the North China Plain and northern Shanxi Province (NCPS) after 2018. Using CO as an indicator, the relative contributions of the different regions to the receptor site (ZJK) were evaluated based on the source-receptor-relationship method (SRR) and an emission inventory. We found that the relative contribution of pollutants from NCPS increased from 33% to 68% during 2019–21. Central Inner Mongolia (CIM) also has an important impact on ZJK under unfavorable weather conditions. This study demonstrated that the effect of pollution control measures in the NCPS and CIM should be strengthened to ensure that the air quality meets the standard during the XXIV OWG.
The Impact of the Numbers of Monitoring Stations on the National and Regional Air Quality Assessment in China During 2013–18
Hongyan LUO, Xiao TANG, Huangjian WU, Lei KONG, Qian WU, Kai CAO, Yating SONG, Xuechun LUO, Yao WANG, Jiang ZHU, Zifa WANG
, Available online   , Manuscript accepted  28 February 2022, doi: 10.1007/s00376-022-1346-5
Abstract:
China national air quality monitoring network has become the core data source for air quality assessment and management in China. However, during network construction, the significant change in numbers of monitoring sites with time is easily ignored, which brings uncertainty to air quality assessments. This study aims to analyze the impact of change in numbers of stations on national and regional air quality assessments in China during 2013–18. The results indicate that the change in numbers of stations has different impacts on fine particulate matter (PM2.5) and ozone concentration assessments. The increasing number of sites makes the estimated national and regional PM2.5 concentration slightly lower by 0.6−2.2 µg m−3 and 1.4−6.0 µg m−3 respectively from 2013 to 2018. The main reason is that over time, the monitoring network expands from the urban centers to the suburban areas with low population densities and pollutant emissions. For ozone, the increasing number of stations affects the long-term trends of the estimated concentration, especially the national trends, which changed from a slight upward trend to a downward trend in 2014−15. Besides, the impact of the increasing number of sites on ozone assessment exhibits a seasonal difference at the 0.05 significance level in that the added sites make the estimated concentration higher in winter and lower in summer. These results suggest that the change in numbers of monitoring sites is an important uncertainty factor in national and regional air quality assessments, that needs to be considered in long-term concentration assessment, trend analysis, and trend driving force analysis.
Observational Subseasonal Variability of the PM2.5 Concentration in the Beijing-Tianjin-Hebei Area During the January 2021 Sudden Stratospheric Warming
Qian LU, Jian RAO, Chunhua SHI, Dong GUO, Ji WANG, Zhuoqi LIANG, Tian WANG
, Available online   , Manuscript accepted  28 February 2022, doi: 10.1007/s00376-022-1393-y
Abstract:
It is still not well understood if subseasonal variability of the local PM2.5 in the Beijing-Tianjin-Hebei (BTH) region is affected by the stratospheric state. Using PM2.5 observations and the ERA5 reanalysis, the evolution of the air quality in BTH during the January 2021 sudden stratospheric warming (SSW) is explored. The subseasonal variability of the PM2.5 concentration after the SSW onset is evidently enhanced. Stratospheric circumpolar easterly anomalies lasted for 53 days during the January–February 2021 SSW with two evident stratospheric pulses arriving at the ground. During the tropospheric wave weakening period and the intermittent period of dormant stratospheric pulses, the East Asian winter monsoon weakened, anomalous temperature inversion developed in the lower troposphere, anomalous surface southerlies prevailed, atmospheric moisture increased, and the boundary layer top height lowered, all of which favor the accumulation of pollutant particulates, leading to two periods of pollution processes in the BTH region. In the phase of strengthened East Asian winter monsoon around the very beginning of the SSW and another two periods when stratospheric pulses had reached the near surface, opposite-signed circulation patterns and meteorological conditions were observed, which helped to dilute and diffuse air pollutants in the BTH region. As a result, the air quality was excellent during the two periods when the stratospheric pulse had reached the near surface. The increased subseasonal variation of the regional pollutant particulates after the SSW onset highlights the important role of the stratosphere in the regional environment and provides implications for the environmental prediction.
Distinguishing the Regional Atmospheric Controls on Precipitation Isotopic Variability in the Central-Southeast Portion of Brazil
Vinícius dos SANTOS, Peter MARSHALL FLEMING, Luís HENRIQUE MANCINI, Stela DALVA SANTOS COTA, Graziele Beatriz de LIMA, Rafaela RODRIGUES GOMES, Roberto Eduardo KIRCHHEIM, Ricardo SANCHÉZ-MURILLO, Didier GASTMANS
, Available online   , Manuscript accepted  15 February 2022, doi: 10.1007/s00376-022-1367-0
Abstract:
Precipitation isotope ratios (O and H) record the history of water phase transitions and fractionation processes during moisture transport and rainfall formation. Here, we evaluated the isotopic composition of precipitation over the central-southeastern region of Brazil at different timescales. Monthly isotopic compositions were associated with classical effects (rainfall amount, seasonality, and continentality), demonstrating the importance of vapor recirculation processes and different regional atmospheric systems (South American Convergence Zone-SACZ and Cold Fronts-CF). While moisture recycling and regional atmospheric processes may also be observed on a daily timescale, classical effects such as the amount effect were not strongly correlated (δ18O-precipitation rate r ≤ –0.37). Daily variability revealed specific climatic features, such as δ18O depleted values (~ –6‰ to –8‰) during the wet season were associated with strong convective activity and large moisture availability. Daily isotopic analysis revealed the role of different moisture sources and transport effects. Isotope ratios combined with d-excess explain how atmospheric recirculation processes interact with convective activity during rainfall formation processes. Our findings provide a new understanding of rainfall sampling timescales and highlight the importance of water isotopes to decipher key hydrometeorological processes in a complex spatial and temporal context in central-southeastern Brazil.
Meshless Surface Wind Speed Field Reconstruction Based on Machine Learning
Nian LIU, Zhongwei YAN, Xuan TONG, Jiang JIANG, Haochen LI, Jiangjiang XIA, Xiao LOU, Rui REN, Yi FANG
, Available online   , Manuscript accepted  15 February 2022, doi: 10.1007/s00376-022-1343-8
Abstract:
We propose a novel machine learning approach to reconstruct meshless surface wind speed fields, i.e., to reconstruct the surface wind speed at any location, based on meteorological background fields and geographical information. The random forest method is selected to develop the machine learning data reconstruction model (MLDRM-RF) for wind speeds over Beijing from 2015–19. We use temporal, geospatial attribute and meteorological background field features as inputs. The wind speed field can be reconstructed at any station in the region not used in the training process to cross-validate model performance. The evaluation considers the spatial distribution of and seasonal variations in the root mean squared error (RMSE) of the reconstructed wind speed field across Beijing. The average RMSE is 1.09 m s−1, considerably smaller than the result (1.29 m s−1) obtained with inverse distance weighting (IDW) interpolation. Finally, we extract the important feature permutations by the method of mean decrease in impurity (MDI) and discuss the reasonableness of the model prediction results. MLDRM-RF is a reasonable approach with excellent potential for the improved reconstruction of historical surface wind speed fields with arbitrary grid resolutions. Such a model is needed in many wind applications, such as wind energy and aviation safety assessments.
How Well Do CMIP6 and CMIP5 Models Simulate the Climatological Seasonal Variations in Ocean Salinity?
Yuanxin LIU, Lijing CHENG, Yuying PAN, Zhetao TAN, John ABRAHAM, Bin ZHANG, Jiang ZHU, Junqiang SONG
, Available online   , Manuscript accepted  15 February 2022, doi: 10.1007/s00376-022-1381-2
Abstract:
This paper includes a comprehensive assessment of 40 models from the Coupled Model Intercomparison Project phase 5 (CMIP5) and 33 models from the CMIP phase 6 (CMIP6) to determine the climatological and seasonal variation of ocean salinity from the surface to 2000 m. The general pattern of the ocean salinity climatology can be simulated by both the CMIP5 and CMIP6 models from the surface to 2000-m depth. However, this study shows an increased fresh bias in the surface and subsurface salinity in the CMIP6 multimodel mean, with a global average of −0.44 g kg−1 for the sea surface salinity (SSS) and −0.26 g kg−1 for the 0–1000-m averaged salinity (S1000) compared with the CMIP5 multimodel mean (−0.25 g kg−1 for the SSS and −0.07 g kg−1 for the S1000). In terms of the seasonal variation, both CMIP6 and CMIP5 models show positive (negative) anomalies in the first (second) half of the year in the global average SSS and S1000. The model-simulated variation in SSS is consistent with the observations, but not for S1000, suggesting a substantial uncertainty in simulating and understanding the seasonal variation in subsurface salinity. The CMIP5 and CMIP6 models overestimate the magnitude of the seasonal variation of the SSS in the tropics in the region 20°S–20°N but underestimate the magnitude of the seasonal change in S1000 in the Atlantic and Indian oceans. These assessments show new features of the model errors in simulating ocean salinity and support further studies of the global hydrological cycle.
The Impact of an Abnormal Zonal Vertical Circulation in Autumn of Super El Niño Years on Non-tropical-cyclone Heavy Rainfall over Hainan Island
Fei WANG, Lifang SHENG, Xiadong AN, Haixia ZHOU, Yingying ZHANG, Xiaodong LI, Yigeng DING, Jing YANG
, Available online   , Manuscript accepted  14 February 2022, doi: 10.1007/s00376-022-1388-8
Abstract:
This study reveals a significant positive connection between autumn non-tropical-cyclone heavy rainfall on Hainan Island and the intensity of Eastern Pacific (EP) El Niño events. That is, the amount of rainfall in super EP El Niño years is more than that in normal EP El Niño years. Comparing EP El Niño years of different intensities, the cooler sea surface temperature in the northwestern Pacific during super EP El Niño years stimulates a negative surface latent heat flux (LHF) anomaly and abnormal anticyclonic circulation at 850 hPa. Under these conditions, an abnormal zonal vertical circulation develops in the northern South China Sea once a positive LHF anomaly and abnormal cyclonic circulation (ACC) at 850 hPa occur in the Beibu Gulf. The abnormal zonal vertical circulation further strengthens the ascending motion over Hainan Island, as the critical factor that leads to excessive rainfall. Further analysis shows that the positive LHF anomaly, which can be attributed to the increased latent heat transfer which resulted from the increased surface wind speed, is an important trigger for the ACC. However, the ACC is also the supplier of favorable moisture conditions because it intensifies vapor convergence over Hainan Island and meridionally transports moisture from the South China Sea to northeastern Hainan Island, thereby generating heavy rainfall. This paper emphasizes that the impact of El Niño events, especially super El Niño events, on rainfall over Hainan Island cannot be ignored, even if the traditional view is that frequent rainfall occurs mainly in La Niña years.
The Asymmetric Connection of SST in the Tasman Sea with Respect to the Opposite Phases of ENSO in Austral Summer
Xueqian SUN, Shuanglin LI, Stefan LIESS
, Available online   , Manuscript accepted  11 February 2022, doi: 10.1007/s00376-022-1421-y
Abstract:
This study uses linear regression and composite analyses to identify a pronounced asymmetric connection of sea surface temperature (SST) in the Tasman Sea with the two opposite phases of El Niño-Southern Oscillation (ENSO) during austral summer. In El Niño years, the SST anomalies (SSTAs) in the Tasman Sea exhibit a dipolar pattern with weak warmth in the northwest and modest cooling in the southeast, while during La Niña years the SSTAs exhibit a basin-scale warmth with greater amplitude. Investigations into the underlying mechanism suggest that this asymmetry arises from a mechanism related to oceanic heat transport, specifically the anomalous Ekman meridional heat transport induced by the zonal wind stress anomalies, rather than the surface heat fluxes on the air-sea interface. Further analysis reveals that the asymmetry of oceanic heat transport between El Niño and La Niña years is driven by the asymmetric atmospheric circulation over the Tasman Sea stimulated by the asymmetric diabatic heating in the tropical Pacific between the two opposite ENSO phases.
The asymmetric connection of SST in the Tasman Sea with respect to the opposite phases of ENSO in austral summer
Xueqian Sun, Shuanglin Li, Stefan Liess
, Available online   , Manuscript accepted  11 February 2022, doi: 10.1007/s00376-022-0421-y
Abstract:
Using linear regression and composite analyses, this study identifies a pronounced asymmetric connection of sea surface temperature (SST) in the Tasman Sea with the two opposite phases of El Niño-Southern Oscillation (ENSO) during austral summer. In El Niño years, the SST anomalies (SSTAs) in the Tasman Sea exhibit a dipolar pattern with weak warmth in the northwest and modest cooling in the southeast, while during La Niña years the SSTAs exhibit a basin-scale warmth with greater amplitude. Investigations on the underlying mechanism suggest that this asymmetry arises from the oceanic heat transport, especially the anomalous Ekman meridional heat fluxes induced by the zonal wind stress anomalies, rather than the surface heat fluxes on the air-sea interface. A further analysis demonstrates that the asymmetry of oceanic heat transport between El Niño and La Niña years is driven by the asymmetric atmospheric circulation over the Tasman Sea stimulated by the asymmetric diabatic heating in the tropical Pacific between the two opposite ENSO phases.
Different Impacts of Intraseasonal Oscillations on Precipitation in Southeast China between Early and Late Summers
Junqi LIU, Riyu LU
, Available online   , Manuscript accepted  11 February 2022, doi: 10.1007/s00376-022-1347-4
Abstract:
This study investigates the influences of boreal summer intraseasonal oscillation (BSISO), which originates from the equatorial Indian Ocean and prevails over the Indo-Pacific region, on precipitation over Southeast China, including South China and Yangtze River Valley. The results indicate that the BSISO-related precipitation anomalies are remarkably different between early summer (May–June) and late summer (July–August). The BSISO-related precipitation anomalies tend to appear more northward in late summer in comparison with early summer. Accordingly, the BSISO is significantly related to precipitation anomalies over South China during many phases in early summer but related to very weak anomalies during all the phases in late summer. Such northward shifts of precipitation anomalies from early summer to late summer are clearest during phases 4 and 7, when the lower-tropospheric anticyclonic and cyclonic circulation anomalies dominate over the subtropical western North Pacific, respectively. Finally, we explain the differences between early and late summers through the seasonal northward migration of climatological equivalent potential temperature gradient, which is located in the South China during early summer but migrates northward to the YRV during late summer.
Microphysical Characteristics of Extreme-Rainfall Convection over the Pearl River Delta Region, South China from Polarimetric Radar Data during the Pre-summer Rainy Season
Hao HUANG, Kun ZHAO, Johnny C. L. CHAN, Dongming HU
, Available online   , Manuscript accepted  09 February 2022, doi: 10.1007/s00376-022-1319-8
Abstract:
During the pre-summer rainy season, heavy rainfall occurs frequently in South China. Based on polarimetric radar observations, the microphysical characteristics and processes of convective features associated with extreme rainfall rates (ERCFs) are examined. In the regions with high ERCF occurrence frequency, sub-regional differences are found in the lightning flash rate (LFR) distributions. In the region with higher LFRs, the ERCFs have larger volumes of high reflectivity factor above the freezing level, corresponding to more active riming processes. In addition, these ERCFs are more organized and display larger spatial coverage, which may be related to the stronger low-level wind shear and higher terrain in the region. In the region with lower LFRs, the ERCFs have lower echo tops and lower-echo centroids. However, no clear differences of the most unstable convective available potential energy (MUCAPE) exist in the ERCFs in the regions with different LFR characteristics. Regardless of the LFRs, raindrop collisional coalescence is the main process for the growth of raindrops in the ERCFs. In the ERCFs within the region with lower LFRs, the main mechanism for the rapid increase of liquid water content with decreasing altitude below 4 km is through the warm-rain processes converting cloud drops to raindrops. However, in those with higher LFRs, the liquid water content generally decreases with decreasing altitude.
A Causality-guided Statistical Approach for Modeling Extreme Mei-yu Rainfall Based on Known Large-scale Modes—A Pilot Study
Kelvin S. NG, Gregor C. LECKEBUSCH, Kevin I. HODGES
, Available online   , Manuscript accepted  05 February 2022, doi: 10.1007/s00376-022-1348-3
Abstract:
Extreme Mei-yu rainfall (MYR) can cause catastrophic impacts to the economic development and societal welfare in China. While significant improvements have been made in climate models, they often struggle to simulate local-to-regional extreme rainfall (e.g., MYR). Yet, large-scale climate modes (LSCMs) are relatively well represented in climate models. Since there exists a close relationship between MYR and various LSCMs, it might be possible to develop causality-guided statistical models for MYR prediction based on LSCMs. These statistical models could then be applied to climate model simulations to improve the representation of MYR in climate models. In this pilot study, it is demonstrated that skillful causality-guided statistical models for MYR can be constructed based on known LSCMs. The relevancy of the selected predictors for statistical models are found to be consistent with the literature. The importance of temporal resolution in constructing statistical models for MYR is also shown and is in good agreement with the literature. The results demonstrate the reliability of the causality-guided approach in studying complex circulation systems such as the East Asian summer monsoon (EASM). Some limitations and possible improvements of the current approach are discussed. The application of the causality-guided approach opens up a new possibility to uncover the complex interactions in the EASM in future studies.
Discrepancies in Simulated Ocean Net Surface Heat Fluxes over the North Atlantic
Chunlei LIU, Yazhu YANG, Xiaoqing LIAO, Ning CAO, Jimmy LIU, Niansen OU, Richard P. ALLAN, Liang JIN, Ni CHEN, Rong ZHENG
, Available online   , Manuscript accepted  20 January 2022, doi: 10.1007/s00376-022-1360-7
Abstract:
The change in ocean net surface heat flux plays an important role in the climate system. It is closely related to the ocean heat content change and ocean heat transport, particularly over the North Atlantic, where the ocean loses heat to the atmosphere, affecting the AMOC (Atlantic Meridional Overturning Circulation) variability and hence the global climate. However, the difference between simulated surface heat fluxes is still large due to poorly represented dynamical processes involving multiscale interactions in model simulations. In order to explain the discrepancy of the surface heat flux over the North Atlantic, datasets from nineteen AMIP6 and eight highresSST-present climate model simulations are analyzed and compared with the DEEPC (Diagnosing Earth's Energy Pathways in the Climate system) product. As an indirect check of the ocean surface heat flux, the oceanic heat transport inferred from the combination of the ocean surface heat flux, sea ice, and ocean heat content tendency is compared with the RAPID (Rapid Climate Change-Meridional Overturning Circulation and Heat flux array) observations at 26°N in the Atlantic. The AMIP6 simulations show lower inferred heat transport due to less heat loss to the atmosphere. The heat loss from the AMIP6 ensemble mean north of 26°N in the Atlantic is about 10 W m–2 less than DEEPC, and the heat transport is about 0.30 PW (1 PW = 1015 W) lower than RAPID and DEEPC. The model horizontal resolution effect on the discrepancy is also investigated. Results show that by increasing the resolution, both surface heat flux north of 26°N and heat transport at 26°N in the Atlantic can be improved.
Recent Decrease in the Difference in Tropical Cyclone Occurrence between the Atlantic and the Western North Pacific
Johnny C. L. CHAN, Kin Sik LIU
, Available online   , Manuscript accepted  18 January 2022, doi: 10.1007/s00376-022-1309-x
Abstract:
Climatologically, among all ocean basins, the western North Pacific (WNP) has the largest annual number of tropical cyclones (TCs) of around 26 while the Atlantic has around 13, yielding a difference of 13. However, the difference is –7 in 2020, with 30 TCs in the Atlantic and 23 in the WNP, which is the most negative difference within the last 46 years. In fact, during the last 26 years, the difference in TC number is below 10 in ten years, with four years being negative. Such a decreasing difference in TC number can be attributed to the natural multidecadal variation of the Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation, as well as other external forcings such as anthropogenic aerosol forcing and increased greenhouse gases, with the additional impact from the La Niña condition. This result has significant implications on climate model projections of future TC activity in the two ocean basins.
Assimilation of the FY-4A AGRI Clear-Sky Radiance Data in a Regional Numerical Model and Its Impact on the Forecast of the “21·7” Henan Extremely Persistent Heavy Rainfall
Lan XU, Wei CHENG, Zhongren DENG, Juanjuan LIU, Bin WANG, Bin LU, Shudong WANG, Li DONG
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1380-3
Abstract:
Assimilation of the Advanced Geostationary Radiance Imager (AGRI) clear-sky radiance in a regional model is performed. The forecasting effectiveness of the assimilation of two water vapor (WV) channels with conventional observations for the “21·7” Henan extremely heavy rainfall is analyzed and compared with a baseline test that assimilates only conventional observations in this study. The results show that the 24-h cumulative precipitation forecast by the assimilation experiment with the addition of the AGRI exceeds 500 mm, compared to a maximum value of 532.6 mm measured by the national meteorological stations, and that the location of the maximum precipitation is consistent with the observations. The results for the short periods of intense precipitation processes are that the simulation of the location and intensity of the 3-h cumulative precipitation is also relatively accurate. The analysis increment shows that the main difference between the two sets of assimilation experiments is over the ocean due to the additional ocean observations provided by FY-4A, which compensates for the lack of ocean observations. The assimilation of satellite data adjusts the vertical and horizontal wind fields over the ocean by adjusting the atmospheric temperature and humidity, which ultimately results in a narrower and stronger WV transport path to the center of heavy precipitation in Zhengzhou in the lower troposphere. Conversely, the WV convergence and upward motion in the control experiment are more dispersed; therefore, the precipitation centers are also correspondingly more dispersed.
Spatiotemporal Variations of Microwave Land Surface Emissivity (MLSE) over China Derived from Four-Year Recalibrated Fengyun 3B MWRI Data
Rui LI, Jiheng HU, Shengli WU, Peng ZHANG, Husi LETU, Yu WANG, Xuewen WANG, Yuyun FU, Renjun ZHOU, Ling SUN
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1314-0
Abstract:
Microwave Land Surface Emissivity (MLSE) over China under both clear and cloudy sky conditions was retrieved using measurements of recalibrated microwave brightness temperatures (Tbs) from Fengyun-3B Microwave Radiation Imager (FY-3B MWRI), combined with cloud properties derived from Himawari-8 Advanced Himawari Imager (AHI) observations. The contributions from cloud particles and atmospheric gases to the upwelling Tbs at the top of atmosphere were calculated and removed in radiative transfer. The MLSEs at horizontal polarizations at 10.65, 18.7, and 36.5 GHz during 7 July 2015 to 30 June 2019 over China showed high values in the southeast vegetated area and low values in the northwest barren, or sparsely vegetated, area. The maximum values were found in the belt area of the Qinling-Taihang Mountains and the eastern edge of the Qinghai-Tibet Plateau, which is highly consistent with MLSEs derived from AMSR-E. It demonstrates that the measurements of FY-3B MWRI Tbs, including its calibration and validation, are reliable, and the retrieval algorithm developed in this study works well. Seasonal variations of MLSE in China are mainly driven by the combined effects of vegetation, rainfall, and snow cover. In tropical and southern forest regions, the seasonal variation of MLSE is small due to the enhancement from vegetation and the suppression from rainfall. In the boreal area, snow causes a significant decrease of MLSE at 36.5 GHz in winter. Meanwhile, the MLSE at lower frequencies experiences less suppression. In the desert region in Xinjiang, increases of MLSEs at all frequencies are observed with increasing snow cover.
A Modified Double-Moment Bulk Microphysics Scheme Geared toward the East Asian Monsoon Region
Jinfang YIN, Donghai WANG, Guoqing ZHAI, Hong WANG, Huanbin XU, Chongjian LIU
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1402-1
Abstract:
Representation of cloud microphysical processes is one of the key aspects of numerical models. An improved double-moment bulk cloud microphysics scheme (named IMY) was created based on the standard Milbrandt-Yau (MY) scheme in the Weather Research and Forecasting (WRF) model for the East Asian monsoon region (EAMR). In the IMY scheme, the shape parameters of raindrops, snow particles, and cloud droplet size distributions are variables instead of fixed constants. Specifically, the shape parameters of raindrop and snow size distributions are diagnosed from their respective shape-slope relationships. The shape parameter for the cloud droplet size distribution depends on the total cloud droplet number concentration. In addition, a series of minor improvements involving detailed cloud processes have also been incorporated. The improved scheme was coupled into the WRF model and tested on two heavy rainfall cases over the EAMR. The IMY scheme is shown to reproduce the overall spatial distribution of rainfall and its temporal evolution, evidenced by comparing the modeled results with surface gauge observations. The simulations also successfully capture the cloud features by using satellite and ground-based radar observations as a reference. The IMY has yielded simulation results on the case studies that were comparable, and in ways superior to MY, indicating that the improved scheme shows promise. Although the simulations demonstrated a positive performance evaluation for the IMY scheme, continued experiments are required to further validate the scheme with different weather events.
A Nonlinear Representation of Model Uncertainty in a Convective-Scale Ensemble Prediction System
Zhizhen XU, Jing CHEN, Mu MU, Guokun DAI, Yanan MA
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1341-x
Abstract:
How to accurately address model uncertainties with consideration of the rapid nonlinear error growth characteristics in a convection-allowing system is a crucial issue for performing convection-scale ensemble forecasts. In this study, a new nonlinear model perturbation technique for convective-scale ensemble forecasts is developed to consider a nonlinear representation of model errors in the Global and Regional Assimilation and Prediction Enhanced System (GRAPES) Convection-Allowing Ensemble Prediction System (CAEPS). The nonlinear forcing singular vector (NFSV) approach, that is, conditional nonlinear optimal perturbation-forcing (CNOP-F), is applied in this study, to construct a nonlinear model perturbation method for GRAPES-CAEPS. Three experiments are performed: One of them is the CTL experiment, without adding any model perturbation; the other two are NFSV-perturbed experiments, which are perturbed by NFSV with two different groups of constraint radii to test the sensitivity of the perturbation magnitude constraint. Verification results show that the NFSV-perturbed experiments achieve an overall improvement and produce more skillful forecasts compared to the CTL experiment, which indicates that the nonlinear NFSV-perturbed method can be used as an effective model perturbation method for convection-scale ensemble forecasts. Additionally, the NFSV-L experiment with large perturbation constraints generally performs better than the NFSV-S experiment with small perturbation constraints in the verification for upper-air and surface weather variables. But for precipitation verification, the NFSV-S experiment performs better in forecasts for light precipitation, and the NFSV-L experiment performs better in forecasts for heavier precipitation, indicating that for different precipitation events, the perturbation magnitude constraint must be carefully selected. All the findings above lay a foundation for the design of nonlinear model perturbation methods for future CAEPSs.
A Sensitivity Study of Arctic Ice-Ocean Heat Exchange to the Three-Equation Boundary Condition Parametrization in CICE6
Lei YU, Jiping LIU, Yongqi GAO, Qi SHU
, Available online   , Manuscript accepted  11 January 2022, doi: 10.1007/s00376-022-1316-y
Abstract:
In this study, we perform a stand-alone sensitivity study using the Los Alamos Sea ice model version 6 (CICE6) to investigate the model sensitivity to two Ice-Ocean (IO) boundary condition approaches. One is the two-equation approach that treats the freezing temperature as a function of the ocean mixed layer (ML) salinity, using two equations to parametrize the IO heat exchanges. Another approach uses the salinity of the IO interface to define the actual freezing temperature, so an equation describing the salt flux at the IO interface is added to the two-equation approach, forming the so-called three-equation approach. We focus on the impact of the three-equation boundary condition on the IO heat exchange and associated basal melt/growth of the sea ice in the Arctic Ocean. Compared with the two-equation simulation, our three-equation simulation shows a reduced oceanic turbulent heat flux, weakened basal melt, increased ice thickness, and reduced sea surface temperature (SST) in the Arctic. These impacts occur mainly at the ice edge regions and manifest themselves in summer. Furthermore, in August, we observed a downward turbulent heat flux from the ice to the ocean ML in two of our three-equation sensitivity runs with a constant heat transfer coefficient (0.006), which caused heat divergence and congelation at the ice bottom. Additionally, the influence of different combinations of heat/salt transfer coefficients and thermal conductivity in the three-equation approach on the model simulated results is assessed. The results presented in this study can provide insight into sea ice model sensitivity to the three-equation IO boundary condition for coupling the CICE6 to climate models.
Seasonal Predictions of Summer Precipitation in the Middle-lower Reaches of the Yangtze River with Global and Regional Models Based on NUIST-CFS1.0
Wushan YING, Huiping YAN, Jing-Jia LUO
, Available online   , Manuscript accepted  10 January 2022, doi: 10.1007/s00376-022-1389-7
Abstract:
Accurate prediction of the summer precipitation over the middle and lower reaches of the Yangtze River (MLYR) is of urgent demand for the local economic and societal development. This study assesses the seasonal forecast skill in predicting summer precipitation over the MLYR region based on the global Climate Forecast System of Nanjing University of Information Science and Technology (NUIST-CFS1.0, previously SINTEX-F). The results show that the model can provide moderate skill in predicting the interannual variations of the MLYR rainbands, initialized from 1 March. In addition, the nine-member ensemble mean can realistically reproduce the links between the MLYR precipitation and tropical sea surface temperature (SST) anomalies, but the individual members show great discrepancies, indicating large uncertainty in the forecasts. Furthermore, the NUIST-CFS1.0 can predict five of the seven extreme summer precipitation anomalies over the MLYR during 1982–2020, albeit with underestimated magnitudes. The Weather Forecast and Research (WRF) downscaling hindcast experiments with a finer resolution of 30 km, which are forced by the large-scale information of the NUIST-CFS1.0 predictions with a spectral nudging method, display improved predictions of the extreme summer precipitation anomalies to some extent. However, the performance of the downscaling predictions is highly dependent on the global model forecast skill, suggesting that further improvements on both the global and regional climate models are needed.
Assimilation of FY-3D MWTS-II Radiance with 3D Precipitation Detection and the Impacts on Typhoon Forecasts
Luyao QIN, Yaodeng CHEN, Gang MA, Fuzhong WENG, Deming MENG, Peng ZHANG
, Available online   , Manuscript accepted  07 January 2022, doi: 10.1007/s00376-022-1252-x
Abstract:
Precipitation detection is an essential step in radiance assimilation because the uncertainties in precipitation would affect the radiative transfer calculation and observation errors. The traditional precipitation detection method for microwave only detects clouds and precipitation horizontally, without considering the three-dimensional distribution of clouds. Extending precipitation detection from 2D to 3D is expected to bring more useful information to the data assimilation without using the all-sky approach. In this study, the 3D precipitation detection method is adopted to assimilate Microwave Temperature Sounder-2 (MWTS-II) onboard the Fengyun-3D, which can dynamically detect the channels above precipitating clouds by considering the near-real-time cloud parameters. Cycling data assimilation and forecasting experiments for Typhoons Lekima (2019) and Mitag (2019) are carried out. Compared with the control experiment, the quantity of assimilated data with the 3D precipitation detection increases by approximately 23%. The quality of the additional MWTS-II radiance data is close to the clear-sky data. The case studies show that the average root-mean-square errors (RMSE) of prognostic variables are reduced by 1.7% in the upper troposphere, leading to an average reduction of 4.53% in typhoon track forecasts. The detailed diagnoses of Typhoon Lekima (2019) further show that the additional MWTS-II radiances brought by the 3D precipitation detection facilitate portraying a more reasonable circulation situation, thus providing more precise structures. This paper preliminarily proves that 3D precipitation detection has potential added value for increasing satellite data utilization and improving typhoon forecasts.
Effects of Drag Coefficients on Surface Heat Flux during Typhoon Kalmaegi (2014)
Lei LIU, Guihua WANG, Ze ZHANG, Huizan WANG
, Available online   , Manuscript accepted  05 January 2022, doi: 10.1007/s00376-022-1285-1
Abstract:
The lack of in situ observations and the uncertainties of the drag coefficient at high wind speeds result in limited understanding of heat flux through the air-sea interface and thus inaccurate estimation of typhoon intensity in numerical models. In this study, buoy observations and numerical simulations from an air-sea coupled model are used to assess the surface heat flux changes and impacts of the drag coefficient parameterization schemes on its simulations during the passage of Typhoon Kalmaegi (2014). Three drag coefficient schemes, which make the drag coefficient increase, level off, and decrease, respectively, are considered. The air-sea coupled model captured both trajectory and intensity changes better than the atmosphere-only model, though with relatively weaker sea surface cooling (SSC) compared to that captured by buoy observations, which led to relatively higher heat flux and thus a stronger typhoon. Different from previous studies, for a moderate typhoon, the coupled simulation with the increasing drag coefficient scheme outputted an intensity most consistent with the observation because of the strongest SSC, reasonable ratio of latent and sensible heat exchange coefficients, and an obvious reduction in the overestimated surface heat flux among all experiments. Results from sensitivity experiments showed that surface heat flux was significantly determined by the drag coefficient-induced SSC rather than the resulting wind speed changes. Only when SSC differs indistinctively (<0.4°C) between the coupled simulations, heat flux showed a weak positive correlation with the drag coefficient-impacted 10-m wind speed. The drag coefficient also played an important role in decreasing heat flux even a long time after the passage of Kalmaegi because of the continuous upwelling from deeper ocean layers driven by the impacted momentum flux through the air-sea interface.
Characterization of Regional Combustion Efficiency using ΔXCO: ΔXCO2 Observed by a Portable Fourier-Transform Spectrometer at an Urban Site in Beijing
Ke CHE, Yi LIU, Zhaonan CAI, Dongxu YANG, Haibo WANG, Denghui JI, Yang YANG, Pucai WANG
, Available online   , Manuscript accepted  05 January 2022, doi: 10.1007/s00376-022-1247-7
Abstract:
Measurements of column-averaged dry-air mole fractions of carbon dioxide and carbon monoxide, CO2 (XCO2) and CO (XCO), were performed throughout 2019 at an urban site in Beijing using a compact Fourier Transform Spectrometer (FTS) EM27/SUN. This data set is used to assess the characteristics of combustion-related CO2 emissions of urban Beijing by analyzing the correlated daily anomalies of XCO and XCO2 (e.g., ΔXCO and ΔXCO2). The EM27/SUN measurements were calibrated to a 125HR-FTS at the Xianghe station by an extra EM27/SUN instrument transferred between two sites. The ratio of ΔXCO over ΔXCO2 (ΔXCO:ΔXCO2) is used to estimate the combustion efficiency in the Beijing region. A high correlation coefficient (0.86) between ΔXCO and ΔXCO2 is observed. The CO:CO2 emission ratio estimated from inventories is higher than the observed ΔXCO:ΔXCO2 (10.46 ± 0.11 ppb ppm−1) by 42.54%–101.15%, indicating an underestimation in combustion efficiency in the inventories. Daily ΔXCO:ΔXCO2 are influenced by transportation governed by weather conditions, except for days in summer when the correlation is low due to the terrestrial biotic activity. By convolving the column footprint [ppm (μmol m–2 s–1)–1] generated by the Weather Research and Forecasting-X-Stochastic Time-Inverted Lagrangian Transport models (WRF-X-STILT) with two fossil-fuel emission inventories (the Multi-resolution Emission Inventory for China (MEIC) and the Peking University (PKU) inventory), the observed enhancements of CO2 and CO were used to evaluate the regional emissions. The CO2 emissions appear to be underestimated by 11% and 49% for the MEIC and PKU inventories, respectively, while CO emissions were overestimated by MEIC (30%) and PKU (35%) in the Beijing area.
Quantitative Precipitation Forecast Experiment Based on Basic NWP Variables Using Deep Learning
Kanghui ZHOU, Jisong SUN, Yongguang ZHENG, Yutao ZHANG
, Available online   , Manuscript accepted  31 December 2021, doi: 10.1007/s00376-021-1207-7
Abstract:
The quantitative precipitation forecast (QPF) performance by numerical weather prediction (NWP) methods depends fundamentally on the adopted physical parameterization schemes (PS). However, due to the complexity of the physical mechanisms of precipitation processes, the uncertainties of PSs result in a lower QPF performance than their prediction of the basic meteorological variables such as air temperature, wind, geopotential height, and humidity. This study proposes a deep learning model named QPFNet, which uses basic meteorological variables in the ERA5 dataset by fitting a non-linear mapping relationship between the basic variables and precipitation. Basic variables forecasted by the highest-resolution model (HRES) of the European Centre for Medium-Range Weather Forecasts (ECMWF) were fed into QPFNet to forecast precipitation. Evaluation results show that QPFNet achieved better QPF performance than ECMWF HRES itself. The threat score for 3-h accumulated precipitation with depths of 0.1, 3, 10, and 20 mm increased by 19.7%, 15.2%, 43.2%, and 87.1%, respectively, indicating the proposed performance QPFNet improved with increasing levels of precipitation. The sensitivities of these meteorological variables for QPF in different pressure layers were analyzed based on the output of the QPFNet, and its performance limitations are also discussed. Using DL to extract features from basic meteorological variables can provide an important reference for QPF, and avoid some uncertainties of PSs.
Energy Paths That Sustain the Warm-Sector Torrential Rainfall over South China and Their Contrasts to the Frontal Rainfall: A Case Study
Shenming FU, Jingping ZHANG, Yali LUO, Wenying YANG, Jianhua SUN
, Available online   , Manuscript accepted  29 December 2021, doi: 10.1007/s00376-021-1336-z
Abstract:
Predicting warm-sector torrential rainfall over South China, which is famous for its destructive power, is one of the most challenging issues of the current numerical forecast field. Insufficient understanding of the key mechanisms underlying this type of event is the root cause. Since understanding the energetics is crucial to understanding the evolutions of various types of weather systems, a general methodology for investigating energetics of torrential rainfall is provided in this study. By applying this methodology to a persistent torrential rainfall event which had concurrent frontal and warm-sector precipitation, the first physical image on the energetics of the warm-sector torrential rainfall is established. This clarifies the energy sources for producing the warm-sector rainfall during this event. For the first time, fundamental similarities and differences between the warm-sector and frontal torrential rainfall are shown in terms of energetics. It is found that these two types of rainfall mainly differed from each other in the lower-tropospheric dynamical features, and their key differences lay in energy sources. Scale interactions (mainly through downscale energy cascade and transport) were a dominant factor for the warm-sector torrential rainfall during this event, whereas, for the frontal torrential rainfall, they were only of secondary importance. Three typical signals in the background environment are found to have supplied energy to the warm-sector torrential rainfall, with the quasi-biweekly oscillation having contributed the most.
Influence of Coriolis Parameter Variation on Langmuir Turbulence in the Ocean Upper Mixed Layer with Large Eddy Simulation
Dongxiao WANG, Guojing LI, Lian SHEN, Yeqiang SHU
, Available online   , Manuscript accepted  29 December 2021, doi: 10.1007/s00376-021-1390-6
Abstract:
Langmuir turbulence is a complex turbulent process in the ocean upper mixed layer. The Coriolis parameter has an important effect on Langmuir turbulence through the Coriolis–Stokes force and Ekman effect, however, this effect on Langmuir turbulence has not been systematically investigated. Here, the impact of the Coriolis parameter on Langmuir turbulence with a change of latitude (LAT) from 20°N to 80°N is studied using a non-hydrostatic large eddy simulation model under an ideal condition. The results show that the ratio of the upper mixed layer depth to Ekman depth scale (RME) RME = 0.266 (LAT = 50°N) is a key value (latitude) for the modulation effect of the Coriolis parameter on the mean and turbulent statistics of Langmuir turbulence. It is found that the rate of change of the sea surface temperature, upper mixed layer depth, entrainment flux, crosswind velocity, downwind vertical momentum flux, and turbulent kinetic energy budget terms associated with Langmuir turbulence are more evident at RME ≤ 0.266 (LAT ≤ 50°N) than at RME ≥ 0.266 (LAT ≥ 50°N). However, the rate of change of the depth-averaged crosswind vertical momentum flux does not have a clear variation between RME ≤ 0.266 and RME ≥ 0.266. The complex changes of both Langmuir turbulence characteristics and influence of Langmuir turbulence on the upper mixed layer with latitude presented here may provide more information for further improving Langmuir turbulence parameterization.
Hybrid Methods for Computing the Streamfunction and Velocity Potential for Complex Flow Fields over Mesoscale Domains
Jie CAO, Qin XU, Haishan CHEN, Shuping MA
, Available online   , Manuscript accepted  10 December 2021, doi: 10.1007/s00376-021-1280-y
Abstract:
Three types of previously used numerical methods are revisited for computing the streamfunction ψ and velocity potential χ from the horizontal velocity v in limited domains. The first type, called the SOR-based method, uses a classical successive over-relaxation (SOR) scheme to compute ψ (or χ) first with an arbitrary boundary condition (BC) and then χ (or ψ) with the BC derived from v . The second type, called the spectral method, uses spectral formulations to construct the inner part of (ψ, χ)—the inversion of (vorticity, divergence) with a homogeneous BC, and then the remaining harmonic part of (ψ, χ) with BCs from v . The third type, called the integral method, uses integral formulas to compute the internally induced (ψ, χ)—the inversion of domain-internal (vorticity, divergence) using the free-space Greenꞌs function without BCs and then the remaining harmonic ψ (or χ) with BCs from v minus the internally-induced part. Although these methods have previously been successfully applied to flows in large-scale and synoptic-scale domains, their accuracy is compromised when applied to complex flows over mesoscale domains, as shown in this paper. To resolve this problem, two hybrid approaches, the integral-SOR method and the integral-spectral method, are developed by combining the first step of the integral method with the second step adopted from the SOR-based and spectral methods, respectively. Upon testing these methods on real-case complex flows, the integral-SOR method is significantly more accurate than the integral-spectral method, noting that the latter is still generally more accurate than the three previously-used methods. The integral-SOR method is recommended for future applications and diagnostic studies of complex flows.
Ocean–atmosphere Teleconnections Play a Key Role in the Interannual Variability of Seasonal Gross Primary Production in China
Kairan YING, Jing PENG, Li DAN, Xiaogu ZHENG
, Available online   , Manuscript accepted  06 December 2021, doi: 10.1007/s00376-021-1226-4
Abstract:
Since the 1950s, the terrestrial carbon uptake has been characterized by interannual variations, which are mainly determined by interannual variations in gross primary production (GPP). Using an ensemble of seven-member TRENDY (Trends in Net Land–Atmosphere Carbon Exchanges) simulations during 1951–2010, the relationships of the interannual variability of seasonal GPP in China with the sea surface temperature (SST) and atmospheric circulations were investigated. The GPP signals that mostly relate to the climate forcing in terms of Residual Principal Component analysis (hereafter, R-PC) were identified by separating out the significant impact from the linear trend and the GPP memory. Results showed that the seasonal GPP over China associated with the first R-PC1 (the second R-PC2) during spring to autumn show a monopole (dipole or tripole) spatial structure, with a clear seasonal evolution for their maximum centers from springtime to summertime. The dominant two GPP R-PC are significantly related to Sea Surface Temperature (SST) variability in the eastern tropical Pacific Ocean and the North Pacific Ocean during spring to autumn, implying influences from the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The identified SST and circulation factors explain 13%, 23% and 19% of the total variance for seasonal GPP in spring, summer and autumn, respectively. A clearer understanding of the relationships of China’s GPP with ocean–atmosphere teleconnections over the Pacific and Atlantic Ocean should provide scientific support for achieving carbon neutrality targets.
The Roles of Barotropic Instability and the Beta Effect in the Eyewall Evolution of Tropical Cyclones
Jie JIANG, Yuqing WANG
, Available online   , Manuscript accepted  25 November 2021, doi: 10.1007/s00376-021-1209-5
Abstract:
Diabatic heating by convection in the eyewall often produces an annular region of high potential vorticity (PV) around the relatively low PV eye in a strong tropical cyclone (TC). Such a PV ring is barotropically unstable and can encourage the exponential growth of PV waves. In this study, such instability and the subsequent nonlinear evolution of three TC-like vortices having PV rings with different degrees of hollowness on an f-plane are first examined using an unforced, inviscid shallow-water-equation model. Results show that the simulated eyewalls evolve similarly to those in the nondivergent barotropic model. It is also found that the polygonal eyewall structure can be decomposed into vortex Rossby waves (VRWs) of different wavenumbers with different amplitudes, allowing for wave-wave interactions to produce complicated behaviors of mesovortices in the TC eyewall. The same set of PV rings has been examined on a beta-plane. Although the beta effect has been rendered unimportant to the eyewall evolution due to the relatively small scale of the inner-core circulation, this study shows that the beta effect may erode the coherent structure of mesovortices in the eyewall of an initially hollow PV-ring vortex. Mesovortices modeled on the beta-plane with a greater beta parameter tend to experience an earlier breakdown and enhanced radial gradients of the basic-state (azimuthally mean) angular velocity, followed by wave-wave, wave-flow interactions, leading to earlier merger and axisymmetrization processes. This result implies that the beta effect could be one of the forcings that shorten the lifetime of quasi-steady mesovortices in the eyewall of real TCs.
Impacts of New Implementing Strategies for Surface and Model Physics Perturbations in TREPS on Forecasts of Landfalling Tropical Cyclones
Xubin ZHANG
, Available online   , Manuscript accepted  23 November 2021, doi: 10.1007/s00376-021-1222-8
Abstract:
To improve the ensemble prediction system of the tropical regional atmosphere model for the South China Sea (TREPS) in predicting landfalling tropical cyclones (TCs), the impacts of three new implementing strategies for surface and model physics perturbations in TREPS were evaluated for 19 TCs making landfall in China during 2014–16. For sea surface temperature (SST) perturbations, spatially uncorrelated random perturbations were replaced with spatially correlated ones. The multiplier f, which is used to form perturbed tendency in the Stochastically Perturbed Parameterization Tendency (SPPT) scheme, was inflated in regions with evident convective activity (f-inflated SPPT). Lastly, the Stochastically Perturbed Parameterization (SPP) scheme with 14 perturbed parameters selected from the planetary boundary layer, surface layer, microphysics, and cumulus convection parameterizations was added. Overall, all these methods improved forecasts more significantly for non-intensifying than intensifying TCs. Compared with f-inflated SPPT, the spatially correlated SST perturbations generally showed comparable performance but were more (less) skillful for intensifying (non-intensifying) TCs. The advantages of the spatially correlated SST perturbations and f-inflated SPPT were mainly present in the deterministic guidance for both TC track and wind and in the probabilistic guidance for reliability of wind. For intensifying TCs, adding SPP led to mixed impacts with significant improvements in probability-matched mean of modest winds and in probabilistic forecasts of rainfall; while for non-intensifying TCs, adding SPP frequently led to positive impacts on the deterministic guidance for track, intensity, strong winds, and moderate rainfall and on the probabilistic guidance for wind and discrimination of rainfall.
Application of a Neural Network to Store and Compute the Optical Properties of Non-Spherical Particles
Jinhe YU, Lei BI, Wei HAN, Xiaoye ZHANG
, Available online   , Manuscript accepted  23 November 2021, doi: 10.1007/s00376-021-1375-5
Abstract:
Radiative transfer simulations and remote sensing studies fundamentally require accurate and efficient computation of the optical properties of non-spherical particles. This paper proposes a deep learning (DL) scheme in conjunction with an optical property database to achieve this goal. Deep neural network (DNN) architectures were obtained from a dataset of the optical properties of super-spheroids with extensive shape parameters, size parameters, and refractive indices. The dataset was computed through the invariant imbedding T-matrix method. Four separate DNN architectures were created to compute the extinction efficiency factor, single-scattering albedo, asymmetry factor, and phase matrix. The criterion for designing these neural networks was the achievement of the highest prediction accuracy with minimal DNN parameters. The numerical results demonstrate that the determination coefficients are greater than 0.999 between the prediction values from the neural networks and the truth values from the database, which indicates that the DNN can reproduce the optical properties in the dataset with high accuracy. In addition, the DNN model can robustly predict the optical properties of particles with high accuracy for shape parameters or refractive indices that are unavailable in the database. Importantly, the ratio of the database size (~127 GB) to that of the DNN parameters (~20 MB) is approximately 6810, implying that the DNN model can be treated as a highly compressed database that can be used as an alternative to the original database for real-time computing of the optical properties of non-spherical particles in radiative transfer and atmospheric models.
The Synergism between Methanogens and Methanotrophs and the Nature of their Contributions to the Seasonal Variation of Methane Fluxes in a Wetland: The Case of Dajiuhu Subalpine Peatland
Luwen WANG, Jiwen GE, Liang FENG, Yaoyao LIU, Yu LI, Jiumei WANG, Xiang XIAO, Zhiqi ZHANG
, Available online   , Manuscript accepted  19 October 2021, doi: 10.1007/s00376-021-1255-z
Abstract:
Wetland ecosystems are the most important natural methane (CH4) sources, whose fluxes periodically fluctuate. Methanogens (methane producers) and methanotrophs (methane consumers) are considered key factors affecting CH4 fluxes in wetlands. However, the symbiotic relationship between methanogens and methanotrophs remains unclear. To help close this research gap, we collected and analyzed samples from four soil depths in the Dajiuhu subalpine peatland in January, April, July, and October 2019 and acquired seasonal methane flux data from an eddy covariance (EC) system, and investigated relationships. A phylogenetic molecular ecological networks (pMENs) analysis was used to identify keystone species and the seasonal variations of the co-occurrence patterns of methanogenic and methanotrophic communities. The results indicate that the seasonal variations of the interactions between methanogenic and methanotrophic communities contributed to CH4 emissions in wetlands. The keystone species discerned by the network analysis also showed their importance in mediating CH4 fluxes. Methane (CH4) emissions in wetlands were lowest in spring; during this period, the most complex interactions between microbes were observed, with intense competition among methanogens while methanotrophs demonstrated better cooperation. Reverse patterns manifested themselves in summer when the highest CH4 flux was observed. Methanoregula formicica was negatively correlated with CH4 fluxes and occupied the largest ecological niches in the spring network. In contrast, both Methanocella arvoryzae and Methylocystaceae demonstrated positive correlations with CH4 fluxes and were better adapted to the microbial community in the summer. In addition, soil temperature and nitrogen were regarded as significant environmental factors to CH4 fluxes. This study was successful in explaining the seasonal patterns and microbial driving mechanisms of CH4 emissions in wetlands.
Ice Nucleation of Cirrus Clouds Related to the Transported Dust Layer Observed by Ground-Based Lidars over Wuhan, China
Yun HE, Fan YI, Fuchao LIU, Zhenping YIN, Jun ZHOU
, Available online   , Manuscript accepted  14 October 2021, doi: 10.1007/s00376-021-1192-x
Abstract:
Cirrus clouds related to transported dust layers were identified on 22 occasions with ground-based polarization lidar from December 2012 to February 2018 over Wuhan (30.5°N, 114.4°E), China. All the events occurred in spring and winter. Cirrus clouds were mostly located above 7.6 km on top of the aloft dust layers. In-cloud relative humidity with respect to ice (RHi) derived from water vapor Raman lidar as well as from ERA5 reanalysis data were used as criteria to determine the possible ice nucleation regimes. Corresponding to the two typical cases shown, the observed events can be classified into two categories: (1) category A (3 cases), in-cloud peak RHi ≥ 150%, indicating competition between heterogeneous nucleation and homogeneous nucleation; and (2) category B (19 cases), in-cloud peak RHi < 150%, revealing that only heterogeneous nucleation was involved. Heterogeneous nucleation generally took place during instances of cirrus cloud formation in the upper troposphere when advected dust particles were present. Although accompanying cloud-top temperatures ranged from –51.9°C to –30.4°C, dust-related heterogeneous nucleation contributed to primary ice nucleation in cirrus clouds by providing ice nucleating particle concentrations on the order of 10-3 L-1 to 102 L-1. Heterogeneous nucleation and subsequent crystal growth reduced the ambient RHi to be less than 150% by consuming water vapor and thus completely inhibited homogeneous nucleation.
The Variability of Air-sea O2 Flux in CMIP6: Implications for Estimating Terrestrial and Oceanic Carbon Sinks
Changyu LI, Jianping HUANG, Lei DING, Yu REN, Linli AN, Xiaoyue LIU, Jiping HUANG
, Available online   , Manuscript accepted  08 October 2021, doi: 10.1007/s00376-021-1273-x
Abstract:
The measurement of atmospheric O2 concentrations and related oxygen budget have been used to estimate terrestrial and oceanic carbon uptake. However, a discrepancy remains in assessments of O2 exchange between ocean and atmosphere (i.e. air-sea O2 flux), which is one of the major contributors to uncertainties in the O2-based estimations of the carbon uptake. Here, we explore the variability of air-sea O2 flux with the use of outputs from Coupled Model Intercomparison Project phase 6 (CMIP6). The simulated air-sea O2 flux exhibits an obvious warming-induced upward trend (~1.49 Tmol yr−2) since the mid-1980s, accompanied by a strong decadal variability dominated by oceanic climate modes. We subsequently revise the O2-based carbon uptakes in response to this changing air-sea O2 flux. Our results show that, for the 1990--2000 period, the averaged net ocean and land sinks are 2.10±0.43 and 1.14±0.52 GtC yr−1 respectively, overall consistent with estimates derived by the Global Carbon Project (GCP). An enhanced carbon uptake is found in both land and ocean after year 2000, reflecting the modification of carbon cycle under human activities. Results derived from CMIP5 simulations also investigated in the study allow for comparisons from which we can see the vital importance of oxygen dataset on carbon uptake estimations.
Changes in Global Vegetation Distribution and Carbon Fluxes in Response to Global Warming: Simulated Results from IAP-DGVM in CAS-ESM2
Xiaofei GAO, Jiawen ZHU, Xiaodong ZENG, Minghua ZHANG, Yongjiu DAI, Duoying JI, He ZHANG
, Available online   , Manuscript accepted  14 September 2021, doi: 10.1007/s00376-021-1138-3
Abstract:
Terrestrial ecosystems are an important part of Earth systems, and they are undergoing remarkable changes in response to global warming. This study investigates the response of the terrestrial vegetation distribution and carbon fluxes to global warming by using the new dynamic global vegetation model in the second version of the Chinese Academy of Sciences (CAS) Earth System Model (CAS-ESM2). We conducted two sets of simulations, a present-day simulation and a future simulation, which were forced by the present-day climate during 1981–2000 and the future climate during 2081–2100, respectively, as derived from RCP8.5 outputs in CMIP5. CO2 concentration is kept constant in all simulations to isolate CO2-fertilization effects. The results show an overall increase in vegetation coverage in response to global warming, which is the net result of the greening in the mid-high latitudes and the browning in the tropics. The results also show an enhancement in carbon fluxes in response to global warming, including gross primary productivity, net primary productivity, and autotrophic respiration. We found that the changes in vegetation coverage were significantly correlated with changes in surface air temperature, reflecting the dominant role of temperature, while the changes in carbon fluxes were caused by the combined effects of leaf area index, temperature, and precipitation. This study applies the CAS-ESM2 to investigate the response of terrestrial ecosystems to climate warming. Even though the interpretation of the results is limited by isolating CO2-fertilization effects, this application is still beneficial for adding to our understanding of vegetation processes and to further improve upon model parameterizations.
Calculating the Climatology and Anomalies of Surface Cloud Radiative Effect Using Cloud Property Histograms and Cloud Radiative Kernels
Chen ZHOU, Yincheng LIU, Quan WANG
, Available online   , Manuscript accepted  08 September 2021, doi: 10.1007/s00376-021-1166-z
Abstract:
Cloud radiative kernels (CRK) built with radiative transfer models have been widely used to analyze the cloud radiative effect on top of atmosphere (TOA) fluxes, and it is expected that the CRKs would also be useful in the analyses of surface radiative fluxes, which determines the regional surface temperature change and variability. In this study, CRKs at the surface and TOA were built using the Rapid Radiative Transfer Model (RRTM). Longwave cloud radiative effect (CRE) at the surface is primarily driven by cloud base properties, while TOA CRE is primarily decided by cloud top properties. For this reason, the standard version of surface CRK is a function of latitude, longitude, month, cloud optical thickness (τ) and cloud base pressure (CBP), and the TOA CRK is a function of latitude, longitude, month, τ and cloud top pressure (CTP). Considering that the cloud property histograms provided by climate models are functions of CTP instead of CBP at present, the surface CRKs on CBP-τ histograms were converted to CTP-τ fields using the statistical relationship between CTP, CBP and τ obtained from collocated CloudSat and MODIS observations. For both climate model outputs and satellites observations, the climatology of surface CRE and cloud-induced surface radiative anomalies calculated with the surface CRKs and cloud property histograms are well correlated with those calculated from surface radiative fluxes. The cloud-induced surface radiative anomalies reproduced by surface CRKs and MODIS cloud property histograms are not affected by spurious trends that appear in Clouds and the Earth's Radiant Energy System (CERES) surface irradiances products.
Multiscale Combined Action and Disturbance Characteristics of Pre-summer Extreme Precipitation Events over South China
Hongbo LIU, Ruojing YAN, Bin WANG, Guanghua CHEN, Jian LING, Shenming FU
, Available online   , Manuscript accepted  08 September 2021, doi: 10.1007/s00376-021-1172-1
Abstract:
The dominant frequency modes of pre-summer extreme precipitation events (EPEs) over South China (SC) between 1998 and 2018 were investigated. The 67 identified EPEs were all characterized by the 3–8-d (synoptic) frequency band. However, multiscale combined modes of the synoptic and three low-frequency bands (10 20-d (quasi-biweekly, QBW); 15–40-d (quasi-monthly, QM); and 20–60-d (intraseasonal)) accounted for the majority (63%) of the EPEs, and the precipitation intensity on the peak wet day was larger than that of the single synoptic mode. It was found that EPEs form within strong southwesterly anomalous flows characterized by either lower-level cyclonic circulation over SC or a deep trough over eastern China. Bandpass-filtered disturbances revealed the direct precipitating systems and their life cycles. Synoptic-scale disturbances are dominated by mid–high latitude troughs, and the cyclonic anomalies originate from downstream of the Tibetan Plateau (TP). Given the warm and moist climate state, synoptic-scale northeasterly flows can even induce EPEs. At the QBW and QM scales, the disturbances originate from the tropical Pacific, downstream of the TP, or mid–high latitudes (QBW only). Each is characterized by cyclonic–anticyclonic wave trains and intense southwesterly flows between them within a region of large horizontal pressure gradient. The intraseasonal disturbances are confined to tropical regions and influence SC by marginal southwesterly flows. It is concluded that low-frequency disturbances provide favorable background conditions for EPEs over SC and synoptic-scale disturbances ultimately induce EPEs on the peak wet days. Both should be simultaneously considered for EPE predictions over SC.
Characteristics of Pre-summer Daytime Cloud Regimes over Coastal South China from the Himawari-8 Satellite
Mingxin LI, Yali LUO, Min MIN
, Available online   , Manuscript accepted  30 August 2021, doi: 10.1007/s00376-021-1148-1
Abstract:
Using the high spatiotemporal resolution (2 km-and-10 min) data from the Advanced Himawari Imager onboard the Himawari-8 satellite, this study documents the fine-scale characteristics of daytime cloud regimes (CRs) over coastal South China during the pre-summer rainy season (April–June). Six CRs (CR1–CR6) are identified based on the joint frequency distribution of cloud top brightness temperature and cloud optical thickness, namely, the optically thin-to-moderate cloud mixture, optically thin warm clouds with cirrus, optically thick warm clouds, weak convective cloud mixture, strong convective clouds, and extreme, deep convective clouds. The optically thick warm clouds are the major CR during April and May, with higher frequencies over land, especially along the urban agglomeration, rather than the offshore which may be an indicator of the higher aerosol concentrations being a contributing factor over the cities. The CRs with weak convective cloud mixtures and strong convective clouds appear more frequently over the land, while the two CRs with optically thinner clouds occur mainly offshore. Synoptic flow patterns (SPs) are objectively identified and examined focusing on those favoring the two major rain-producing CRs (CR5 and CR6) and the highly reflective CR with optically thick warm clouds (CR3). The two SPs favoring CR5 and CR6 are characterized by abundant moisture with low-level jets after monsoon onset, and a northwest high-southeast low pattern with strong dynamic convergence along the coastline, respectively. The non-convective CR3 with high reflectance is related to a SP that features the western North Pacific subtropical high extending more westward, leading to a moderate moisture supply and a wide range of convective available potential energy, but also, large convective inhibition.
A Machine Learning-based Cloud Detection Algorithm for the Himawari-8 Spectral Image
Chao LIU, Shu YANG, Di DI, Yuanjian YANG, Chen ZHOU, Xiuqing HU, Byung-Ju SOHN
, Available online   , Manuscript accepted  20 April 2021, doi: 10.1007/s00376-021-0366-x
Abstract:
Cloud Masking is one of the most essential products for satellite remote sensing and downstream applications. This study develops machine learning-based (ML-based) cloud detection algorithms using spectral observations for the Advanced Himawari Imager (AHI) onboard the Himawari-8 geostationary satellite. Collocated active observations from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) are used to provide reference labels for model development and validation. We introduce both daytime and nighttime algorithms that differ according to whether solar band observations are included, and the artificial neural network (ANN) and random forest (RF) techniques are adopted for comparison. To eliminate the influences of surface conditions on cloud detection, we introduce three models with different treatments of the surface. Instead of developing independent ML-based algorithms, we add surface variables in a binary way that enhances the ML-based algorithm accuracy by ~5%. Validated against CALIOP observations, we find that our daytime RF-based algorithm outperforms the AHI operational algorithm by improving the accuracy of cloudy pixel detection by ~5%, while at the same time, reducing misjudgment by ~3%. The nighttime model with only infrared observations is also slightly better than the AHI operational product but may tend to overestimate cloudy pixels. Overall, our ML-based algorithms can serve as a reliable method to provide cloud mask results for both daytime and nighttime AHI observations. We furthermore suggest treating the surface with a set of independent variables for future ML-based algorithm development.
Evaluating the Impacts of Cloud Microphysical and Overlap Parameters on Simulated Clouds in Global Climate Models
Haibo WANG, Hua ZHANG, Bing XIE, Xianwen JING, Jingyi HE, Yi LIU
, Available online   , Manuscript accepted  18 March 2021, doi: 10.1007/s00376-021-0369-7
Abstract:
The improvement of the accuracy of simulated cloud-related variables, such as the cloud fraction, in global climate models (GCMs) is still a challenging problem in climate modeling. In this study, the influence of cloud microphysics schemes (one-moment versus two-moment schemes) and cloud overlap methods (observation-based versus a fixed vertical decorrelation length) on the simulated cloud fraction was assessed in the BCC_AGCM2.0_CUACE/Aero. Compared with the fixed decorrelation length method, the observation-based approach produced a significantly improved cloud fraction both globally and for four representative regions. The utilization of a two-moment cloud microphysics scheme, on the other hand, notably improved the simulated cloud fraction compared with the one-moment scheme; specifically, the relative bias in the global mean total cloud fraction decreased by 42.9%–84.8%. Furthermore, the total cloud fraction bias decreased by 6.6% in the boreal winter (DJF) and 1.64% in the boreal summer (JJA). Cloud radiative forcing globally and in the four regions improved by 0.3%−1.2% and 0.2%−2.0%, respectively. Thus, our results showed that the interaction between clouds and climate through microphysical and radiation processes is a key contributor to simulation uncertainty.
Letter and Notes
Understanding the development of 2018/19 central Pacific El Niño
Chengyang Guan, Xin Wang, Haijun Yang
, Available online   , Manuscript accepted  18 May 2022, doi: 10.1007/s00376-022-1410-1
Abstract:
A central Pacific (CP) El Niño occurs in 2018-19. Previous studies discovered different mechanisms responsible for sub-types of CP El Niño (CP-I El Niño and CP-II El Niño). By comparing the evolutions of surface winds, ocean temperature and heat budget of CP-I, CP-II and 2018/19 El Niño, it is illustrated that the subtropical westerly anomalies in the North Pacific, which led to anomalous convergence of Ekman flow and surface warming in the central equatorial Pacific, played an important role in the 2018/19 El Niño event as well as in CP-II El Niño. Although the off-equatorial forcing played a vital role, it is found that the equatorial forcing acted as a driving (damping) term in boreal spring (summer) of 2018/19 El Niño. The 2018/19 El Niño provided us a timely and vivid example to understand the proposed mechanism of the CP El Niño, which could help us improve the El Niño predictability.
Alternation of the atmospheric teleconnections associated with the Northeast China spring rainfall during recent 60 years
Zhiwei Zhu , Rui Lu, Shanshan FU, Hua CHEN
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-2024-3
Abstract:
Northeast China (NEC) is the national grain production base, and the local precipitation is vital for agriculture during the springtime. Therefore, understanding the dynamic origins of the NEC spring rainfall (NECSR) variability is of socioeconomic importance. This study reveals an interdecadal change in the atmospheric teleconnections associated with the NECSR during recent 60 years (1961-2020). The NECSR is related to the Rossby wave train which is coupled with extratropical North Atlantic sea surface temperature (SST) before the mid-1980s, whereas it is linked to a quite different Rossby wave train that coupled with tropical North Atlantic SST after the mid-1980s. Both Rossby wave trains could lead to the enhanced NECSR through the anomalous cyclones over East Asia. The weakening of the westerly jet over around North America is mainly responsible for alternation of the atmospheric teleconnections towards NECSR during two epochs.
News & Views
COP26: Progress, Challenges, and Outlook
Yi WANG, Yuxuan LIU, Baihe GU
, Available online   , Manuscript accepted  07 May 2022, doi: 10.1007/s00376-022-2097-z
Abstract:
The 26th Conference of the Parties (COP26) to the United Nations Framework Convention on Climate Change (UNFCCC) was held in Glasgow a year later than scheduled, with expected outcomes achieved under a post-pandemic background. Based on the Issue-Actor-Mechanism Framework, this paper systematically evaluates the outcomes achieved at COP26 and analyzes the tendency of post-COP26 climate negotiations. Overall, with the concerted efforts of all parties, COP26 has achieved a balanced and inclusive package of outcomes and concluded six years of negotiations on the Paris Rulebook. It is fair to say that COP26 is another milestone in climate governance following the implementation of the Paris Agreement. Meanwhile, the Glasgow Climate Pact has cemented the consensus on a global commitment to accelerating climate action over the next decade and reached a breakthrough consensus on reducing coal, controlling methane, and halting deforestation. In the post-COP26 era, we still need to take concrete actions to implement the outcomes of the Paris Agreement and the Glasgow Climate Pact, innovate ways to speed up CO2 emissions reduction, and continue to strive for breakthroughs in important issues such as finance, technology, adaptation, and collaboration. In addition to avoiding the escalation of international conflicts, we need to collectively and properly handle the relationship between energy security, carbon reduction, and development and facilitate the efforts of countries to achieve their Sustainable Development Goals (SDGs), including climate-related goals. China will continue to maintain the existing multilateral mechanisms and processes for climate governance, unremittingly take concrete actions to address climate change, promote a domestic comprehensive green transition and global cooperation on carbon neutrality, and contribute constructively to global climate governance.
An Unprecedented Record Low Antarctic Sea-ice Extent during Austral Summer 2022
Jinfei WANG, Hao LUO, Qinghua YANG, Jiping LIU, Lejiang YU, Qian SHI, Bo HAN
, Available online   , Manuscript accepted  06 April 2022, doi: 10.1007/s00376-022-2087-1
Abstract:
Seasonal minimum Antarctic sea ice extent (SIE) in 2022 hit a new record low since recordkeeping began in 1978 of 1.9 million km2 on 25 February, 0.17 million km2 lower than the previous record low set in 2017. Significant negative anomalies in the Bellingshausen/Amundsen Seas, the Weddell Sea, and the western Indian Ocean sector led to the new record minimum. The sea ice budget analysis presented here shows that thermodynamic processes dominate sea ice loss in summer through enhanced poleward heat transport and albedo–temperature feedback. In spring, both dynamic and thermodynamic processes contribute to negative sea ice anomalies. Specifically, dynamic ice loss dominates in the Amundsen Sea as evidenced by sea ice thickness (SIT) change, while positive surface heat fluxes contribute most to sea ice melt in the Weddell Sea.
2021: A Year of Unprecedented Climate Extremes in Eastern Asia, North America, and Europe
Tianjun ZHOU, Wenxia ZHANG, Lixia ZHANG, Robin CLARK, Cheng QIAN, Qinghong ZHANG, Hui QIU, Jie JIANG, Xing ZHANG
, Available online   , Manuscript accepted  29 March 2022, doi: 10.1007/s00376-022-2063-9
Abstract:
The year 2021 was recorded as the 6th warmest since 1880. In addition to large-scale warming, 2021 will be remembered for its unprecedented climate extremes. Here, a review of selected high-impact climate extremes in 2021, with a focus on China, along with an extension to extreme events in North America and Europe is presented. Nine extreme events that occurred in 2021 in China are highlighted, including a rapid transition from cold to warm extremes and sandstorms in spring, consecutive drought in South China and severe thunderstorms in eastern China in the first half of the year, extremely heavy rainfall over Henan Province and Hubei Province during summer, as well as heatwaves, persistent heavy rainfall, and a cold surge during fall. Potential links of extremes in China to four global-scale climate extremes and the underlying physical mechanisms are discussed here, providing insights to understand climate extremes from a global perspective. This serves as a reference for climate event attribution, process understanding, and high-resolution modeling of extreme events.
Volcanoes and Climate: Sizing up the Impact of the Recent Hunga Tonga-Hunga Ha'apai Volcanic Eruption from a Historical Perspective
Meng ZUO, Tianjun ZHOU, Wenmin MAN, Xiaolong CHEN, Jian LIU, Fei LIU, Chaochao GAO
, Available online   , Manuscript accepted  23 February 2022, doi: 10.1007/s00376-022-2034-1
Abstract:
An undersea volcano at Hunga Tonga-Hunga Ha'apai (HTHH) near the South Pacific island nation of Tonga, erupted violently on 15 January 2022. Potential climate impact of the HTHH volcanic eruption is of great concern to the public; here, we intend to size up the impact of the HTHH eruption from a historical perspective. The influence of historical volcanic eruptions on the global climate are firstly reviewed, which are thought to have contributed to decreased surface temperature, increased stratospheric temperature, suppressed global water cycle, weakened monsoon circulation and El Niño-like sea surface temperature. Our understanding of the impacts of past volcanic eruptions on global-scale climate provides potential implication to evaluate the impact of the HTHH eruption. Based on historical simulations, we estimate that the current HTHH eruption with an intensity of 0.4 Tg SO2 injection will decrease the global mean surface temperature by only 0.004°C in the first year after eruption, which is within the amplitude of internal variability at the interannual time scale and thus not strong enough to have significant impacts on the global climate.
Data Description Article
CAS FGOALS-f3-H Dataset for the High-Resolution Model Intercomparison Project (HighResMIP) Tier 2
Bo AN, Yongqiang YU, Qing BAO, Bian HE, Jinxiao LI, Yihua LUAN, Kangjun CHEN, Weipeng ZHENG
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-2030-5
Abstract:
Following the High-Resolution Model Intercomparison Project (HighResMIP) Tier 2 protocol under the Coupled Model Intercomparison Project Phase 6 (CMIP6), three numerical experiments are conducted with the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere-Land System Model, version f3-H (CAS FGOALS-f3-H), and a 101-year (1950–2050) global high-resolution simulation dataset is presented in this study. The basic configuration of the FGOALS-f3-H model and numerical experiments design are briefly described, and then the historical simulation is validated. Forced by observed radiative agents from 1950 to 2014, the coupled model essentially reproduces the observed long-term trends of temperature, precipitation, and sea ice extent, as well as the large-scale pattern of temperature and precipitation. With an approximate 0.25º horizontal resolution in the atmosphere and 0.1\begin{document}$ \text{°} $\end{document} in the ocean, the coupled models also simulate energetic western boundary currents and the Antarctic Circulation Current (ACC), reasonable characteristics of extreme precipitation, and realistic frontal scale air-sea interaction. The dataset and supporting detailed information have been published in the Earth System Grid Federation (ESGF, https://esgf-node.llnl.gov/projects/cmip6/).
High-resolution Projection Dataset of Agroclimatic Indicators over Central Asia
Yuan QIU, Jinming FENG, Zhongwei YAN, Jun WANG
, Available online   , Manuscript accepted  14 March 2022, doi: 10.1007/s00376-022-2008-3
Abstract:
To understand the potential impacts of projected climate change on the vulnerable agriculture in Central Asia (CA), six agroclimatic indicators are calculated based on the 9-km-resolution dynamical downscaled results of three different global climate models from Phase 5 of the Coupled Model Intercomparison Project (CMIP5), and their changes in the near-term future (2031–50) are assessed relative to the reference period (1986–2005). The quantile mapping (QM) method is applied to correct the model data before calculating the indicators. Results show the QM method largely reduces the biases in all the indicators. Growing season length (GSL, day), summer days (SU, day), warm spell duration index (WSDI, day), and tropical nights (TR, day) are projected to significantly increase over CA, and frost days (FD, day) are projected to decrease. However, changes in biologically effective degree days (BEDD, °C) are spatially heterogeneous. The high-resolution projection dataset of agroclimatic indicators over CA can serve as a scientific basis for assessing the future risks to local agriculture from climate change and will be beneficial in planning adaption and mitigation actions for food security in this region.
The Super-large Ensemble Experiments of CAS FGOALS-g3
Pengfei LIN, Bowen ZHAO, Jilin WEI, Hailong LIU, Wenxia ZHANG, Xiaolong CHEN, Jie JIANG, Mengrong DING, Wenmin MAN, Jinrong JIANG, Xu ZHANG, Yuewen DING, Wenrong BAI, Chenyang JIN, Zipeng YU, Yiwen LI, Weipeng ZHENG, Tianjun ZHOU
, Available online   , Manuscript accepted  11 March 2022, doi: 10.1007/s00376-022-1439-1
Abstract:
A super-large ensemble simulation dataset with 110 members has been produced by the fully coupled model FGOALS-g3 developed by researchers at the Institute of Atmospheric Physics, Chinese Academy of Sciences. This is the first dataset of large ensemble simulations with a climate system model developed by a Chinese modeling center. The simulation has the largest realizations up to now worldwide in terms of single-model initial-condition large ensembles. Each member includes a historical experiment (1850–2014) and an experiment (2015–99) under the very high greenhouse gas emissions Shared Socioeconomic Pathway scenario (SSP5-8.5). The dataset includes monthly and daily temperature, precipitation, and other variables, requiring storage of 275 TB. Additionally, the surface air temperature (SAT) and land precipitation simulated by the FGOALS-g3 super-large ensemble have been validated and projected. The ensemble can capture the response of SAT and land precipitation to external forcings well, and the internal variabilities can be quantified. The availability of more than 100 realizations will help researchers to study rare events and improve the understanding of the impact of internal variability on forced climate changes.
Observed Frequent Occurrences of Marine Heatwaves in Most Ocean Regions during the Last Two Decades
Xiaojuan ZHANG, Fei ZHENG, Jiang ZHU, Xing-Rong CHEN
, Available online   , Manuscript accepted  18 January 2022, doi: 10.1007/s00376-022-1291-3
Abstract:
Marine heatwaves (MHWs) are prolonged high-temperature extreme events in the ocean that can be devastating to marine life and seriously impact climate systems and economies. This paper describes the accessibility, content, characteristics, and potential applications of an MHW dataset to facilitate its use in scientific research. Daily intensities of global MHWs from 1982 to 2020 were analyzed using gridded SST data sourced from the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation (OI) SST V2 high-resolution (0.25°) dataset. The analysis shows a linear increase in the frequency of MHWs in most ocean regions of the world as well as significant interdecadal changes. This data product can be used as a basic dataset to study the seasonal to decadal changes in extreme ocean events and explore the effects of global warming on the surface layers of oceans during the last 40 years.
Review
Frontiers of CO2 Capture and Utilization (CCU) towards Carbon Neutrality
Lingyun ZHANG, Yanfang SONG, Jialin SHI, Qun SHEN, Deng HU, Qiang GAO, Wei CHEN, Kien-Woh KOW, Cheng Heng PANG, Nannan SUN, Wei WEI
, Available online   , Manuscript accepted  12 April 2022, doi: 10.1007/s00376-022-1467-x
Abstract:
CO2 capture, utilization, and storage (CCUS) technology is a rare option for the large-scale use of fossil fuels in a low-carbon way, which will definitely play a part in the journey towards carbon neutrality. Within the CCUS nexus, CCU is especially interesting because these processes will establish a new “atmosphere-to-atmosphere” carbon cycle and thus indirectly offer huge potential in carbon reduction. This study focuses on the new positioning of CCUS in the carbon neutrality scenario and aims to identify potential cutting-edge/disruptive CCU technologies that may find important application opportunities during the decarbonization of the energy and industrial system. To this end, direct air capture (DAC), flexible metal-framework materials (MOFs) for CO2 capture, integrated CO2 capture and conversion (ICCC), and electrocatalytic CO2 reduction (ECR) were selected, and their general introduction, the importance to carbon neutrality, and most up-to-date research progress are summarized.
A Concise Overview on Solar Resource Assessment and Forecasting
Dazhi YANG, Wenting WANG, Xiang'ao XIA
, Available online   , Manuscript accepted  18 November 2021, doi: 10.1007/s00376-021-1372-8
Abstract:
China’s recently announced directive on tackling climate change, namely, to reach carbon peak by 2030 and to achieve carbon neutrality by 2060, has led to an unprecedented nationwide response among the academia and industry. Under such a directive, a rapid increase in the grid penetration rate of solar in the near future can be fully anticipated. Although solar radiation is an atmospheric process, its utilization, as to produce electricity, has hitherto been handled by engineers. In that, it is thought important to bridge the two fields, atmospheric sciences and solar engineering, for the common good of carbon neutrality. In this überreview, all major aspects pertaining to solar resource assessment and forecasting are discussed in brief. Given the size of the topic at hand, instead of presenting technical details, which would be overly lengthy and repetitive, the overarching goal of this review is to comprehensively compile a catalog of some recent, and some not so recent, review papers, so that the interested readers can explore the details on their own.
PERSPECTIVES
Climate Warming Mitigation from Nationally Determined Contributions
Bo FU, Jingyi LI, Thomas GASSER, Philippe CIAIS, Shilong PIAO, Shu TAO, Guofeng SHEN, Yuqin LAI, Luchao HAN, Bengang LI
, Available online   , Manuscript accepted  30 March 2022, doi: 10.1007/s00376-022-1396-8
Abstract:
Individual countries are requested to submit nationally determined contributions (NDCs) to alleviate global warming in the Paris Agreement. However, the global climate effects and regional contributions are not explicitly considered in the countries’ decision-making process. In this study, we evaluate the global temperature slowdown of the NDC scenario (∆T = 0.6°C) and attribute the global temperature slowdown to certain regions of the world with a compact earth system model. Considering reductions in CO2, CH4, N2O, BC, and SO2, the R5OECD (the Organization for Economic Co-operation and Development in 1990) and R5ASIA (Asian countries) are the top two contributors to global warming mitigation, accounting for 39.3% and 36.8%, respectively. R5LAM (Latin America and the Caribbean) and R5MAF (the Middle East and Africa) followed behind, with contributions of 11.5% and 8.9%, respectively. The remaining 3.5% is attributed to R5REF (the Reforming Economies). Carbon Dioxide emission reduction is the decisive factor of regional contributions, but not the only one. Other greenhouse gases are also important, especially for R5MAF. The contribution of short-lived aerosols is small but significant, notably SO2 reduction in R5ASIA. We argue that additional species beyond CO2 need to be considered, including short-lived pollutants, when planning a route to mitigate climate change. It needs to be emphasized that there is still a gap to achieve the Paris Agreement 2-degree target with current NDC efforts, let alone the ambitious 1.5-degree target. All countries need to pursue stricter reduction policies for a more sustainable world.
Notes & Letters
Causes and Predictability of the 2021 Spring Southwestern China Severe Drought
Yunyun LIU, Zeng-Zhen HU, Renguang WU, Xing YUAN
, Available online   , Manuscript accepted  14 March 2022, doi: 10.1007/s00376-022-1428-4
Abstract:
In the spring of 2021, southwestern China (SWC) experienced extreme drought, accompanied by the highest seasonal-mean temperature record since 1961. This drought event occurred in the decaying phase of a La Niña event with negative geopotential height anomalies over the Philippine Sea, which is distinct from the historical perspective. Historically, spring drought over SWC is often linked to El Niño and strong western North Pacific subtropical high. Here, we show that the extreme drought in the spring of 2021 may be mainly driven by the atmospheric internal variability and amplified by the warming trend. Specifically, the evaporation increase due to the high temperature accounts for about 30% of drought severity, with the contributions of its linear trend portion being nearly 20% and the interannual variability portion being about 10%. Since the sea surface temperature forcing from the tropical central and eastern Pacific played a minor role in the occurrence of drought, it is a challenge for a climate model to capture the 2021 SWC drought beyond one-month lead times.
Data-driven Estimation of Cloud Effects on Surface Irradiance at Xianghe, a Suburban Site on the North China Plain
Mengqi LIU, Jinqiang ZHANG, Hongrong SHI, Disong FU, Xiangao XIA
, Available online   , Manuscript accepted  10 March 2022, doi: 10.1007/s00376-022-1414-x
Abstract:
Clouds are a dominant modulator of the energy budget. The cloud shortwave radiative effect at the surface (CRE) is closely related to the cloud macro- and micro-physical properties. Systematic observation of surface irradiance and cloud properties are needed to narrow uncertainties in CRE. In this study, 1-min irradiance and Total Sky Imager measurements from 2005 to 2009 at Xianghe in North China Plain are used to estimate cloud types, evaluate cloud fraction (CF), and quantify the sensitivities of surface irradiance with respect to changes in CF whether clouds obscure the sun or not. The annual mean CF is 0.50, further noting that CF exhibits a distinct seasonal variation, with a minimum in winter (0.37) and maximum in summer (0.68). Cumulus occurs more frequently in summer (32%), which is close to the sum of the occurrence of stratus and cirrus. The annual CRE is –54.4 W m–2, with seasonal values ranging from –29.5 W m–2 in winter and –78.2 W m–2 in summer. When clouds do not obscure the sun, CF is a dominant factor affecting diffuse irradiance, which in turn affects global irradiance. There is a positive linear relationship between CF and CRE under sun-unobscured conditions, the mean sensitivity of CRE for each CF 0.1 increase is about 1.2 W m–2 [79.5° < SZA (Solar Zenith Angle) < 80.5°] to 7.0 W m–2 (29.5° < SZA < 30.5°). When clouds obscure the sun, CF affects both direct and diffuse irradiance, resulting in a non-linear relationship between CF and CRE, and the slope decreases with increasing CF. It should be noted that, although only data at Xianghe is used in this study, our results are representative of neighboring areas, including most parts of the North China Plain.
Importance of Air-Sea Coupling in Simulating Tropical Cyclone Intensity at Landfall
Charlie C. F. LOK, Johnny C. L. CHAN, Ralf TOUMI
, Available online   , Manuscript accepted  09 March 2022, doi: 10.1007/s00376-022-1326-9
Abstract:
An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones (TCs) along the South China coast is upgraded by including ocean and wave models. A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea. Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size. In particular, interaction with the ocean does not necessarily imply a weakening of the TC, with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC. These results suggest that to simulate the evolution of TC structure more accurately, it is essential to use an air-sea coupled model instead of an atmosphere-only model.
Identification of Convective and Stratiform Clouds Based on the Improved DBSCAN Clustering Algorithm
Yuanyuan ZUO, Zhiqun HU, Shujie YUAN, Jiafeng ZHENG, Xiaoyan YIN, Boyong LI
, Available online   , Manuscript accepted  09 November 2021, doi: 10.1007/s00376-021-1223-7
Abstract:
A convective and stratiform cloud classification method for weather radar is proposed based on the density-based spatial clustering of applications with noise (DBSCAN) algorithm. To identify convective and stratiform clouds in different developmental phases, two-dimensional (2D) and three-dimensional (3D) models are proposed by applying reflectivity factors at 0.5° and at 0.5°, 1.5°, and 2.4° elevation angles, respectively. According to the thresholds of the algorithm, which include echo intensity, the echo top height of 35 dBZ (ET), density threshold, and ε neighborhood, cloud clusters can be marked into four types: deep-convective cloud (DCC), shallow-convective cloud (SCC), hybrid convective-stratiform cloud (HCS), and stratiform cloud (SFC) types. Each cloud cluster type is further identified as a core area and boundary area, which can provide more abundant cloud structure information. The algorithm is verified using the volume scan data observed with new-generation S-band weather radars in Nanjing, Xuzhou, and Qingdao. The results show that cloud clusters can be intuitively identified as core and boundary points, which change in area continuously during the process of convective evolution, by the improved DBSCAN algorithm. Therefore, the occurrence and disappearance of convective weather can be estimated in advance by observing the changes of the classification. Because density thresholds are different and multiple elevations are utilized in the 3D model, the identified echo types and areas are dissimilar between the 2D and 3D models. The 3D model identifies larger convective and stratiform clouds than the 2D model. However, the developing convective clouds of small areas at lower heights cannot be identified with the 3D model because they are covered by thick stratiform clouds. In addition, the 3D model can avoid the influence of the melting layer and better suggest convective clouds in the developmental stage.
Perspectives
The Chinese Carbon-Neutral Goal: Challenges and Prospects
Ning ZENG, Kejun JIANG, Pengfei HAN, Zeke HAUSFATHER, Junji CAO, Daniel KIRK-DAVIDOFF, Shaukat ALI, Sheng ZHOU
, Available online   , Manuscript accepted  21 December 2021, doi: 10.1007/s00376-021-1313-6
Abstract:
On 22 September 2020, within the backdrop of the COVID-19 global pandemic, China announced its climate goal for peak carbon emissions before 2030 and to reach carbon neutrality before 2060. This carbon-neutral goal is generally considered to cover all anthropogenic greenhouse gases. The planning effort is now in full swing in China, but the pathway to decarbonization is unclear. The needed transition towards non-fossil fuel energy and its impact on China and the world may be more profound than its reform and development over the past 40 years, but the challenges are enormous. Analysis of four representative scenarios shows significant differences in achieving the carbon-neutral goal, particularly the contribution of non-fossil fuel energy sources. The high target values for nuclear, wind, and bioenergy have approached their corresponding resource limitations, with solar energy being the exception, suggesting solar's critical role. We also found that the near-term policies that allow for a gradual transition, followed by more drastic changes after 2030, can eventually reach the carbon-neutral goal and lead to less of a reduction in cumulative emissions, thus inconsistent with the IPCC 1.5°C scenario. The challenges and prospects are discussed in the historical context of China's socio-economic reform, globalization, international collaboration, and development.