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
The Linkage Between Midwinter Suppression of North Pacific Storm Track and Atmospheric Circulation Features in the Northern Hemisphere
Minghao Yang, Chongyin Li, Xin Li, Xiong Chen, Lifeng Li
, Available online   , Manuscript accepted  17 September 2021, doi: 10.1007/s00376-021-1145-4
The midwinter suppression (MWS) of the North Pacific storm track (NPST) has been an active research topic for decades. Based on the daily-mean NCEP/NCAR reanalysis from 1948 to 2018, this study investigates the MWS-related atmospheric circulation characteristics in the Northern Hemisphere by regression analysis with respect to a new MWS index, which may shed more light on this difficult issue. The occurrence frequency of the MWS of the upper-tropospheric NPST is more than 0.8 after the mid-1980s. The MWS is accompanied by significantly positive sea level pressure anomalies in Eurasia and negative anomalies over the North Pacific, which correspond to the strengthened East Asian winter monsoon. The intensified East Asian trough and North Pacific atmospheric blocking as well as significantly negative low-level air temperature anomalies accompanied by anomalous northerlies lying upstream of the MNPST are expected to be associated with the distinct MWS. However, the relationship between the MWS and low-level atmospheric baroclinicity is somewhat puzzling. From the diagnostics of the eddy energy budget, it is identified that the inefficiency of the barotropic energy conversion related to the barotropic governor mechanism does not favor the occurrence of the MWS. In contrast, the strengthened baroclinic energy conversion, buoyancy conversion, and generation of eddy available potential energy by diabatic heating are conducive to the occurrence of the MWS. In addition, Ural blocking in the upstream region of the MNPST may be another candidate associated with the MWS.
A New Method of Significance Testing for Correlation-Coefficient Fields and Its Application
Xiaojuan Sun, Siyan Li, Julian X. L. Wang, Panxing Wang, Guo Dong
, Available online   , Manuscript accepted  15 September 2021, doi: 10.1007/s00376-021-1196-6
Correlation-coefficient fields are widely used in short-term climate prediction research. The most frequently used significance-test method for the correlation-coefficient field was proposed by Livezey, in which the number of significant-correlation lattice (station) points on the correlation coherence map is used as the statistic. However, the method relies on two assumptions: (1) the spatial distribution of the lattice (station) points is uniform; and (2) there is no correlation between the physical quantities in the correlation-coefficient field. However, in reality, the above two assumptions are not valid. Therefore, we designed a more reasonable method for significance testing of the correlation-coefficient field. Specifically, a new statistic, the significant-correlation area, is introduced to eliminate the inhomogeneity of the grid (station)-point distribution, and an empirical Monte Carlo method is employed to eliminate the spatial correlation of the matrix. Subsequently, the new significance test was used for simultaneous correlation-coefficient fields between intensities of the atmospheric activity center in the Northern Hemisphere and temperature/precipitation in China. The results show that the new method is more reasonable than the Livezey method.
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
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 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, the present-day simulation and the future simulation, which were forced by the present-day climate during 1981–2000 and the future climate during 2081–2100, respectively, derived from RCP8.5 outputs in CMIP5. 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 CAS-ESM2 to investigate the response of terrestrial ecosystems to climate warming, and this application is favorable to better understand vegetation processes and to further improve 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
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 surface and TOA were built using the Rapid Radiative Transfer Model (RRTM). Longwave cloud radiative effect (CRE) at surface is primarily driven by cloud base properties, while TOA CRE is primarily decided by cloud top properties, so 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 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
The dominant frequency modes of pre-summer (April–June) 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 modes. It was found that EPEs form within strong southwesterly anomalous flows of 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 climatic 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 with a large horizontal pressure gradient. The intraseasonal disturbances are confined to tropical regions and influence SC by their 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 day. Both synoptic-scale disturbances and subseasonal oscillations should be simultaneously considered for EPE predictions over SC.
On the Two Successive Supercold Waves Straddling the End of 2020 and the Beginning of 2021
Cholaw Bueh, Jingbei Peng, Dawei Lin, Bomin Chen
, Available online   , Manuscript accepted  07 September 2021, doi: 10.1007/s00376-021-1107-x
Two supercold waves straddling 2020 and 2021 successively hit China and caused record-breaking extremely low temperature. In this study, we analyse the distinct features of these two supercold waves on the medium-range time scale. The blocking pattern from the Kara Sea to Lake Baikal characterized the first cold wave, while the large-scale tilted ridge and trough over the Asian continent featured the second cold wave. Prior to the cold waves, both the northwest and hyperpolar paths of cold air contributed to a zonally extensive cold air accumulation in the key region of Siberia. This might be the primary reason why strong and extensive supercold waves occur even under the Arctic amplification background. The two cold waves straddling 2020 and 2021 exhibited distinct features: (1) the blocking circulation occurred to the north or the east of the Ural Mountains, not only confined to the Ural Mountains as for the earlier cold waves; (2) the collocation of the Asian blocking pattern and the polar vortex deflection towards East Asia, we emphasize, preferred the hyperpolar path of cold air accumulation and the subsequent southward outburst; and (3) both high- and low-frequency processes worked in concert for the superstrong cold waves. The cold air advance along the northwest path, which coincides with the southeastward intrusion of the SH front edge, is associated with the high-frequency process, while the cold air movement along the hyperpolar path, which is close to the eastern edge of SH, is controlled by the low-frequency process.
Seasonal cumulative effect of Ural blocking episodes on the frequent cold events in China during the early winter of 2020/21
Yao Yao, Wenqi Zhang, Dehai Luo, Linhao Zhong, Lin Pei
, Available online   , Manuscript accepted  07 September 2021, doi: 10.1007/s00376-021-1100-4
Starting in mid-November, China was hit by several cold events during the early winter of 2020/21. The lowest temperature observed at Beijing station on 7 January reached −19.6°C. In this paper, we show that the outbreak of the record-breaking extreme cold event can be attributed to a huge merging Ural blocking (UB) ridge over the Eurasian region. The sea-ice cover in the Kara and East Siberia Seas (KESS) in autumn was at its lowest value since 1979, which could have served as a precursor signal. Further analysis shows that several successive UB episodes occurred from 1 September 2020 to 10 January 2021. The persistent UB that occurred in late September/early October 2020 may have made an important contribution to the October historical minimum of sea ice in the KESS region. Our results also show that, after each UB episode in winter, significant upward propagation of wave activity occurred around 60°E, which resulted in weakening of the stratospheric vortex. Meanwhile, each UB episode also caused a significant reduction in sea-ice extent in KESS and a significant weakening of the westerly jet in mid–high-latitude Eurasia. Results suggest that the Arctic vortex, which is supposed to enhance seasonally, became weaker and more unstable than the climatic mean under the seasonal cumulative effects of UB episodes, KESS warming, and long-lasting negative-phase North Atlantic Oscillation (NAO-). Those seasonal cumulative effects, combined with the impact of La Niña winter, led to the frequent occurrence of extreme cold events.
Hydro-climatic characteristics of Yarlung Tsangpo River Basin since the Last Glacial Maximum
Shuang Liu, Kaiheng Hu, Weiming Liu, Paul CARLING
, Available online   , Manuscript accepted  01 September 2021, doi: 10.1007/s00376-021-1150-7
Climate changes significantly impact the water condition of big rivers in glacierized high mountains. However, there is a lack of studies on hydrological changes within river basins over a geological timescale due to the impossibility of direct observations. In this study, we examine the hydro-climatic variation of the Yarlung Tsangpo River Basin in the Tibet Plateau since the Last Glacial Maximum (LGM) by combining δ18O proxy with the simulations via climate model and glacier model. The mean river runoff was kept around a low level of 145 billion cubic meters per year until an abrupt increase at a rate of 8.7 million cubic meters per year in the Bølling-Allerød interval (BA). The annual runoff reached a maximum of 250 billion cubic meters in the early Holocene and then reduced to the current value of 180 billion cubic meters at a rate of 6.4 million cubic meters per year. The low runoff in the LGM and Heinrich Stadial 1 (HS1) is likely attributed to a small contribution of precipitation to runoff and large glacier cover. The percentage of precipitation to runoff was only 20% during the LGM and HS1. Comparison of glacier area among different periods indicates that the fastest deglaciation occurred during the late HS1, when nearly 60% of glacier disappeared in the middle reach, 50% in the upper reach, and 30% in the lower reach. The rapid deglaciation and increasing runoff between the late HS1 and BA may accelerate ice-dam breaches and lead to outburst flood events.
Climatology of Tropical Cyclone Extreme Rainfall over China from 1960 to 2019
YING LI, Dajun Zhao
, Available online   , Manuscript accepted  01 September 2021, doi: 10.1007/s00376-021-1080-4
Tropical cyclone extreme rainfall (TCER) causes devastating floods and severe damage in China and it is therefore important to determine its long-term climatological distribution for both disaster prevention and operational forecasting. Based on the tropical cyclone (TC) best-track dataset and TC precipitation data from 1960 to 2019, the spatiotemporal distribution of TCER affecting China is analyzed. Results showed that there were large regional differences in the threshold for TCER in China, which decreased from the southeastern coast to the northwest inland, with the maximum precipitation (578 mm) about 35 times higher than the minimum (16.1 mm). TCER occurred infrequently in northern China, but had a high intensity and was highly localized. TCER was characterized by group occurrence. The frequency and intensity of TCER showed slightly increasing trends over time and was most likely to occur in August (41.0%). Most of the TC precipitation processes with extreme rainfall lasted for four to six days, with TCER mainly occurring on the third to fourth days. TC with wide areas of extreme rainfall showed two types of prevailing landfalling tracks: a northwestward track and a westward track. On average, stronger (weaker) TC corresponded to more (less) extreme precipitation. However, there were exceptions and some weak TCs led to strong extreme rainfall. A total of 64.7% (35.3%) of the TCER samples occurred when the TC was centered over the land (sea). TCER≥250 mm was located within 3° of the center of the TC.
On the second-year warming in late 2019 over the tropical Pacific and its triggering mechanism attributed to Indian Ocean effects
Licheng Feng, Rong-Hua Zhang, Fei Liu, Bo Yu, Xue Han, Gao Chuan
, Available online   , Manuscript accepted  01 September 2021, doi: 10.1007/s00376-021-1234-4
After its mature stage, El Niño usually decays rapidly in next summer and evolves into La Niña. However, it is not the case for 2018/19 El Niño event. Based on multiple reanalysis data sets, the space-time evolution and triggering mechanism for the unusual second-year warming in late 2019 after the 2018/19 El Niño event are investigated in the tropical Pacific. After a short decaying period associated with the 2018/19 El Niño condition, positive sea surface temperature anomalies (SSTAs) reintensified in the eastern equatorial Pacific in late 2019. Compared with the composite pattern of El Niño, two key differences are evident in the SSTAs evolution in 2019: one is the persistence of the surface warming over the central equatorial Pacific in May, and the other is the reintensification of the positive SSTAs over the eastern equatorial Pacific in September. Results imply that the reintensified anomalous westerly winds over the western and central Pacific, induced remotely by extreme Indian Ocean Dipole (IOD) event, acted as a triggering mechanism for the second-year warming. That is, the IOD-related cold SSTAs in the eastern Indian Ocean sustained anomalous surface westerly winds over the western equatorial Pacific, which induced downwelling Kelvin waves propagating eastward along the equator. At the same time, subsurface ocean provided plenty of warm water in the western and central equatorial Pacific. Mixed-layer heat budget analyses further confirm that positive zonal advection induced by the anomalous westerly winds and thermocline feedback played important roles in leading to the second-year warming in late 2019.
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
Using the high spatiotemporal resolution (2km-and-10min) data from the Advanced Himawari Imager onboard the Himawari-8 satellite, this study s 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 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 the urban agglomeration than the offshore which may be contributed by the higher aerosol concentrations over the cities. The CRs with weak convective cloud mixture and strong convective clouds appear more on 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 the 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 with the western North Pacific subtropical high extending more westward, leading to moderate moisture and a wide range of convective available potential energy but large convection inhibition.
Interannual influences of the surface potential vorticity forcing over the Tibetan Plateau on East Asian summer rainfall
Chen Sheng, Bian He, Guoxiong Wu, Yimin Liu, Shaoyu Zhang
, Available online   , Manuscript accepted  27 August 2021, doi: 10.1007/s00376-021-1218-4
The influences of interannual surface potential vorticity forcing over the Tibetan Plateau (TP) on East Asian summer rainfall (EASR) and upper-level circulation are explored in this study. The results show that the interannual EASR and associated circulations are closely related to the surface potential vorticity negative uniform leading mode (PVNUM) over the TP. When the PVNUM is in the positive phase, more rainfall occurs in the Yangtze River valley, South Korea, Japan and part of northern China, less rainfall occurs in southeastern China, and vice versa. A possible mechanism by which PVNUM affects EASR is proposed. Unstable air induced by the positive phase of PVNUM could stimulate significant upward motion and lower-level anomalous cyclone over the TP. As a result, a dipole heating mode with anomalous cooling over the southwestern TP and anomalous heating over the southeastern TP is generated. Sensitivity experimental results regarding this dipole heating mode indicate that anomalous cooling over the southwestern TP leads to local and northeastern Asian negative height anomalies, while anomalous heating over the southeastern TP leads to local positive height anomalies. These results greatly resemble the realistic circulation pattern associated with EASR. Further analysis indicates that the anomalous water vapor transport associated with this anomalous circulation pattern is responsible for the anomalous EASR. Consequently, changes in surface potential vorticity forcing over the TP can induce changes in EASR.
An Adaptive Nonhydrostatic Atmospheric Dynamical Core Using Multi-moment Constrained Finite Volume Method
Pei Huang, Chungang Chen, Xingliang Li, XueShun Shen, Feng Xiao
, Available online   , Manuscript accepted  26 August 2021, doi: 10.1007/s00376-021-1185-9
An adaptive 2D nonhydrostatic dynamical core is proposed by using the multi-moment constrained finite-volume (MCV) scheme and the Berger-Oliger’s adaptive mesh refinement (AMR) algorithm. MCV scheme takes several point-wise values within each computational cell as the predicted variables to build high-order schemes based on single-cell reconstruction. Two types of moments, such as volume-integrated average (VIA) and point value (PV), are defined as constraint conditions to derive the updating formulations of the unknowns and the constraint condition on VIA guarantees the rigorous conservation of the proposed model. In this study, MCV scheme is implemented on height-based terrain-following grid with variable resolution to solve the nonhydrostatic governing equations for atmospheric dynamics. Berger-Oliger’s AMR grid consists of several groups of blocks with different resolutions, where the MCV model developed on fixed structured mesh can be used directly. Numerical formulations are designed to implement the coarse-fine interpolation and the flux correction for properly exchanging the solution information among different blocks. Widely used benchmark tests are carried out to evaluate the proposed model. The numerical experiments on uniform and AMR grids indicate that the adaptive model has promising potential in improving the computational efficiency without losing the accuracy.
Diagnosing SST Error Growth during ENSO Developing Phase in BCC_CSM1.1(m) Prediction System
Ben Tian, Hongli Ren
, Available online   , Manuscript accepted  25 August 2021, doi: 10.1007/s00376-021-1189-5
In this study, the predictability of the El Niño-South Oscillation (ENSO) in an operational prediction model from the view of initial errors is diagnosed using the seasonal hindcasts of the Beijing Climate Center System Model, BCC_CSM1.1(m). Forecast skills during the different ENSO phases are analyzed and it is shown that the ENSO forecasts appear to be more challenging during the developing phase, compared with the decay phase. During the ENSO development, the SST prediction errors are significantly negative and cover a large area in the central and eastern tropical Pacific, thus limiting the model in predicting the intensity of El Niño. The large-scale SST errors, at their early stage, are generated gradually in terms of negative anomalies in the subsurface ocean temperature over the central-western equatorial Pacific, featuring an error evolution process similar to the El Niño decay and transition to the La Niña growth phase. Meanwhile, for short lead-time ENSO predictions, the initial wind errors begin to play the increasing roles, particularly in linking with the subsurface heat content errors in central-western Pacific. By comparing the multiple samples of initial fields in the model, it is clearly found that worse SST predictions of Niño3.4 are largely due to contributions of the initial errors in some specific locations in tropical Pacific. This demonstrates that the sensitive areas for initial fields in ENSO prediction are fairly consistent in both previous ideal experiments and our operational predictions, indicating implication for target observations to further improve operational forecasts of ENSO.
Analysis of atmospheric disturbance characteristics in the lower stratosphere by the observation of round-trip intelligent sounding system in China
Yang He, Zheng Sheng, Wei Ge, Mingyuan He, Xiaoran Zhao
, Available online   , Manuscript accepted  24 August 2021, doi: 10.1007/s00376-021-1110-2
Through multi-order structure function analysis and singularity measurement, the Hurst index and intermittent parameter are obtained to quantitatively describe the characteristics of atmospheric disturbance based on the round-trip intelligent sounding system in the middle stratosphere. According to the third-order structure function, small-scale gravity waves are classified into three states: stable, unstable, and broken. The evolution of gravity waves is reflected by the variation of third-order structure function over timevariation characteristics of the third-order structure function in different segments, and the generation of turbulence is also observed. Atmospheric disturbance intensity parameter is defined in this paper, containing both wave disturbance (H_1) and random intermittency (C_1), which is considered to reflect the characteristics of atmospheric disturbance more reasonably. In addition, by obtaining the vertical wavenumber spectrum from the horizontal wavenumber spectrum from the flat-floating stage and the vertical wavenumber spectrum from the ascending and descending stages at the height range of 18-24 kmheight interval below the flat-floating level, we found that when the gravity wave activity is significantly enhanced in the horizontal direction, the amplitude of the vertical wavenumber spectrum below is significantly larger, which shows a significant impact of gravity wave activity on the atmospheric environment below.
Moisture origins and transport processes for the 2020 Yangtze River Valley record-breaking Mei-yu rainfall
Lixia Zhang, Dan Zhao, Tian-Jun ZHOU, Dongdong Peng, Chan Xiao
, Available online   , Manuscript accepted  24 August 2021, doi: 10.1007/s00376-021-1097-8
The summer of 2020 underwent a record-breaking flood falling over the Yangtze River Valley (YRV). Using the Lagrangian model FLEXPART, this paper investigates moisture sources and transport processes behind this extreme event. Based on data from 1979 to 2019, the air-particle trajectories reaching YRV show five main moisture sources sectors: the Indian monsoon region (IND, 27.5% of the total rainfall), the local evaporation (27.4%), Western Pacific Ocean (WPO, 21.3%), Eurasian continent (8.5%) and Northeast Asia (4.4%). In the 2020 Mei-yu season, moisture from all source origins was above normal except that from the Northeast Asia. A record-breaking moisture source from the IND and WPO dominated this extreme Mei-yu flood in 2020, which was 1.5 and 1.6 times of the climate mean, respectively. This study reveals a significant relationship between the moisture source with three moisture transport processes, i.e., trajectory density, moisture content and moisture uptake of air-particle. A broad anomalous anticyclonic circulation over the Indo-Northwestern Pacific (Indo-NWP) benefits more moisture sources from the IND and WPO transported to the YRV. In 2020 Mei-yu season, a record-breaking Indo-NWP anomalous anticyclonic circulation contributed to higher trajectories density, higher moisture content and moisture uptake of air-particle from the IND and WPO region. They collectively resulted in an unprecedented moisture from source origins and thus Mei-yu flood over the YRV in 2020.
Growing Threat of Rapidly-intensifying Tropical Cyclones in East Asia
Kin Sik Liu, Johnny CHAN
, Available online   , Manuscript accepted  24 August 2021, doi: 10.1007/s00376-021-1126-7
This study examines the long-term change in the threat of landfalling tropical cyclones (TCs) in East Asia over the period 1975-2020 with a focus on the rapidly-intensifying (RI) TCs. The increase in the annual number of RI-TCs over the western North Pacific and the northwestward shift of their genesis location lead to an increasing trend in the annual number of landfalling RI-TCs along the coast of East Asia. The annual power dissipation index (PDI), a measure of the destruction potential of RI-TCs at landfall, also shows a significant increasing trend due to increases in the annual frequency and mean landfall intensity of landfalling RI-TCs. The increase in mean landfall intensity is related to the higher lifetime maximum intensity (LMI) and the LMI location of the landfalling RI-TCs being closer to the coast. The increase in annual PDI of East Asia is mainly contributed by those in the southern (the Philippines, south China and Vietnam) and northern parts (Japan and the Korean Peninsula) of East Asia through long-term changes in vertical wind shear and TC heat potential. The former leads to a northwestward shift in the favorable environment for TC genesis and intensification, resulting in the northwestward shift in the genesis, RI and LMI locations of RI-TCs. The latter provides more heat energy from the ocean for TC intensification, increasing its chance to undergo RI.
Influence of Coastal Marine Boundary Layer Jets on Rainfall in South China
Yu Du, Yian Shen, Guixing Chen
, Available online   , Manuscript accepted  24 August 2021, doi: 10.1007/s00376-021-1195-7
Coastal marine boundary layer jets (CMBLJs) play an important role in coastal and inland rainfall in South China. Using 21-yr ERA5 and CMORPH rainfall data, two main CMBLJs are found on the either side of Hainan Island (named as BLJ-WEST vs BLJ-EAST), which are always strengthened jointly. Both the two CMBLJs often occur in the pre-summer rainy season and exhibit evident diurnal cycle with a maximum at night. With the emergence of the CMBLJs, rainfall is significantly enhanced in South China, particularly downstream of each CMBLJ. The response of rainfall to the CMBLJs is mainly attributed to the convergence at the terminus of the CMBLJ, terrain-induced lifting and relevant atmospheric stratification. Coastal rainfall at the downstream of the BLJ-WEST is much weaker than that of the BLJ-EAST because of higher CIN over Beibu Gulf, which is caused by lower temperature lapse rate and adiabatic heating in lee of the Annamite Range. The inland rainfall increases along with the CMBLJs’ intensity, whereas coastal rainfall reaches a maximum in the presence of moderate CMBLJs rather than stronger CMBLJs. Stronger CMBLJs induce stronger dynamic lifting but higher CIN near the coastal area. Additionally, CAPE near the coast does not become highest with strongest CMBLJs, because the CAPE generation contributed by coastal dynamic lifting can be offset by the negative contribution caused by the horizontal advection of the cold and dry air from Indochina Peninsula. Therefore, anomalous dynamic lifting, moisture flux convergence, and CAPE/CIN associated with the CMBLJs’ intensity jointly result in anomalous rainfall.
A New X-band Weather Radar System with Distributed Phased-Array Front-ends: Development and Preliminary Observation Results
Xiaoqiong Zhen, Shuqing Ma, Hongbin Chen, Guorong Wang, Xiaoping Xu, siteng li
, Available online   , Manuscript accepted  24 August 2021, doi: 10.1007/s00376-021-1114-y
A novel weather radar system with distributed phased-array front-ends was developed. The specifications and preliminary data synthesis of this system are presented, which comprises one back-end and three or more front-ends. Each front-end, which utilizes a phased-array digital beamforming technology, sequentially transmits four 22.5°-width beams to cover the 0-90° elevational scan within about 0.05 s. The azimuthal detection is completed by one mechanical scan of 0–360° azimuths within about 12 s volume-scan update time. In the case of three front-ends, they are deployed according to an acute triangle to form a fine detection area (FDA). Because of the triangular deployment of multiple phased-array front-ends and a unique synchronized azimuthal scanning (SAS) rule, this new radar system is named Array Weather Radar (AWR). The back-end controls the front-ends to scan strictly in accordance with the SAS rule that assures the data time differences (DTD) among the three front-ends being less than 2 s for the same detection point in the FDA. The SAS can maintain DTD < 2 s of an expanded seven-front-end AWR. With the smallest DTD, gridded wind fields are derived from AWR data, by sampling of the interpolated grid, into a rectangular grid of 100 m × 100 m × 100 m at a 12 s temporal resolution in the FDA. In addition, the first X-band single-polarized three-front-end AWR was deployed in field experiments in 2018 at Huanghua International Airport, China. After data synthesis and processing, the preliminary observation results of the first AWR are described finally.
Comparison of the Anthropogenic Emission Inventory for CMIP6 models with a Country-Level Inventory over China and the Simulations of the Aerosol Properties
Tianyi Fan, Xiaohong Liu, Cheng-Lai Wu, Qiang Zhang, Chuanfeng Zhao, Xin YANG, Yanglian LI
, Available online   , Manuscript accepted  23 August 2021, doi: 10.1007/s00376-021-1119-6
Anthropogenic emission inventory for aerosols and reactive gases is crucial to the estimation of aerosol radiative forcing and climate effects. Here the anthropogenic emission inventory for AerChemMIP endorsed by CMIP6 is briefly introduced. The CMIP6 inventory is compared with a country-level inventory (i.e., MEIC) over China from 1986 to 2015. Discrepancies are found in the yearly trends of the two inventories especially after 2006. The yearly trends of the aerosol burdens simulated by CESM2 using the two inventories follow their emission trends and deviate after the mid-2000s, while the simulated aerosol optical depths (AODs) show similar trends. The difference between the simulated AODs is much smaller than the difference between model and observation. Although the simulated AODs agree with the MODIS satellite retrievals for country-wide average, the good agreement is an offset between the underestimation in eastern China and the overestimation in western China. Low-biased precursor gas of SO<sub>2</sub>, too strong convergence of wind field, too strong dilution and transport by summer monsoon circulation, too much wet scavenging by precipitation, and too weak aerosol swelling due to low-biased relative humidity are suggested to be responsible for the underestimated AOD in eastern China. This indicates that the influence of the emission inventory uncertainties on simulated aerosol properties can be overwhelmed by model biases of meteorology and aerosol processes. It is necessary for climate models to perform reasonably well in the dynamical, physical and chemical processes that would influence aerosol simulations.
The Extraordinary Rainfall over the Eastern Periphery of the Tibetan Plateau in August 2020
Xuelin Hu, Weihua Yuan, Rucong Yu
, Available online   , Manuscript accepted  23 August 2021, doi: 10.1007/s00376-021-1134-7
A large amount of accumulated precipitation was recorded over the eastern periphery of the Tibetan Plateau (EPTP) in August 2020. Using hourly rain gauge records and the ERA5 reanalysis dataset, we analyzed the unique characteristics of rainfall in August and the accompanying circulation conditions, as well as conducting a comparison with previous data. This record-breaking amount of accumulated rainfall was centered on the northern slope of the EPTP. This location was in contrast with the historical records of the concentration of rainfall over the middle and southern slopes. The hourly rainfall in August 2020 was both more frequent and more intense than the climatological mean rainfall. An amplification effect of the topography was observed, with the precipitation over the EPTP showing a more significant change with terrain height in August 2020. A circulation analysis showed that cold (warm) anomalies existed over the north (south) at approximately 35° N compared with those in the years when the southern EPTP received more rain. The western Pacific subtropical high was more intense and extended to the west, and the low-level cold air from the north was more active. The enhanced low-level southerly winds on the periphery of the subtropical high injected warm-wet air further north than the climatological mean. These winds changed easterly near the northern EPTP and were forced to ascend by the steep terrain.
The surface energy budget and its impact on the freeze-thaw processes of active layer in the permafrost regions of the Qinghai-Tibetan Plateau
Junjie Ma, Ren Li, Hongchao Liu, Zhongwei Huang, Tonghua Wu, Guojie Hu, Yao Xiao, Lin Zhao, Yizhen Du, Shuhua Yang
, Available online   , Manuscript accepted  23 August 2021, doi: 10.1007/s00376-021-1066-2
Surface energy budget is closely related to freeze-thaw processes and also a key issue for land surface process research in permafrost region. In this study, data in situ collected from 2005 to 2015 at the Tanggula site were used to analyze surface energy regimes, the interaction between surface energy budget and freeze-thaw processes. The results confirmed that surface energy flux in the permafrost region of the Qinghai-Tibetan Plateau exhibited obvious seasonal variations. Annual average net radiation (Rn) for 2010 was 86.5 Wm-2, with largest in July and smallest in November. Surface soil heat flux (G0) was positive during warm seasons but negative in cold seasons with annual average value of 2.7 Wm-2. Variations in Rn and G0 were closely related to freeze-thaw processes. Sensible heat flux (H) was the main energy budget component during cold seasons, whereas latent heat flux (LE) dominated surface energy distribution in warm seasons. Freeze-thaw processes, snow cover, precipitation, and surface conditions were important influencing factors for surface energy flux. Albedo was strongly dependent on soil moisture content and ground surface state, increasing significantly when land surface was covered with deep snow, and exhibited negative correlation with surface soil moisture content. Energy variation was significantly related to active layer thaw depth. Soil heat balance coefficient K was greater than 1 during the investigation time period, indicating the permafrost in the Tanggula area tended to degrade.
Ocean response to climate change heat-flux perturbation in an ocean model and its corresponding coupled model
Jiangbo Jin, Xiao Dong, Hailong LIU, Juanxiong He, minghua Zhang, Qing-Cun Zeng, He Zhang, Xin Gao, Guangqing Zhou, Yaqi Wang
, Available online   , Manuscript accepted  17 August 2021, doi: 10.1007/s00376-021-1167-y
State-of-the-art coupled general circulation models (CGCMs) are used to predict ocean heat uptake (OHU) and global mean sea level change due to thermal expansion under anthropogenic global warming. However, the projections of different models vary, resulting in high uncertainty. Much of the inter-model spread is driven by responses to surface heat perturbations. This study mainly focused on the ocean response to surface heat flux perturbation F prescribed by the Flux-Anomaly-Forced Model Intercomparison Project in an ocean model and its corresponding coupled model. The warming trends in both the OGCM and CGCM are mainly driven by anomalous surface heating F. Differences in changes in ocean temperature, OHC and SSL between the two models primarily occur in the Arctic and Atlantic Ocean (AA) and the Southern Ocean (SO) basins. The variations between models are much smaller in the Indo-Pacific Ocean (IP). In addition to the differences in surface heat flux anomalies between the two models, differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins. The greater weakening of the AMOC in the CGCM is associated with the atmosphere–ocean feedback and the lack of restoring salinity boundary condition. Furthermore, differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes that occur in the Atlantic and Southern Oceans.
Lightning Nowcasting with an Algorithm of Thunderstorm Tracking based on Lightning Location Data over Beijing Area
Abhay Srivastava, Dongxia Liu, Chen Xu, Shanfeng Yuan, Dongfang Wang, Ogunsua Babalola, Zhuling Sun, Xiong Chen, Hongbo Zhang
, Available online   , Manuscript accepted  17 August 2021, doi: 10.1007/s00376-021-0398-2
A thunderstorm tracking algorithm is proposed to nowcast the possibility of lightning activity over an area of concern by using the total lightning data and neighborhood technique. The lightning radiation sources observed from the Beijing Lightning Network (BLNET) were used to obtain the thunderstorm cells, which are significantly valuable in real-time. The boundaries of thunderstorm cells were obtained through the neighborhood technique. After smoothing, these boundaries were used to track the movement of thunderstorm and extrapolated to nowcast the lightning approaching in an area of concern. The algorithm can deliver creditable results before a thunderstorm arriving at the area of concern, with accuracies of 62.8%, 80%, and 91% for lead time of 30, 15, and 5 minutes, respectively. The real-time observations of total lightning seem to be significant for thunderstorm tracking and lightning nowcasting, as total lightning tracking could help to fill the gaps in radar reflectivity due to the blockage by hills or other obstacles. The lightning data used in the algorithm can well track the active thunderstorm cells associated with lightning activities.
A Comparison of Two Bulk Microphysics Parameterizations for the Study of Aerosol Impacts on an Idealized Supercell
Wanchen WU, Wei HUANG, Baode CHEN
, Available online   , Manuscript accepted  12 August 2021, doi: 10.1007/s00376-021-1187-7
Idealized supercell storms are simulated with two aerosol-aware bulk microphysics schemes (BMSs), the Thompson and the Chen-Liu-Reisner (CLR), using the Weather Research and Forecast (WRF) model. The objective of this study is to investigate the parameterizations of aerosol effects on cloud and precipitation characteristics and assess the necessity of introducing aerosols into a weather prediction model at fine grid resolution. The results show that aerosols play a decisive role in the composition of clouds in terms of the mixing ratios and number concentrations of liquid and ice hydrometeors in an intense supercell storm. The storm consists of a large amount of cloud water and snow in the polluted environment, but a large amount of rainwater and graupel instead in the clean environment. The total precipitation and rain intensity are suppressed in the CLR scheme more than in the Thompson scheme in the first three hours of storm simulations. The critical processes explaining the differences are the auto-conversion rate in the warm-rain process at the beginning of storm intensification and the low-level cooling induced by large ice hydrometeors. The cloud condensation nuclei (CCN) activation and auto-conversion processes of the two schemes exhibit considerable differences, indicating the inherent uncertainty of the parameterized aerosol effects among different BMSs. Beyond the aerosol effects, the fall speed characteristics of graupel in the two schemes play an important role in the storm dynamics and precipitation via low-level cooling. The rapid intensification of storms simulated with the Thompson scheme is attributed to the production of hail-like graupel.
Impact of the western Pacific tropical easterly jet on tropical cyclone genesis frequency over the western North Pacific
Ruifen Zhan, Yuqing Wang, Yihui Ding
, Available online   , Manuscript accepted  12 August 2021, doi: 10.1007/s00376-021-1103-1
Although it is well known that the tropical easterly jet (TEJ) has a significant impact on summer weather and climate over India and Africa, whether the TEJ exerts an important impact on tropical cyclone (TC) activity over the western North Pacific (WNP) remains unknown. In this study, we examined the impact of the TEJ on the interannual variability of TC genesis frequency over the WNP in the TC season (June-September) during 19802020. The results show a significant positive correlation between TC genesis frequency over the WNP and the jet intensity in the entrance region of the TEJ over the tropical western Pacific (in brief WP_TEJ), with a correlation coefficient as high as 0.66. The intensified WP_TEJ results in strong ageostrophic northerly winds in the entrance region and thus the upper-level divergence to the north over the main TC genesis region over the WNP. This would lead to an increase in upward motion in the troposphere with enhanced low-level convergence, which is the most important factors to the increases in low-level vorticity, mid-level humidity and low-level eddy kinetic energy, and a decrease in sea level pressure and vertical wind shear in the region. All these changes are favorable for TC genesis over the WNP and vice versa. Further analyses indicate that the interannual variability of the WP_TEJ intensity is likely to be linked to the local diabatic heating over the western Pacific and the central Pacific El Niño-Southern Oscillation.
Quantifying the Contribution of Track Change to Interannual variations of North Atlantic Intense Hurricanes
Jun Lu, Liguang Wu, Shunwu Zhou
, Available online   , Manuscript accepted  12 August 2021, doi: 10.1007/s00376-021-1116-9
Previous studies linked the interannual variability of tropical cyclone (TC) intensity in the North Atlantic basin to the Sahelian rainfall, vertical shear of the environmental flow and relative SST. In this study, the contribution of the TC track change to the interannual variations of intense hurricane activity in the North Atlantic basin is evaluated through numerical experiments. It is found that that the observed interannual variations of the frequency of intense hurricanes during the period 1958-2017 are dynamically consistent with changes in the large-scale ocean/atmosphere environment. The track change can account for ~50% of the interannual variability of intense hurricanes, while no significant difference is found for individual environmental parameters between the active and inactive years. The only significant difference between the active and inactive years occurs to the duration of TC intensification in the region east of 60°W. The duration increase is not due to the slow-down of TC translation. In the active years the southeastward shift of the formation location in the region east of 60°W makes TCs take a westward prevailing track, which allows TCs to have longer time for intensification. On the other hand, most TCs in the inactive years take a recurving track, leading to the shorter duration of intensification. This study suggests that the influence of track change should be considered to understand the basin-wide intensity change in the North Atlantic basin on the interannual time scale.
Federico Otero, Diego Araneo
, Available online   , Manuscript accepted  12 August 2021, doi: 10.1007/s00376-021-1007-0
Zonda wind is a typical downslope windstorm over the eastern slopes of Central Andes, in Argentina, which produces extremely warm and dry conditions creating substantial socioeconomic impacts. The aim of this work is to obtain an index for predicting the probability of Zonda wind occurrence. The Principal Component Analysis (PCA) is applied to the vertical soundings data on both sides of the Andes. Through the use of a binary logistic regression, we seek to apply the PCA to discriminate those soundings associated to Zonda wind events from those that are not, obtaining a probabilistic forecasting tool for Zonda occurrence. This index model is capable to discriminate between Zonda and Non-Zonda events with an effectiveness close to 91%. The best model consists in four variables each side of the Andes. From an event-based statistic, the probability of detection locates the mixed model above 97% with a probability of false detection lower than 7% and a missing ratio below 1%. From an alarm-based view, models present values below 7% in the false alarm rate, lower than 1.5% in the missing alarm ratio and higher than 93% in the correct alarm ratio. The zonal component of the wind on both sides and the windward temperature are the key variables in class discrimination. This vertical structure present two wind maximums and a lapse rate that tend to becomes unstable at med-levels on the leeside and a wind maximum at 700 hPa accompanied by a relatively more stable layer near the top of the barrier.
CLDASSD: Reconstructing Fine Textures of the Temperature Field Using Super-Resolution Technology
Ruian TIE, Chunxiang SHI, Gang WAN, Xingjie HU, Lihua KANG, Lingling GE
, Available online   , Manuscript accepted  11 August 2021, doi: 10.1007/s00376-021-0438-y
Before 2008, the n umber of surface observation stations in China was small. Thus, the surface observation data were too sparse to effectively support the High-resolution China Meteorological Administration's Land Assimilation System (HRCLDAS) which ultimately inhibited the output of high-resolution and high-quality gridded products. This paper proposes a statistical downscaling model based on a deep learning algorithm in super-resolution to research the above problem. Specifically, we take temperature as an example. The model is used to downscale the 0.0625° × 0.0625°, 2-m temperature data from the China Meteorological Administration's Land Data Assimilation System (CLDAS) to 0.01° × 0.01°, named CLDASSD. We performed quality control on the paired data from CLDAS and HRCLDAS, using data from 2018 and 2019. CLDASSD was trained on the data from 31 March 2018 to 28 February 2019, and then tested with the remaining data. Finally, extensive experiments were conducted in the Beijing-Tianjin-Hebei region which features complex and diverse geomorphology. Taking the HRCLDAS product and surface observation data as the “true values” and comparing them with the results of bilinear interpolation, especially in complex terrain such as mountains, the root mean square error (RMSE) of the CLDASSD output can be reduced by approximately 0.1°C, and its structural similarity (SSIM) was approximately 0.2 higher. CLDASSD can estimate detailed textures, in terms of spatial distribution, with greater accuracy than bilinear interpolation and other sub-models and can perform the expected downscaling tasks.
Dissimilarity among Ocean Reanalyses in Equatorial Pacific Upper-Ocean Heat Content and Its Relationship with ENSO
Paxson K. Y. CHEUNG, Wen ZHOU, Dongxiao WANG, Marco Y.T. LEUNG
, Available online   , Manuscript accepted  04 August 2021, doi: 10.1007/s00376-021-1109-8
This study focuses on the temporal variation of dissimilarity in heat content (HC) anomalies in the upper 300 m of ocean (HC300A) in the equatorial Pacific (±10°N) and its response to the El Niño-Southern Oscillation (ENSO). The HC300A anomalies are derived from four ocean reanalyses that are commonly used in ENSO studies and are compared using a simple differencing method. The dissimilarity in HC300A is found to vary closely with the magnitude of ENSO (regardless of phase), meaning that it tends to be greater during strong ENSO events. However, the dissimilarity among ocean reanalyses persists after the event decays. This effect is more pronounced after strong events. The persistence of the dissimilarity after ENSO events is a result of a late maturation of the ENSO signal, its persistence, and the interruption of the signal decay due to follow-up ENSO events. The combined effect of these three factors slows down the decay of HC300A in the region and hence results in the slow decay of dissimilarity. It is also found that areas with a significant spread in vertical temperature profiles collocate with the ENSO signal during warm ENSO phases. Thus, differences in subsurface process reconstruction are a significant factor in the dissimilarity among ocean reanalyses during warm ENSO events.
Atmospheric Rivers and Mei-yu Rainfall in China: A Case Study of Summer 2020
Ting WANG, Ke WEI, Jiao MA
, Available online   , Manuscript accepted  16 July 2021, doi: 10.1007/s00376-021-1096-9
Atmospheric rivers (ARs) are long, narrow, and transient filaments of strong horizontal water vapor transport that can lead to extreme precipitation. To investigate the relationship between ARs and mei-yu rainfall in China, the mei-yu season of 2020 in the Yangtze-Huaihe River basin is taken as an example. An adjusted AR-detection algorithm is applied on integrated water vapor transport (IVT) of the ERA5 reanalysis. The JRA-55 reanalysis and the data from Integrated Multi-satellite Retrievals for GPM (IMERG) are also utilized to study the impacts of ARs on mei-yu rainfall in 2020. The results reveal that ARs in East Asia have an average length of 5400 km, a width of 600 km, a length/width ratio of 9.3, and a northeastward orientation of 30°. ARs are modulated by the western North Pacific subtropical high. The IVT core is located at the south side of low pressure systems, moving eastward with a speed of 10° d−1. For the cross sections of ARs in the Yangtze-Huaihe River basin, 75% of the total flux is concentrated below 4 km with low-level jets near AR cores. Moreover, ARs occur mainly in the mei-yu period with a frequency of 20%–60%. The intensity of AR-related precipitation is 6–12 times that of AR-unrelated precipitation, and AR-related precipitation contributes about 50%–80% to total mei-yu precipitation. As shown in this case study of summer 2020, ARs are an essential part of the mei-yu system and have great impacts on mei-yu rainfall. Thus, ARs should receive more attention in research and weather forecast practices.
Magnitude, Scale, and Dynamics of the 2020 Mei-yu Rains and Floods over China
, Available online   , Manuscript accepted  13 July 2021, doi: 10.1007/s00376-021-1085-z
Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/1999. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with a halted northward progression of the EASMF and prolonged mei-yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12−16 June and 4−8 July 2020) are selected from this period for dynamical characterisation, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualizes heavy precipitation in Asia in summer 2020 and showcases several analysis tools developed by the authors for the study of such events.
The Extreme Mei-yu Season in 2020: Role of the Madden-Julian Oscillation and the Cooperative Influence of the Pacific and Indian Oceans
Ping LIANG, Zeng-Zhen HU, Yihui DING, Qiwen QIAN
, Available online   , Manuscript accepted  01 July 2021, doi: 10.1007/s00376-021-1078-y
The middle and lower reaches of the Yangtze River in eastern China during summer 2020 suffered the strongest mei-yu since 1961. In this work, we comprehensively analyzed the mechanism of the extreme mei-yu season in 2020, with focuses on the combined effects of the Madden-Julian Oscillation (MJO) and the cooperative influence of the Pacific and Indian Oceans in 2020 and from a historical perspective. The prediction and predictability of the extreme mei-yu are further investigated by assessing the performances of the climate model operational predictions and simulations.   It is noted that persistent MJO phases 1−2 during June−July 2020 played a crucial role for the extreme mei-yu by strengthening the western Pacific subtropical high. Both the development of La Niña conditions and sea surface temperature (SST) warming in the tropical Indian Ocean exerted important influences on the long-lived MJO phases 1−2 by slowing down the eastward propagation of the MJO and activating convection related to the MJO over the tropical Indian Ocean. The spatial distribution of the 2020 mei-yu can be qualitatively captured in model real-time forecasts with a one-month lead. This can be attributed to the contributions of both the tropical Indian Ocean warming and La Niña development. Nevertheless, the mei-yu rainfall amounts are seriously underestimated. Model simulations forced with observed SST suggest that internal processes of the atmosphere play a more important role than boundary forcing (e.g., SST) in the variability of mei-yu anomaly, implying a challenge in quantitatively predicting an extreme mei-yu season, like the one in 2020.
Tropical Cyclones over the Western North Pacific Strengthen the East Asia–Pacific Pattern during Summer
Sining LING, Riyu LU
, Available online   , Manuscript accepted  01 July 2021, doi: 10.1007/s00376-021-1171-2
The contribution of tropical cyclones (TCs) to the East Asia–Pacific (EAP) teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to 2018. The results showed that the TCs over the western North Pacific (WNP) correspond to a strengthened EAP pattern: During the summers of strong convection over the tropical WNP, TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere, an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere, and a more northward shift of the East Asian westerly jet in the upper troposphere. Further analyses indicated that two types of TCs with distinctly different tracks, i.e., westward-moving TCs and northward-moving TCs, both favor the EAP pattern. The present results imply that TCs over the WNP, as extreme weather, can contribute significantly to summer-mean climate anomalies over the WNP and East Asia.
Three-Dimensional Wind Field Retrieved from Dual-Doppler Radar Based on a Variational Method: Refinement of Vertical Velocity Estimates
Chenbin XUE, Zhiying DING, Xinyong SHEN, Xian CHEN
, Available online   , Manuscript accepted  30 June 2021, doi: 10.1007/s00376-021-1035-9
In this paper, a scheme of dual-Doppler radar wind analysis based on a three-dimensional variational method is proposed and performed in two steps. First, the horizontal wind field is simultaneously recovered through minimizing a cost function defined as a radial observation term with the standard conjugate gradient method, avoiding a weighting parameter specification step. Compared with conventional dual-Doppler wind synthesis approaches, this variational method minimizes errors caused by interpolation from radar observation to analysis grid in the iterative solution process, which is one of the main sources of errors. Then, through the accelerated Liebmann method, the vertical velocity is further re-estimated as an extra step by solving the Poisson equation with impermeable conditions imposed at the ground and near the tropopause. The Poisson equation defined by the second derivative of the vertical velocity is derived from the mass continuity equation. Compared with the method proposed by O'Brien, this method is less sensitive to the uncertainty of the boundary conditions and has better stability and reliability. Furthermore, the method proposed in this paper is applied to Doppler radar observation of a squall line process. It is shown that the retrieved vertical wind profile agrees well with the vertical profile obtained with the velocity–azimuth display (VAD) method, and the retrieved radial velocity as well as the analyzed positive and negative velocity centers and horizontal wind shear of the squall line are in accord with radar observations. There is a good correspondence between the divergence field of the derived wind field and the vertical velocity. And, the horizontal and vertical circulations within and around the squall line, as well as strong updrafts, the associated downdrafts, and associated rear inflow of the bow echo, are analyzed well. It is worth mentioning that the variational method in this paper can be applied to simultaneously synthesize the three-dimensional wind field from multiple-Doppler radar observations.
The Impact of Moist Physics on the Sensitive Area Identification for Heavy Rainfall Associated Weather System
Huizhen YU, Zhiyong MENG
, Available online   , Manuscript accepted  28 June 2021, doi: 10.1007/s00376-021-0278-9
The impact of moist physics on the sensitive areas identified by conditional nonlinear optimal perturbation (CNOP) is examined based on four typical heavy rainfall cases in northern China through performing numerical experiments with and without moist physics. Results show that the CNOP with moist physics identifies sensitive areas corresponding to both the lower- (850−700 hPa) and upper-level (300−100 hPa) weather systems, while the CNOP without moist physics fails to capture the sensitive area at lower levels. The reasons for the CNOP peaking at different levels can be explained in both algorithm and physics aspects. Firstly, the gradient of the cost function with respect to initial perturbations peaks at the upper level without moist physics which results in the upper-level peak of the CNOP, while it peaks at both the upper and lower levels with moist physics which results in both the upper- and lower-level peaks of the CNOP. Secondly, the upper-level sensitive area is associated with high baroclinicity, and these dynamic features can be captured by both CNOPs with and without moist physics. The lower-level sensitive area is associated with moist processes, and this thermodynamic feature can be captured only by the CNOP with moist physics. This result demonstrates the important contribution of the initial error of lower-level systems that are related to water vapor transportation to the forecast error of heavy rainfall associated weather systems, which could be an important reference for heavy rainfall observation targeting.
The Anomalous Mei-yu Rainfall of Summer 2020 from a Circulation Clustering Perspective: Current and Possible Future Prevalence
Robin T. CLARK, Peili WU, Lixia ZHANG, Chaofan LI
, Available online   , Manuscript accepted  25 June 2021, doi: 10.1007/s00376-021-1086-y
Highly unusual amounts of rainfall were seen in the 2020 summer in many parts of China, Japan, and South Korea. At the intercontinental scale, case studies have attributed this exceptional event to a displacement of the climatological western North Pacific subtropical anticyclone, potentially associated Indian Ocean sea surface temperature patterns and a mid-latitude wave train emanating from the North Atlantic.Using clusters of spatial patterns of sea level pressure, we show that an unprecedented 80% of the 2020 summer days in East Asia were dominated by clusters of surface pressure greater than normal over the South China Sea. By examining the rainfall and water vapor fluxes in other years when these clusters were also prevalent, we find that the frequency of these types of clusters was likely to have been largely responsible for the unusual rainfall of 2020. From two ensembles of future climate projections, we show that summers like 2020 in East Asia may become more frequent and considerably wetter in a warmer world with an enhanced moisture supply.
Influence of the NAO on Wintertime Surface Air Temperature over East Asia: Multidecadal Variability and Decadal Prediction
Jianping LI, Tiejun XIE, Xinxin TANG, Hao WANG, Cheng SUN, Juan FENG, Fei ZHENG, Ruiqiang DING
, Available online   , Manuscript accepted  16 June 2021, doi: 10.1007/s00376-021-1075-1
In this paper, we investigate the influence of the winter NAO on the multidecadal variability of winter East Asian surface air temperature (EASAT) and EASAT decadal prediction. The observational analysis shows that the winter EASAT and East Asian minimum SAT (EAmSAT) display strong in-phase fluctuations and a significant 60–80-year multidecadal variability, apart from a long-term warming trend. The winter EASAT experienced a decreasing trend in the last two decades, which is consistent with the occurrence of extremely cold events in East Asia winters in recent years. The winter NAO leads the detrended winter EASAT by 12–18 years with the greatest significant positive correlation at the lead time of 15 years. Further analysis shows that ENSO may affect winter EASAT interannual variability, but does not affect the robust lead relationship between the winter NAO and EASAT. We present the coupled oceanic-atmospheric bridge (COAB) mechanism of the NAO influences on winter EASAT multidecadal variability through its accumulated delayed effect of ~15 years on the Atlantic Multidecadal Oscillation (AMO) and Africa–Asia multidecadal teleconnection (AAMT) pattern. An NAO-based linear model for predicting winter decadal EASAT is constructed on the principle of the COAB mechanism, with good hindcast performance. The winter EASAT for 2020–34 is predicted to keep on fluctuating downward until ~2025, implying a high probability of occurrence of extremely cold events in coming winters in East Asia, followed by a sudden turn towards sharp warming. The predicted 2020/21 winter EASAT is almost the same as the 2019/20 winter.
Changes in typhoon regional heavy precipitation events over China from 1960 to 2018
Yangmei Tian, John L. MCBRIDE, Fumin Ren, Guoping Li, Tian Feng
, Available online   , Manuscript accepted  10 June 2021, doi: 10.1007/s00376-021-1015-0
In a number of earlier studies the authors have used objective techniques to determine the contribution of tropical cyclones to precipitation (TCP) in a region (a tropical cyclone that produces precipitation over land is called Tropical cyclone Precipitation Event [TPE] in this study), and to define Regional Heavy Precipitation events (RHPEs). This study combines the two approaches to evaluate the contribution of tropical cyclones to regional extreme rainfall events through the objective determination of Typhoon Regional Heavy Precipitation Events (TRHPEs). Based on the Objective Identification Technique for Regional Extreme Events (OITREE, Ren et al., 2012) and the Objective Synoptic Analysis Technique (OSAT, Ren et al., 2007), temporal and spatial overlap indices were developed to identify the combined events. With daily precipitation data and TC best-track data over the western North Pacific from 1960 to 2018, 86 TRHPEs have been identified. TRHPEs contribute as 20% of the RHPEs, but 100% of events with extreme individual precipitation intensities. The major TRHPEs continue for approximately a week after the tropical cyclone landfall, indicating a major role of post landfall precipitation. The frequency and extreme intensity of TRHPEs display increasing trends, while the ratios of TRHPEs to RHPEs and TPEs exhibit upward trends with the TPEs’ being significant. More frequent landfalling Southeast and South China TCs induce more serious impacts in coastal areas in the Southeast and the South during 1990-2018 than 1960-1989.
Toward Understanding the Extreme Floods over Yangtze River Valley in June−July 2020: Role of Tropical Oceans
Shaolei TANG, Jing-Jia LUO, Jiaying HE, Jiye WU, Yu ZHOU, Wushan YING
, Available online   , Manuscript accepted  01 June 2021, doi: 10.1007/s00376-021-1036-8
The extreme floods in the Middle/Lower Yangtze River Valley (MLYRV) during June−July 2020 caused more than 170 billion Chinese Yuan direct economic losses. Here, we examine the key features related to this extreme event and explore relative contributions of SST anomalies in different tropical oceans. Our results reveal that the extreme floods over the MLYRV were tightly related to a strong anomalous anticyclone persisting over the western North Pacific, which brought tropical warm moisture northward that converged over the MLYRV. In addition, despite the absence of a strong El Niño in 2019/2020 winter, the mean SST anomaly in the tropical Indian Ocean during June−July 2020 reached its highest value over the last 40 years, and 43% (57%) of it is attributed to the multi-decadal warming trend (interannual variability). Based on the NUIST CFS1.0 model that successfully predicted the wet conditions over the MLYRV in summer 2020 initiated from 1 March 2020 (albeit the magnitude of the predicted precipitation was only about one-seventh of the observed), sensitivity experiment results suggest that the warm SST condition in the Indian Ocean played a dominant role in generating the extreme floods, compared to the contributions of SST anomalies in the Maritime Continent, central and eastern equatorial Pacific, and North Atlantic. Furthermore, both the multi-decadal warming trend and the interannual variability of the Indian Ocean SSTs had positive impacts on the extreme floods. Our results imply that the strong multi-decadal warming trend in the Indian Ocean needs to be taken into consideration for the prediction/projection of summer extreme floods over the MLYRV in the future.
Radar-based Characteristics and Formation Environment of Supercells in the Landfalling Typhoon Mujigae in 2015
Lanqiang BAI, Zhiyong MENG, Ruilin ZHOU, Guixing CHEN, Naigeng WU, Wai-Kin WONG
, Available online   , Manuscript accepted  10 May 2021, doi: 10.1007/s00376-021-1013-2
This study presents the radar-based characteristics and formation environment of supercells spawned by the tornadic landfalling Typhoon Mujigae (2015) in October 2015. More than 100 supercells were identified within a 24-hour period around the time of the typhoon’s landfall, of which three were tornadic with a rotational intensity clearly stronger than those of non-tornadic supercells. The identified supercells were concentrated within a relatively small area in the northeast quadrant beyond 140 km from the typhoon center. These supercells were found more likely to form over flat topography and were difficult to maintain in mountainous regions. During the study period, more supercells formed offshore than onshore. The mesocyclones of the identified supercells were characterized by a small diameter generally less than 5 km and a shallow depth generally less than 4 km above ground level. An environmental analysis revealed that the northeast quadrant had the most favorable conditions for the genesis of supercell in this typhoon case. The nondimensional supercell composite parameter (SCP) and entraining-SCP (E-SCP) were effective in separating supercell from non-supercell environment. Even though the atmosphere in the typhoon’s northeast quadrant was characterized by an E-SCP/SCP value supportive of supercell organization, orography was an impeditive factor for the supercell development. These findings support the use of traditional parameters obtained from midlatitude supercells to assess the supercell potential in a tropical cyclone envelope.
The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020
Chaofan LI, Riyu LU, Nick DUNSTONE, Adam A. SCAIFE, Philip E. BETT, Fei ZHENG
, Available online   , Manuscript accepted  06 May 2021, doi: 10.1007/s00376-021-1092-0
During June and July of 2020, the Yangtze River basin suffered from extreme mei-yu rainfall and catastrophic flooding. This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event, based on forecasts from the Met Office GloSea5 operational forecast system. The forecasts successfully predicted above-average rainfall over the Yangtze River basin, which arose from the successful reproduction of the anomalous western North Pacific subtropical high (WNPSH). Our results indicate that both the Indian Ocean warm sea surface temperature (SST) and local WNP SST gradient were responsible for the westward extension of the WNPSH, and the forecasts captured these tropical signals well. We explore extratropical drivers but find a large model spread among the forecast members regarding the meridional displacements of the East Asian mid-latitude westerly jet (EAJ). The forecast members with an evident southward displacement of the EAJ favored more extreme Yangtze River rainfall. However, the forecast Yangtze River rainfall anomaly was weaker compared to that was observed and no member showed such strong rainfall. In observations, the EAJ displayed an evident acceleration in summer 2020, which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin, and favor more mei-yu rainfall. The model forecast failed to satisfactorily reproduce these processes. This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall, hindering a better forecast of the intensity of the event and disaster mitigation.
Cause of Extreme Heavy and Persistent Rainfall over Yangtze River in Summer 2020
Xiao PAN, Tim LI, Ying SUN, Zhiwei ZHU
, Available online   , Manuscript accepted  27 April 2021, doi: 10.1007/s00376-021-0433-3
Record-breaking heavy and persistent precipitation occurred over the Yangtze River Valley (YRV) in June-July (JJ) 2020. An observational data analysis has indicated that the strong and persistent rainfall arose from the confluence of southerly wind anomalies to the south associated with an extremely strong anomalous anticyclone over the western North Pacific (WNPAC) and northeasterly anomalies to the north associated with a high-pressure anomaly over Northeast Asia. A further observational and modeling study has shown that the extremely strong WNPAC was caused by both La Niña-like SST anomaly (SSTA) forcing in the equatorial Pacific and warm SSTA forcing in the tropical Indian Ocean (IO). Different from conventional central Pacific (CP) El Niños that decay slowly, a CP El Niño in early 2020 decayed quickly and became a La Niña by early summer. This quick transition had a critical impact on the WNPAC. Meanwhile, an unusually large area of SST warming occurred in the tropical IO because a moderate interannual SSTA over the IO associated with the CP El Niño was superposed by an interdecadal/long-term trend component. Numerical sensitivity experiments have demonstrated that both the heating anomaly in the IO and the heating anomaly in the tropical Pacific contributed to the formation and maintenance of the WNPAC. The persistent high-pressure anomaly in Northeast Asia was part of a stationary Rossby wave train in the midlatitudes, driven by combined heating anomalies over India, the tropical eastern Pacific, and the tropical Atlantic.
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
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.
Improving the Analyses and Forecasts of a Tropical Squall Line Using Upper Tropospheric Infrared Satellite Observations
Man-Yau CHAN, Xingchao CHEN
, Available online   , Manuscript accepted  20 April 2021, doi: 10.1007/s00376-021-0449-8
The advent of modern geostationary satellite infrared radiance observations has noticeably improved numerical weather forecasts and analyses. However, compared to midlatitude weather systems and tropical cyclones, research into using infrared radiance observations for numerically predicting and analyzing tropical mesoscale convective systems remain mostly fallow. Since tropical mesoscale convective systems play a crucial role in regional and global weather, this deficit should be addressed. This study is the first of its kind to examine the potential impacts of assimilating all-sky upper tropospheric infrared radiance observations on the prediction of a tropical squall line. Even though these all-sky infrared radiance observations are not directly affected by lower-tropospheric winds, the high-frequency assimilation of these all-sky infrared radiance observations improved the analyses of the tropical squall line’s outflow position. Aside from that, the assimilation of all-sky infrared radiance observations improved the analyses and prediction of the squall line’s cloud field. Finally, reducing the frequency of assimilating these all-sky infrared radiance observations weakened these improvements to the analyzed outflow position, as well as the analyses and predictions of cloud fields.
Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Niño?
Congxi FANG, Yu LIU, Qiufang CAI, Huiming SONG
, Available online   , Manuscript accepted  15 April 2021, doi: 10.1007/s00376-021-1009-y
In summer 2020, extreme rainfall occurred throughout the Yangtze River basin, Huaihe River basin, and southern Yellow River basin, which are defined here as the central China (CC) region. However, only a weak central Pacific (CP) El Niño happened during winter 2019/20, so the correlations between the El Niño–Southern Oscillation (ENSO) indices and ENSO-induced circulation anomalies were insufficient to explain this extreme precipitation event. In this study, reanalysis data and numerical experiments are employed to identify and verify the primary ENSO-related factors that cause this extreme rainfall event. During summer 2020, unusually strong anomalous southwesterlies on the northwest side of an extremely strong Northwest Pacific anticyclone anomaly (NWPAC) contributed excess moisture and convective instability to the CC region, and thus, triggered extreme precipitation in this area. The tropical Indian Ocean (TIO) has warmed in recent decades, and consequently, intensified TIO basinwide warming appears after a weak El Niño, which excites an extremely strong NWPAC via the pathway of the Indo-western Pacific Ocean capacitor (IPOC) effect. Additionally, the ENSO event of 2019/20 should be treated as a fast-decaying CP El Niño rather than a general CP El Niño, so that the circulation and precipitation anomalies in summer 2020 can be better understood. Last, the increasing trend of tropospheric temperature and moisture content in the CC region after 2000 is also conducive to producing heavy precipitation.
The Impact of Tibetan Plateau Snow Cover on the Summer Temperature in Central Asia
Xuke LIU, Xiaojing JIA, Min WANG, Qifeng QIAN
, Available online   , Manuscript accepted  14 April 2021, doi: 10.1007/s00376-021-1011-4
The current work examines the impact of the snow cover extent (SCE) of the Tibetan Plateau (TP) on the interannual variation in the summer (June−July−August) surface air temperature (SAT) over Central Asia (CA) (SAT_CA) during the 1979−2019 period. The leading mode of the summer SAT_CA features a same-sign temperature anomalies in CA and explains 62% of the total variance in SAT_CA. The atmospheric circulation associated with a warming SAT_CA is characterized by a pronounced high-pressure system dominating CA. The high-pressure system is accompanied by warm advection as well as descending motion over CA, favoring the warming of the SAT_CA. Analysis shows that the interannual variation in the summer SAT_CA is significantly positively correlated with the April SCE over the central-eastern TP. In April, higher than normal SCE over the central-eastern TP has a pronounced cooling effect on the column of the atmosphere above the TP and can persist until the following early summer. Negative and positive height anomalies appear above and to the west of the TP. In the following months, the perturbation forcing generated by the TP SCE anomalies lies near the western center of the Asian subtropical westerly jet (SWJ), which promotes atmospheric waves in the zonal direction guided by the Asian SWJ. Associated with this atmospheric wave, in the following summer, a significant high-pressure system dominates CA, which is a favorable condition for a warm summer SAT_CA.
Evaluating the Ozone Valley over the Tibetan Plateau in CMIP6 Models
Kequan ZHANG, Jiakang DUAN, Siyi ZHAO, Jiankai ZHANG, James KEEBLE, Hongwen LIU
, Available online   , Manuscript accepted  08 April 2021, doi: 10.1007/s00376-021-0442-2
Total column ozone (TCO) over the Tibetan Plateau (TP) is lower than that over other regions at the same latitude, particularly in summer. This feature is known as the “TP ozone valley”. This study evaluates long-term changes in TCO and the ozone valley over the TP from 1984 to 2100 using Coupled Model Intercomparison Project Phase 6 (CMIP6). The TP ozone valley consists of two low centers, one is located in the upper troposphere and lower stratosphere (UTLS), and the other is in the middle and upper stratosphere. Overall, the CMIP6 models simulate the low ozone center in the UTLS well and capture the spatial characteristics and seasonal cycle of the TP ozone valley, with spatial correlation coefficients between the modeled TCO and the Multi Sensor Reanalysis version 2 (MSR2) TCO observations greater than 0.8 for all CMIP6 models. Further analysis reveals that models which use fully coupled and online stratospheric chemistry schemes simulate the anticorrelation between the 150 hPa geopotential height and zonal anomaly of TCO over the TP better than models without interactive chemistry schemes. This suggests that coupled chemical-radiative-dynamical processes play a key role in the simulation of the TP ozone valley. Most CMIP6 models underestimate the low center in the middle and upper stratosphere when compared with the Microwave Limb Sounder (MLS) observations. However, the bias in the middle and upper stratospheric ozone simulations has a marginal effect on the simulation of the TP ozone valley. Most CMIP6 models predict the TP ozone valley in summer will deepen in the future.
Seasonal and Diurnal Variations of Cloud Systems over the Eastern Tibetan Plateau and East China: A Cloud-resolving Model Study
Jinghua CHEN, Xiaoqing WU, Chunsong LU, Yan YIN
, Available online   , Manuscript accepted  08 April 2021, doi: 10.1007/s00376-021-0391-9
The seasonal and diurnal variations of cloud systems are profoundly affected by the large-scale and local environments. In this study, a one-year-long simulation was conducted using a two-dimensional cloud-resolving model over the Eastern Tibetan Plateau (ETP) and two subregions of Eastern China: Southern East China and Central East China. Deep convective clouds (DCCs) rarely occur in the cold season over ETP, whereas DCCs appear in Eastern China throughout the year, and the ETP DCCs are approximately 20%−30% shallower than those over Eastern China. Most strong rainfall events (precipitation intensity, PI> 2.5 mm h−1) in Eastern China are related to warm-season DCCs with ice cloud processes. Because of the high elevation of the ETP, the warm-season freezing level is lower than in Eastern China, providing favorable conditions for ice cloud processes. DCCs are responsible for the diurnal variations of warm-season rainfall in all three regions. Warm-season DCCs over the ETP have the greatest total cloud water content and frequency in the afternoon, resulting in an afternoon rainfall peak. In addition, rainfall events in the ETP also exhibit a nocturnal peak in spring, summer, and autumn due to DCCs. Strong surface heat fluxes around noon can trigger or promote DCCs in spring, summer, and autumn over the ETP but produce only cumulus clouds in winter due to the cold and dry environment.
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
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.
Projection of the Future Changes in Tropical Cyclone Activity Affecting East Asia over the Western North Pacific Based on Multi-RegCM4 Simulations
Jie WU, Xuejie GAO, Yingmo ZHU, Ying SHI, Filippo GIORGI
, Available online   , Manuscript accepted  03 February 2021, doi: 10.1007/s00376-021-0286-9
Future changes in tropical cyclone (TC) activity over the western North Pacific (WNP) under the representative concentration pathway RCP4.5 are investigated based on a set of 21st century climate change simulations over East Asia with the regional climate model RegCM4 driven by five global models. The RegCM4 reproduces the major features of the observed TC activity over the region in the present-day period of 1986−2005, although with the underestimation of the number of TC genesis and intensity. A low number of TCs making landfall over China is also simulated. By the end of the 21st century (2079−98), the annual mean frequency of TC genesis and occurrence is projected to increase over the WNP by 16% and 10%, respectively. The increase in frequency of TC occurrence is in good agreement among the simulations, with the largest increase over the ocean surrounding Taiwan Island and to the south of Japan. The TCs tend to be stronger in the future compared to the present-day period of 1986−2005, with a large increase in the frequency of strong TCs. In addition, more TCs landings are projected over most of the China coast, with an increase of ~18% over the whole Chinese territory.
The Record-breaking Mei-yu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies
Yihui DING, Yunyun LIU, Zeng-Zhen HU
, Available online   , Manuscript accepted  11 January 2021, doi: 10.1007/s00376-021-0361-2
The record-breaking mei-yu in the Yangtze-Huaihe River valley (YHRV) in 2020 was characterized by an early onset, a delayed retreat, a long duration, a wide meridional rainbelt, abundant precipitation, and frequent heavy rainstorm processes. It is noted that the East Asian monsoon circulation system presented a significant quasi-biweekly oscillation (QBWO) during the mei-yu season of 2020 that was associated with the onset and retreat of mei-yu, a northward shift and stagnation of the rainbelt, and the occurrence and persistence of heavy rainstorm processes. Correspondingly, during the mei-yu season, the monsoon circulation subsystems, including the western Pacific subtropical high (WPSH), the upper-level East Asian westerly jet, and the low-level southwesterly jet, experienced periodic oscillations linked with the QBWO. Most notably, the repeated establishment of a large southerly center, with relatively stable latitude, led to moisture convergence and ascent which was observed to develop repeatedly. This was accompanied by a long-term duration of the mei-yu rainfall in the YHRV and frequent occurrences of rainstorm processes. Moreover, two blocking highs were present in the middle to high latitudes over Eurasia, and a trough along the East Asian coast was also active, which allowed cold air intrusions to move southward through the northwestern and/or northeastern paths. The cold air frequently merged with the warm and moist air from the low latitudes resulting in low-level convergence over the YHRV. The persistent warming in the tropical Indian Ocean is found to be an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia along with an intensified and southerly displaced WPSH, which was observed to be favorable for excessive rainfall over YHRV.
Correlation Structures between Satellite All-Sky Infrared Brightness Temperatures and the Atmospheric State at Storm Scales
, Available online   , Manuscript accepted  06 January 2021, doi: 10.1007/s00376-021-0352-3
This study explores the structures of the correlations between infrared (IR) brightness temperatures (BTs) from the three water vapor channels of the Advanced Baseline Imager (ABI) onboard the GOES-16 satellite and the atmospheric state. Ensemble-based data assimilation techniques such as the ensemble Kalman filter (EnKF) rely on correlations to propagate innovations of BTs to increments of model state variables. Because the three water vapor channels are sensitive to moisture in different layers of the troposphere, the heights of the strongest correlations between these channels and moisture in clear-sky regions are closely related to the peaks of their respective weighting functions. In cloudy regions, the strongest correlations appear at the cloud tops of deep clouds, and ice hydrometeors generally have stronger correlations with BT than liquid hydrometeors. The magnitudes of the correlations decrease from the peak value in a column with both vertical and horizontal distance. Just how the correlations decrease depend on both the cloud scenes and the cloud structures, as well as the model variables. Horizontal correlations between BTs and moisture, as well as hydrometeors, in fully cloudy regions decrease to almost 0 at about 30 km. The horizontal correlations with atmospheric state variables in clear-sky regions are broader, maintaining non-zero values out to ~100 km. The results in this study provide information on the proper choice of cut-off radii in horizontal and vertical localization schemes for the assimilation of BTs. They also provide insights on the most efficient and effective use of the different water vapor channels.
News & Views
FY-3E: The First Operational Meteorological Satellite Mission in an Early Morning Orbit
Peng ZHANG, Xiuqing HU, Qifeng LU, Aijun ZHU, Manyun LIN, Ling SUN, Lin CHEN, Na XU
, Available online   , Manuscript accepted  30 August 2021, doi: 10.1007/s00376-021-1304-7
Fengyun-3E (FY-3E), the world’s first early-morning-orbit meteorological satellite for civil use, was launched successfully at the Jiuquan Satellite Launch Center on 5 July 2021. The FY-3E satellite will fill the vacancy of the global early-morning-orbit satellite observation, working together with the FY-3C and FY-3D satellites to achieve the data coverage of early morning, morning, and afternoon orbits. The combination of these three satellites will provide global data coverage for numerical weather prediction (NWP) at 6-hour intervals, effectively improving the accuracy and time efficiency of global NWP, which is of great significance to perfect the global earth observing system. In this article, the background and meteorological requirements for the early-morning-orbit satellite are reviewed, and the specifications of the FY-3E satellite, as well as the characteristics of the onboard instrumentation for earth observations, are also introduced. In addition, the ground segment and the retrieved geophysical products are also presented. It is believed that the NWP communities will significantly benefit from an optimal temporal distribution of observations provided by the early morning, mid-morning, and afternoon satellite missions. Further benefits are expected in numerous applications such as the monitoring of severe weather/climate events, the development of improved sampling designs of the diurnal cycle for accurate climate data records, more efficient monitoring of air quality by thermal infrared remote sensing, and the quasi-continuous monitoring of the sun for space weather and climate.
Satellite All-sky Infrared Radiance Assimilation: Recent Progress and Future Perspectives
Jun LI, Alan J. GEER, Kozo OKAMOTO, Jason A. OTKIN, Zhiquan LIU, Wei HAN, Pei WANG
, Available online   , Manuscript accepted  12 August 2021, doi: 10.1007/s00376-021-1088-9
Satellite infrared (IR) sounder and imager measurements have become one of the main sources of data used by data assimilation systems to generate initial conditions for numerical weather prediction (NWP) models and atmospheric analysis/reanalysis. This paper reviews the development of satellite IR data assimilation in NWP in recent years, especially the assimilation of all-sky satellite IR observations. The major challenges and future directions are outlined and discussed.
Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track
Guidi ZHOU, Xuhua CHENG
, Available online   , Manuscript accepted  02 August 2021, doi: 10.1007/s00376-021-0408-4
This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies, emphasizing the Kuroshio-Oyashio Extension (KOE) region. Oceanic perturbations of interest consist of sharp oceanic fronts, temperature anomalies associated with mesoscale eddies, and to some extent even higher-frequency sub-mesoscale variability. The focus is on the free atmosphere above the boundary layer. As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track, the response of both the time-mean flow and the storm track is assessed. The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations. Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed, followed by the role of oceanic fronts in maintaining the storm track as a whole. KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track. Relevant studies are summarized and intercompared. Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors. Recent modeling studies highlighted the importance of mesoscale eddies and probably even sub-mesoscale processes in maintaining the storm track but confirmation and validation are still needed. Moreover, the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate. By reviewing the above aspects, we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes (fronts, eddies, submesoscale features) and atmospheric processes (fronts, extratropical cyclones etc.), in an integrated way, within the context of different climate background states.
Notes & Letters
Two Types of Diurnal Variations in Heavy Rainfall during July over Korea
Chang-Kyun PARK, Minhee CHANG, Chang-Hoi HO, Kyung-Ja HA, Jinwon KIM, Byung-Ju SOHN
, Available online   , Manuscript accepted  12 July 2021, doi: 10.1007/s00376-021-1178-8
This study examined the characteristics of the diurnal variations of heavy rainfall (≥110 mm in 12 hours) in Korea and the related atmospheric circulation for July from 1980−2020. During the analysis period, two dominant pattens of diurnal variation of the heavy rainfall emerged: all-day heavy rainfall (AD) and morning only heavy rainfall (MO) types. For the AD-type, the heavy rainfall is caused by abundant moisture content in conjunction with active convection in the morning (0000−1200, LST; LST = UTC + 9) and the afternoon hours (1200−2400 LST). These systems are related to the enhanced moisture inflow and upward motion induced by the strengthening of the western North Pacific subtropical high and upper-tropospheric jet. For the MO-type, heavy rainfall occurs mostly in the morning hours; the associated atmospheric patterns are similar to the climatology. We find that the atmospheric pattern related to severe heavy rainfalls in 2020 corresponds to a typical AD-type and resembles the 1991 heavy-rainfall system in its overall synoptic/mesoscale circulations. The present results imply that extremely heavy rainfall episodes in Korea during the 2020 summer may occur again in the future associated with the recurring atmospheric phenomenon related to the heavy rainfall.
Seasonal Rainfall Forecasts for the Yangtze River Basin in the Extreme Summer of 2020
Philip E. BETT, Gill M. MARTIN, Nick DUNSTONE, Adam A. SCAIFE, Hazel E. THORNTON, Chaofan LI
, Available online   , Manuscript accepted  11 June 2021, doi: 10.1007/s00376-021-1087-x
Seasonal forecasts for Yangtze River basin rainfall in June, May–June–July (MJJ), and June–July–August (JJA) 2020 are presented, based on the Met Office GloSea5 system. The three-month forecasts are based on dynamical predictions of an East Asian Summer Monsoon (EASM) index, which is transformed into regional-mean rainfall through linear regression. The June rainfall forecasts for the middle/lower Yangtze River basin are based on linear regression of precipitation. The forecasts verify well in terms of giving strong, consistent predictions of above-average rainfall at lead times of at least three months. However, the Yangtze region was subject to exceptionally heavy rainfall throughout the summer period, leading to observed values that lie outside the 95% prediction intervals of the three-month forecasts. The forecasts presented here are consistent with other studies of the 2020 EASM rainfall, whereby the enhanced mei-yu front in early summer is skillfully forecast, but the impact of midlatitude drivers enhancing the rainfall in later summer is not captured. This case study demonstrates both the utility of probabilistic seasonal forecasts for the Yangtze region and the potential limitations in anticipating complex extreme events driven by a combination of coincident factors.
News & Views
Extreme Cold Events from East Asia to North America in Winter 2020/21: Comparisons, Causes, and Future Implications
Xiangdong ZHANG, Yunfei FU, Zhe HAN, James E. OVERLAND, Annette RINKE, Han TANG, Timo VIHMA, Muyin WANG
, Available online   , Manuscript accepted  05 July 2021, doi: 10.1007/s00376-021-1229-1
Three striking and impactful extreme cold weather events successively occurred across East Asia and North America during the mid-winter of 2020/21. These events open a new window to detect possible underlying physical processes. The analysis here indicates that the occurrences of the three events resulted from integrated effects of a concurrence of anomalous thermal conditions in three oceans and interactive Arctic-lower latitude atmospheric circulation processes, which were linked and influenced by one major sudden stratospheric warming (SSW). The North Atlantic warm blob initiated an increased poleward transient eddy heat flux, reducing the Barents-Kara seas sea ice over a warmed ocean and disrupting the stratospheric polar vortex (SPV) to induce the major SSW. The Rossby wave trains excited by the North Atlantic warm blob and the tropical Pacific La Nina interacted with the Arctic tropospheric circulation anomalies or the tropospheric polar vortex to provide dynamic settings, steering cold polar air outbreaks. The long memory of the retreated sea ice with the underlying warm ocean and the amplified tropospheric blocking highs from the midlatitudes to the Arctic intermittently fueled the increased transient eddy heat flux to sustain the SSW over a long time period. The displaced or split SPV centers associated with the SSW played crucial roles in substantially intensifying the tropospheric circulation anomalies and moving the jet stream to the far south to cause cold air outbreaks to a rarely observed extreme state. The results have significant implications for increasing prediction skill and improving policy decision making to enhance resilience in “One Health, One Future”.
The Nature and Predictability of the East Asian Extreme Cold Events of 2020/21
Guokun DAI, Chunxiang LI, Zhe HAN, Dehai LUO, Yao YAO
, Available online   , Manuscript accepted  31 March 2021, doi: 10.1007/s00376-021-1057-3
Three extreme cold events invaded China during the early winter period between December 2020 to mid-January 2021 and caused drastic temperature drops, setting new low-temperature records at many stations during 6−8 January 2021. These cold events occurred under background conditions of low Arctic sea ice extent and a La Niña event. This is somewhat expected since the coupled effect of large Arctic sea ice loss in autumn and sea surface temperature cooling in the tropical Pacific usually favors cold event occurrences in Eurasia. Further diagnosis reveals that the first cold event is related to the southward movement of the polar vortex and the second one is related to a continent-wide ridge, while both the southward polar vortex and the Asian blocking are crucial for the third event. Here, we evaluate the forecast skill for these three events utilizing the operational forecasts from the ECMWF model. We find that the third event had the highest predictability since it achieves the best skill in forecasting the East Asian cooling among the three events. Therefore, the predictability of these cold events, as well as their relationships with the atmospheric initial conditions, Arctic sea ice, and La Niña deserve further investigation.
The 2020/21 Extremely Cold Winter in China Influenced by the Synergistic Effect of La Niña and Warm Arctic
Fei ZHENG, Yuan YUAN, Yihui DING, Kexin LI, Xianghui FANG, Yuheng ZHAO, Yue SUN, Jiang ZHU, Zongjian KE, Ji WANG, Xiaolong JIA
, Available online   , Manuscript accepted  01 February 2021, doi: 10.1007/s00376-021-1033-y
In the first half of winter 2020/21, China has experienced an extremely cold period across both northern and southern regions, with record-breaking low temperatures set in many stations of China. Meanwhile, a moderate La Niña event which exceeded both oceanic and atmospheric thresholds began in August 2020 and in a few months developed into its mature phase, just prior to the 2020/21 winter. In this report, the mid−high-latitude large-scale atmospheric circulation anomalies in the Northern Hemisphere, which were forced by the negative phase of Arctic Oscillation, a strengthened Siberian High, an intensified Ural High and a deepened East Asian Trough, are considered to be the direct reason for the frequent cold surges in winter 2020/21. At the same time, the synergistic effect of the warm Arctic and the cold tropical Pacific (La Niña) provided an indispensable background, at a hemispheric scale, to intensify the atmospheric circulation anomalies in middle-to-high latitudes. In the end, a most recent La Niña prediction is provided and the on-coming evolution of climate is discussed for the remaining part of the 2020/21 winter for the purpose of future decision-making and early warning.
Letters & Notes
What Drives the Decadal Variability of Global Tropical Storm Days from 1965 to 2019?
Yifei DAI, Bin WANG, Weiyi SUN
, Available online   , Manuscript accepted  26 January 2021, doi: 10.1007/s00376-021-0354-1
The tropical storm day (TSD) is a combined measure of genesis and lifespan. It reflects tropical cyclone (TC) overall activity, yet its variability has rarely been studied, especially globally. Here we show that the global total TSDs exhibit pronounced interannual (3−6 years) and decadal (10 years) variations over the past five-to-six decades without a significant trend. The leading modes of the interannual and decadal variability of global TSD feature similar patterns in the western Pacific and Atlantic, but different patterns in the Eastern Pacific and the Southern Indian Ocean. The interannual and decadal leading modes are primarily linked to El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), respectively. The TSDs-ENSO relationship has been steady during the entire 55-year period, but the TSDs-PDO relationship has experienced a breakdown in the 1980s. We find that the decadal variation of TSD in the Pacific is associated with the PDO sea surface temperature (SST) anomalies in the tropical eastern Pacific (PDO-E), while that in the Atlantic and the Indian Ocean is associated with the PDO SST anomalies in the western Pacific (PDO-W). However, the PDO-E and PDO-W SST anomalies are poorly coupled in the 1980s, and this “destructive PDO” pattern results in a breakdown of the TSDs-PDO relationship. The results here have an important implication for seasonal to decadal predictions of global TSD.