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
Discrepancies in simulated ocean net surface heat fluxes over the North Atlantic
Chunlei Liu, Yazhu Yang, Xiaoqing Liao, Ning Cao, Jimmy Liu, Niansen Ou, Richard Allan, Liang Jin, Ni Chen, Rong Zheng
, Available online   , Manuscript accepted  20 January 2022, doi: 10.1007/s00376-022-1360-7
The change in ocean net surface heat flux plays an important role in the climate system. It is closely related to the ocean heat content change and ocean heat transport, particularly over the North Atlantic, where the ocean loses heat to the atmosphere, affecting the AMOC (Atlantic Meridional Overturning Circulation) variability and hence the global climate. However, the difference between simulated surface heat fluxes is still large due to poorly represented dynamical processes involving multiscale interactions in model simulations. In order to explain the discrepancy of the surface heat flux over the North Atlantic, data sets from nineteen AMIP6 and eight highresSST-present climate model simulations are analyzed and compared with the DEEPC (Diagnosing Earth's Energy Pathways in the Climate system) product. As an indirect check of the ocean surface heat flux, the oceanic heat transport inferred from the combination of the ocean surface heat flux, sea ice and ocean heat content tendency is compared with the RAPID (Rapid Climate Change-Meridional Overturning Circulation and Heat flux array) observations at 26°N in the Atlantic. The AMIP6 simulations show lower inferred heat transport due to less heat loss to the atmosphere. The heat loss from the AMIP6 ensemble mean north of 26°N in Atlantic is about 10 Wm-2 less than DEEPC, and the heat transport is about 0.30 PW lower than RAPID and DEEPC. The model horizontal resolution effect on the discrepancy is also investigated. Results show that by increasing the resolution, both surface heat flux north of 26°N and heat transport at 26°N of the Atlantic can be improved.
Recent decrease in the difference in tropical cyclone occurrence between the Atlantic and the western North Pacific
Johnny CHAN, Kin Sik Liu
, Available online   , Manuscript accepted  18 January 2022, doi: 10.1007/s00376-022-1309-x
Climatologically, among all ocean basins, the western North Pacific (WNP) has the largest annual number of tropical cyclones (TCs) of around 26 while the Atlantic has around 13, giving a difference of 13. However, the difference is -7 in 2020, with 30 TCs in the Atlantic and 23 in the WNP, which is the most negative within the last 46 years. In fact, during the last 26 years, the difference in TC number is below 10 in ten years, with four years being negative. Such a decreasing difference in TC number can be attributed to the natural multidecadal variation of the Atlantic Multidecadal Oscillation and Interdecadal Pacific Oscillation as well as other external forcings such as anthropogenic aerosol forcing and increased greenhouse gases, with the additional impact from the La Niña condition. This result has significant implication on climate model projections of future TC activity in the two ocean basins.
Observed frequent occurrences of marine heatwaves in most ocean regions during the last two decades
Xiaojuan Zhang, Fei Zheng, Jiang Zhu, Xingrong Chen
, Available online   , Manuscript accepted  18 January 2022, doi: 10.1007/s00376-022-1291-3
Marine heatwaves (MHWs) are prolonged high temperature extreme events in the ocean that can cause devastating impacts on marine life in addition to serious impacts on climate systems and social economies. This paper describes the access, content, characteristics, and potential applications of an MHW dataset, with the goal of facilitating its use in scientific research. Using daily global remotely sensed National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation (OI) SST V2 high-resolution (1/4°) gridded SST data over 1982–2020, the daily intensity of global MHWs from 1982 to 2020 was analyzed. The analysis of the dataset showed that the frequency of MHWs has increased linearly in most ocean regions of the world and has experienced significant interdecadal changes. This data product can be used as a basic dataset to study the seasonal to decadal changes in extreme ocean events and to explore the effects of global warming on the surface layers of oceans during the last 40 years.
Assimilation of the FY-4A AGRI Clear-Sky Radiance Data in a Regional Numerical Model and its Impact on the Forecast of the “7·20” Severe Rainstorm in Henan
Lan Xu, Wei Cheng, Zhongren Deng, Juanjuan Liu, Bin Wang, Bing Lu, Shudong Wang, Li Dong
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1380-3
Assimilation of the Advanced Geostationary Radiance Imager (AGRI) clear-sky radiance in a regional model is performed, and the forecasting effectiveness of the assimilation of two water vapor (WV) channels with conventional observations for the severe rainstorm event of "7·20" in Henan is analyzed and compared with a benchmark test that assimilates only conventional observations in this study. The results show that the 24-h cumulative precipitation forecast by the assimilation experiment with the addition of the AGRI exceeds 500 mm, which is close to a maximum value of 532.6 mm measured by the national meteorological stations, and the location of the maximum precipitation is consistent with the observations. The results for the short periods of intense precipitation processes found that the simulation of the location and intensity of the 3-h cumulative precipitation is also relatively accurate. The analysis increment shows that the main difference between the two sets of assimilation experiments is over the ocean due to the additional ocean observations provided by FY-4A, which compensates for the lack of ocean observations. The assimilation of satellite data adjusts the vertical and horizontal wind fields over the ocean by adjusting the atmospheric temperature and humidity, which ultimately results in a narrower and stronger WV transport path to the center of heavy precipitation in Zhengzhou in the lower troposphere. Conversely, the WV convergence and upward motion in the control experiment are more dispersed; therefore, the precipitation centers are also correspondingly more dispersed.
Spatiotemporal Variations of Microwave Land Surface Emissivity (MLSE) over China derived from 4-Year Recalibrated Fengyun 3B MWRI data
Rui Li, Jiheng Hu, Shengli Wu, Peng Zhang, Husi Letu, Yu WANG, Xuewem Wang, Yuyun Fu, Renjun Zhou, Ling Sun
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1314-0
Microwave Land Surface Emissivity (MLSE) over China under both clear sky condition and cloudy sky condition were retrieved using measurements of recalibrated microwave brightness temperatures (Tbs) from Fengyun-3B Microwave Radiation Imager (FY-3B MWRI) combined with cloud properties derived from Himawari-8 Advanced Himawari Imager (AHI) observations. The contributions from cloud particles and atmospheric gases to the upwelling Tbs at the top of atmosphere were taken into account. The MLSEs at horizontal polarizations at 10.65GHz, 18.7GHz and 36.5GHz during July 7th, 2015 to June 30th, 2019 over China showed high values in southeast vegetated area and low values in northwest barren or sparsely vegetated area. The maximum values of MLSEs were found in the southwest-northeast oriented belt area of Qinling-Taihang Mountains and the eastern edge of the Qinghai-Tibet Plateau, which also is the dividing belt between high and low MLSE in China. This spatial pattern is highly consistent with what is derived from AMSR-E. It demonstrates the measurements of Tbs by FY-3B MWRI, including its calibration and validation, are reliable, and the retrieving algorithm developed in this study works well. Seasonal variations of MLSE in China are mainly driven by the combined effects of vegetation, rainfall and snow cover. In tropical and southern forest regions, seasonal variation of MLSE is small due to the enhancement from vegetation and the suppression from rainfall. In boreal area, snow causes a significant decrease of MLSE at 36.5GHz in winter. Meanwhile, the MLSE at lower frequencies receives less suppression. In desert region in Xinjiang, increases of MLSEs at all frequencies are observed with increasing snow cover.
A Modified Double-Moment Bulk Microphysics Scheme toward the East Asia Monsoon Region
Jinfang Yin, Donghai Wang, Guoqing Zhai, Hong Wang, Huanbin Xu, Chongjian Liu
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1402-1
Representation of cloud microphysical processes is one of the key aspects of numerical models. An improved double-moment bulk cloud microphysics scheme (named IMY) has been achieved based on the standard Milbrandt-Yau (MY) scheme in Weather Research and Forecasting (WRF) model toward East Asia monsoon region (EAMR). In the IMY scheme, the shape parameters of raindrop, snow particle, and cloud droplet size distributions are variable, instead of fixed constants. Specifically, the shape parameters of raindrop and snow size distributions are diagnosed from shape-slope relationships accordingly. As for the cloud droplet size distribution, the shape parameter depends on the total cloud number concentration. Besides, a series of minor improvements in terms of detailed cloud processes have been accomplished. The improved scheme has been coupled into the WRF model and tested with two heavy rainfall cases over EAMR. The results show that the IMY scheme can reproduce the overall spatial distribution of rainfalls and their temporal evolutions by comparing with the surface gauge observations, and the simulations successfully capture the cloud features by taking satellite and ground-based radar observations as reference. Comparatively speaking, the IMY scheme provides better simulations of the two selected events than the MY scheme. Good results have been gained from the two real-data case simulations with the IMY scheme, indicating that the improved scheme is potentially promising. Although the simulations show a positive evaluation of the performance of the IMY scheme, continued experiments are required to further validate the scheme with different weather events.
A sensitivity study of Arctic Ice-Ocean heat exchanges to 3-equation boundary condition parametrization in CICE6
Lei yu, Jiping Liu, Yongqi Gao, Qi Shu
, Available online   , Manuscript accepted  11 January 2022, doi: 10.1007/s00376-022-1316-y
In this study, we perform a stand-alone sensitivity study using use the Los Alamos Sea ice model version 6 (CICE6) to investigate the model sensitivity to two Ice-Ocean (IO) boundary condition approaches. One is the 2-equation approach, it treats the freezing temperature as a function of the ocean mixed layer (ML) salinity, using two equations to parametrize the IO heat exchanges. Another approach, use salinity of the IO interface to define the actual freezing temperature, so an equation describing the salt flux at the IO interface is added to the 2-equation approach, forming the so-called 3-equation approach. Compared with the 2-equation simulation, our 3-equation simulation shows reduced oceanic turbulent heat flux, weakened basal melt, increased ice thickness and reduced the sea surface temperature (SST) in the Arctic. These impacts occur mainly at the ice edge regions and are manifested in summer. Furthermore, we observe a downward turbulent heat flux from the ice to ocean ML in August in two of our 3-equation sensitivity runs with a constant heat transfer coefficient (0.006), which caused heat divergence and congelation at the ice bottom. Additionally, the influence of different combinations of heat/salt transfer coefficients and thermal conductivity in the 3-equation approach on the model simulated results is assessed. The results presented in this study can provide an insight on sea ice model sensitivity to the 3-eqation IO boundary condition for coupling the CICE6 to climate models.
Another Record: Ocean Warming Continues through 2021 despite La Niña Conditions
Lijing CHENG, John ABRAHAM, Kevin E. TRENBERTH, John FASULLO, Tim BOYER, Michael E. MANN, Jiang ZHU, Fan WANG, Ricardo LOCARNINI, Yuanlong LI, Bin ZHANG, Zhetao TAN, Fujiang YU, Liying WAN, Xingrong CHEN, Xiangzhou SONG, Yulong LIU, Franco RESEGHETTI, Simona SIMONCELLI, Viktor GOURETSKI, Gengxin CHEN, Alexey MISHONOV, Jim REAGAN
, Available online   , Manuscript accepted  10 January 2022, doi: 10.1007/s00376-022-1461-3
The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021 from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by 16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in this study should be incorporated into climate risk assessments, adaptation, and mitigation.
Seasonal Predictions of Summer Precipitation in the Middle-Lower Reaches of Yangtze River with Global and Regional Models based on NUIST-CFS1.0
Wushan Ying, Huiping Yan, Jing-Jia Luo
, Available online   , Manuscript accepted  10 January 2022, doi: 10.1007/s00376-022-1389-7
Accurate prediction of the summer precipitation over the Middle and Low Reaches of the Yangtze River (MLYR) is of urgent demand for the local economic and societal development. This study assessed the seasonal forecast skill in predicting summer precipitation over the MLYR region based on the global Climate Forecast System of Nanjing University of Information Science and Technology (NUIST-CFS1.0, previously SINTEX-F). The results show that the model can provide a moderate skill in predicting the interannual variations of the MLYR rainbands, initialized from 1st March. In addition, 9-member ensemble mean can realistically reproduce the links between the MLYR precipitation and tropical sea surface temperature (SST) anomalies, but the individual member shows great discrepancies, indicating a large uncertainty in the forecasts. Furthermore, the NUIST-CFS1.0 can predict five of the seven extreme summer precipitation anomalies over MLYR during 1982-2020, albeit with underestimated magnitudes. The Weather Forecast and Research (WRF) downscaling hindcast experiments with a finer resolution of 30 km, which is forced by the large-scale information of the NUIST-CFS1.0 predictions with a spectral nudging method, display improved predictions of the extreme summer precipitation anomalies to some extent. However, the performance of the downscaling predictions is highly dependent on the global model forecast skill, suggesting that further improvements on both the global and regional climate models are needed.
Assimilation of FY-3D MWTS-2 Radiance with 3-D Precipitation Detection and the Impacts on Typhoon Forecasts
Luyao Qin, Yaodeng Chen, Gang Ma, Fuzhong Weng, Deming Meng, Peng Zhang
, Available online   , Manuscript accepted  07 January 2022, doi: 10.1007/s00376-022-1252-x
Precipitation detection is an essential step in radiance assimilation because the uncertainties in precipitation would affect the radiative transfer calculation and observation errors. The traditional precipitation detection method for microwave only detects clouds and precipitation horizontally without considering the three-dimensional distribution of clouds. Extending precipitation detection from 2-D to 3-D is expected to bring more useful information to the data assimilation without using the all-sky approach. In this study, the 3-D precipitation detection method is adopted to assimilate Microwave Temperature Sounder-2 (MWTS-2) onboard the Fengyun-3D, which can dynamically detect the channels above precipitating clouds by considering the near real-time cloud parameters. Cycling data assimilation and forecasting experiments for Typhoon Lekima (2019) and Mitag (2019) were carried out. Compared with the control experiment, the quantity of assimilated data with the 3-D precipitation detection increased by approximately 23% on average. The quality of the additional MWTS-2 radiance is close to the clear-sky data. The case studies show that the average root-mean-square errors (RMSE) of prognostic variables are reduced by 1.7% in the upper troposphere, leading to an average reduction of 4.53% in typhoon tracks forecasts. The detailed diagnoses of Typhoon Lekima (2019) further show that the additional MWTS-2 radiances brought by the 3-D precipitation detection facilitate portraying a more reasonable circulation situation, thus providing more precise structures. This paper preliminarily proves that 3-D precipitation detection has the potential values in increasing satellite data utilization and improving typhoon forecasts.
Initiation and evolution of long-lived eastward-propagating mesoscale convective systems over the second-step terrain along Yangtze-Huai River Valley
Yuanchun Zhang, Jianhua Sun, Ruyi Yang, Ruoyun Ma
, Available online   , Manuscript accepted  05 January 2022, doi: 10.1007/s00376-022-1303-3
Based on the previous statistical analysis of mesoscale convective systems (MCSs) over the second-step terrain along Yangtze-Huai River Valley, eight representative long-lived eastward-propagating MCSs are selected to investigate the initiation and evolution of this-type MCSs as well as their impact on downstream areas using semi-idealized (CNTL) and sensitivity (NOLH) simulations. CNTL experiment indicates that convection forms in the region with prevailing southwesterly wind associated with short-wave trough to the east of the Tibetan Plateau and the western Pacific subtropical high. Merging with other convective systems, convection develops into an MCS, which propagates eastward under the influence of westerlies in the middle troposphere, and moves out of the second-step terrain. Then the MCS merges with preexisting local convection over the plains; the merged convection reinforces cyclonic wind perturbation into the mesoscale vortex at 850 hPa. While the mesoscale vortex moves eastward to regions with local vortex at 850 hPa, this intensified local vortex merges with the other vortex at 925 hPa and finally develops into a mature mesoscale convective vortex(MCV). NOLH experiment with diabatic heating in convection formation region turned off indicates that no MCSs form and move eastward near the second-step terrain when no diabatic heating is available. In the absence of eastward-propagating MCSs, convection and mesoscale vortices still appear in the plains, but the vortex strength and precipitation intensity weaken markedly. It is indicated that the eastward-propagating MCSs over the second-step terrain has non-negligible impact on the development and enhancement of convection and vortices in the downstream areas.
Effects of drag coefficients on surface heat flux during typhoon Kalmaegi (2014)
Lei Liu, Guihua Wang, Ze Zhang, Huizan Wang
, Available online   , Manuscript accepted  05 January 2022, doi: 10.1007/s00376-022-1285-1
The lack of in-situ observations and uncertainties of drag coefficient (Cd) at high winds mainly result in pale understandings of heat flux through the air-sea interface and thus inaccurate estimation of typhoon intensity in numerical models. In this study, buoy observations and numerical simulations from an air-sea coupled model are used to access the surface heat flux changes and impacts of Cd parameterization schemes on it during the passage of typhoon Kalmaegi (2014). Three Cd schemes, which make Cd increase, level off, and decrease respectively are considered. The air-sea coupled model captured both trajectory and intensity changes better than the atmosphere-only model, though with relatively weaker sea surface cooling (SSC) than buoy observations which led to higher heat flux and thus stronger typhoon. Different from previous studies, for a moderate typhoon, the coupled simulation with increasing Cd scheme outputted an intensity most consistent with the observation because of the strongest SSC, reasonable Cq/Ch, and an obvious reduction in the overestimated surface heat flux among all experiments. Results from sensitivity experiments showed that surface heat flux was significantly determined by Cd-induced SSC rather than the resulting wind speed changes. Only when SSC differs indistinctively (<0.4℃) between the coupled simulations, heat flux showed a weak positive correlation with the Cd-impacted 10-m wind speed. Cd also played an important role in declining heat flux even a long time after the passage of Kalmaegi because of the continuous upwelling from deeper ocean layers driven by the impacted momentum flux through the air-sea interface.
Characterization of regional combustion efficiency using ΔXCO:ΔXCO2 observed by a portable Fourier-Transform Spectrometer at an urban site in Beijing
Ke Che, Yi Liu, cai zhaonan, Dongxu Yang, Haibo Wang, Denghui Ji, Yang Yang, Pucai Wang
, Available online   , Manuscript accepted  05 January 2022, doi: 10.1007/s00376-022-1247-7
Measurements of dry air column of CO2 (XCO2) and CO (XCO) were performed throughout 2019 at an urban site in Beijing using a compact Fourier Transform Spectrometer (FTS) EM27/SUN. This data set is used to assess the characteristics of combustion-related CO2 emission of urban Beijing by analyzing the correlated daily anomalies of XCO and XCO2 (e.g. ΔXCO and ΔXCO2). The EM27/SUN measurements were calibrated to a 125HR-FTS at Xianghe station by an extra EM27/SUN instrument transferred between two sites. The ratio of ΔXCO over ΔXCO2 (ΔXCO:ΔXCO2) is used to estimate the combustion efficiency in the Beijing region. High correlation coefficient (0.86) between ΔXCO and ΔXCO2 is observed. CO:CO2 emission ratio estimated from inventories is higher than the observed ΔXCO:ΔXCO2 (10.46 ± 0.11 ppb ppm−1) by 42.54–101.15% , indicating an overestimation in combustion efficiency in the inventories. Daily ΔXCO:ΔXCO2 are influenced by transportation governed by weather conditions, except for days in summer when the correlation is low due to the terrestrial biotic activity. By convolving the column footprint (ppm (μmol m-2s-1)-1) generated by Weather Research and Forecasting-X-Stochastic Time-Inverted Lagrangian Transport models (WRF-X-STILT) with two fossil-fuel emission inventories (the Multi-resolution Emission Inventory for China (MEIC) and the Peking University (PKU) inventory), the observed enhancement of CO2 and CO was used to evaluate the regional emission. The CO2 emission are estimated to be 11%, 49% underestimation for MEIC and PKU, respectively. For CO emission, MEIC and PKU overestimates about 30% and 35% in Beijing area.
Quantitative precipitation forecast experiment based on basic NWP variables using deep learning
Kanghui Zhou, Jisong Sun, Yongguang ZHENG, yutao Zhang
, Available online   , Manuscript accepted  31 December 2021, doi: 10.1007/s00376-021-1207-7
The quantitative precipitation forecast (QPF) performance of numerical weather prediction (NWP) methods depends fundamentally on the physical parameterization schemes (PS) adopted. However, due to the complexity of the physical mechanisms of precipitation processes, the uncertainties of PSs result in worse QPF performance than the basic meteorological variables such as air temperature, wind, geopotential height, and humidity. In this study, we propose a deep learning model named QPFNet using basic meteorological variables in the ERA5 dataset by fitting a non-linear mapping relationship between the basic variables and precipitation. Basic variables forecast by the European Centre for Medium-Range Weather Forecasts (ECMWF)'s highest-resolution model (HRES) were fed into QPFNet to forecast precipitation. Evaluation results show that QPFNet achieved better QPF performance than ECMWF HRES itself. The threat score for 3 h of accumulated precipitation with depths of 0.1, 3, 10, and 20 mm increased by 19.7%, 15.2%, 43.2%, and 87.1%, indicating that the performance of the proposed QPFNet improved with increasing levels of precipitation. The sensitivities of these meteorological variables for QPF in different pressure layers was analyzed based on the output of the QPFNet, and its performance limitations are also discussed. Using DL to extract features from basic meteorological variables can provide an important reference for QPF, and avoid some uncertainties of PSs.
Energy paths that sustain the warm-sector torrential rainfall over South China and their contrasts to the frontal rainfall: A case study
Shenming Fu, Jingping Zhang, Yali Luo, Wenting Yang, Jianhua Sun
, Available online   , Manuscript accepted  29 December 2021, doi: 10.1007/s00376-021-1336-z
The warm-sector torrential rainfall over South China, which is famous for its huge destructive power, is one of the most challenging issues in current numerical forecast field. Insufficient understanding of the key mechanisms underlying this type of event is the root cause. Since energetics is crucial to understand evolutions of various types of weather systems, we provided a general methodology for investigating energetics of torrential rainfall. By applying this methodology to a persistent torrential rainfall event which had concurrent frontal and warm-sector precipitation, we established the first physical image on the energetics of the warm-sector torrential rainfall. This clarified energy sources for producing the warm-sector rainfall during this event. For the first time, fundamental similarities and differences between the warm-sector and frontal torrential rainfall were shown in terms of energetics. It was found that these two types of rainfall mainly differed from each other in the lower-tropospheric dynamical features and their key differences lay in energy sources. Scale interactions (mainly through downscale energy cascade and transport) were a dominant factor for the warm-sector torrential rainfall during this event, whereas, for the frontal torrential rainfall, they were only of the secondary importance. Three typical signals in the background environment were found to supply energy to the warm-sector torrential rainfall, with the quasi-biweekly oscillation contributed the most.
Influence of Coriolis parameter variation on Langmuir turbulence in the ocean upper mixed layer with Large Eddy Simulation
Dongxiao Wang, Guojing Li, SHEN Lian, shu qiang
, Available online   , Manuscript accepted  29 December 2021, doi: 10.1007/s00376-021-1390-6
Langmuir turbulence is a complex turbulent process in the ocean upper mixed layer. The Coriolis parameter has an important effect on Langmuir turbulence through Coriolis-Stokes force and Ekman effect, which has not been systematically investigated. Here, impact of Coriolis parameter on Langmuir turbulence with a change of latitude (LAT) from 20oN to 80oN is studied using a non-hydrostatic large eddy simulation model under an ideal condition. The results show that fh/u_* = 0.266 (LAT = 50 oN) is a key value (latitude) for the modulation effect of Coriolis parameter on the mean and turbulent statistics of Langmuir turbulence. The f, h, and represent the Coriolis parameter, the initial upper mixed layer depth and the friction velocity of sea surface induced by wind, respectively. It is found that the rate of change of the sea surface temperature, upper mixed layer depth, entrainment flux, crosswind velocity, downwind vertical momentum flux and turbulent kinetic energy budget terms associated with Langmuir turbulence are more evident at fh/u_*≤ 0.266 (LAT ≤ 50 oN) than that at fh/u_*≥ 0.266 (LAT ≥ 50 oN). However, the rate of change of the depth-averaged crosswind vertical momentum flux does not have a clear variation between fh/u_*≤ 0.266 and fh/u_*≥ 0.266. The complex changes of both Langmuir turbulence characteristics and influence of Langmuir turbulence on the upper mixed layer with latitude presented here may provide more information for further improving Langmuir turbulence parameterization.
Numerical Simulations of a Florida Sea Breeze and Its Interactions with Associated Convection: Effects of Geophysical Representations and Model Resolution
Nessa Hock, Feimin Zhang, Zhaoxia Pu
, Available online   , Manuscript accepted  10 December 2021, doi: 10.1007/s00376-021-1216-6
The Florida peninsula in the USA has a frequent occurrence of sea breeze (SB) thunderstorms. In this study, the numerical simulation of a Florida SB and its associated convective initiation (CI) is simulated using the mesoscale community Weather Research and Forecasting (WRF) model in one-way nested domains at different horizontal resolutions. Results are compared with observations to examine the accuracy of model-simulated SB convection as well as factors that influence SB CI within the simulation. It is found that the WRF model is able to realistically reproduce the observed SB CI. Differences are found in the timing, location, and intensity of the convective cells at different domains with various spatial resolutions. With increasing spatial resolution, the simulation improvements are manifested mainly in the timing of CI and the orientation of the convection after the sea breeze front (SBF) merger into the squall line over the peninsula. Diagnoses indicate that accurate representation of geophysical variables (e.g., coastline and bay shape, small lakes measuring 10-30 km2) play a significant role in improving the simulations through its impacts on the location and timing of SB CI due to changes in low-level atmospheric convergence and surface sensible heating. More importantly, it enables Florida lakes (30 km2 and larger) to produce noticeable lake breezes (LBs) that collide with the SBFs to produce CI. Furthermore, improving the representation of geophysical variables helps the model reproduce a stronger convective squall line caused by merging SBs, leading to accurate locations of post-frontal convective systems.
Hybrid Methods for Computing Streamfunction and Velocity Potential for Complex Flow Fields over Mesoscale Domains
Jie Cao, Qin Xu, Haishan Chen, Shuping Ma
, Available online   , Manuscript accepted  10 December 2021, doi: 10.1007/s00376-021-1280-y
Three types of previous methods are revisited for computing streamfunction  and velocity potential  from horizontal velocity v in limited domains. The first type, called SOR-based method, uses classical successive over-relaxation (SOR) scheme to compute  (or ) first with an arbitrary boundary condition (BC) and then  (or ) with the BC derived from v. The second type, called spectral method, uses spectral formulations to construct the inner part of (, ) – the inversion of (vorticity, divergence) with homogeneous BC, and then the remaining harmonic part of (, ) with BCs from v. The third type, called integral method, uses integral formulas to compute the internally induced (, ) – the inversion of domain-internal (vorticity, divergence) without BC and then the remaining harmonic  (or ) with BC from v minus the internally induced part. Although these previous methods have been successfully applied to flows in large-scale and synoptic-scale domains, they fail to retain adequate accuracies when applied to complex flows over mesoscale domains as shown in this paper. To solve this problem, two hybrid methods, integral-SOR and integral-spectral methods, are developed by optimally combining the first step of integral method with the second step adopted from SOR-based and spectral methods, respectively. Tested with real-case complex flows, the integral-SOR method is found to be significantly more accurate than integral-spectral method, while the latter is generally more accurate than the three previous methods. The integral-SOR method is recommended for future applications and diagnostic studies of complex flows.
Ocean-atmosphere teleconnections play a key role in the interannual variability of seasonal gross primary production in China
Kairan Ying, Jing Peng, Li Dan, Xiaogu Zheng
, Available online   , Manuscript accepted  06 December 2021, doi: 10.1007/s00376-021-1226-4
Since 1950s, the terrestrial carbon uptake has been characterized by large interannual variations, which are mainly determined by interannual variations in GPP. However, at the regional scale, the sources behind these changes – especially those from the ocean-atmosphere teleconnections – are still unclear. Using an ensemble of TRENDY simulations during 1951–2010, the relationships of the interannual variability of seasonal GPP in China with SST and atmospheric circulations were investigated. The GPP signals that mostly related to the climate forcings were identified by firstly separating out the significant impact from the linear trend and the GPP memory, for the seasons of MAM, JJA and SON. Results showed that the seasonal GPP over China associated with the R-PC1s during MAM to SON show a monopole spatial structure, with a clear seasonal evolution for their maximum centers from MAM to SON. The dominant two GPP R-PCs are significantly related to the SST variability in the eastern tropical Pacific Ocean and the North Pacific Ocean during MAM to SON, implying the influences from ENSO and PDO. The identified SST and circulation factors explain 13%, 23% and 19% of the total variance for seasonal GPP for MAM, JJA and SON, respectively. More importantly, these major SST and circulation factors are mostly associated with the GPP variations for the regions of central China, southwestern China, northeastern China and southern China during MAM–JJA–SON. Understanding for the relationships of China’s GPP with ocean-atmosphere teleconnections provides a way to estimate and project changes in the carbon cycle.
A hybrid neural network model for ENSO prediction in combination with principal oscillation pattern analyses
Lu Zhou, Rong-Hua Zhang
, Available online   , Manuscript accepted  06 December 2021, doi: 10.1007/s00376-021-1368-4
El Nino-Southern Oscillation (ENSO) can be currently predicted reasonably well six months and longer, but large biases and uncertainties remain in its real-time prediction. Various approaches have been taken to improve understanding of ENSO processes, and different models for ENSO predictions have been developed, including linear statistical models based on principal oscillation pattern (POP) analyses, convolutional neural networks (CNNs), and so on. Here, we develop a novel hybrid model, named as POP-Net, by combining the POP analysis procedure with CNN-long short-term memory (LSTM) algorithm to predict the Nino 3.4 sea surface temperature (SST) index. ENSO predictions are compared with each other from the corresponding three models: POP model, CNN-LSTM model and POP-Net, respectively. The POP-based pre-processing acts to enhance ENSO-related signals of interest while filtering unrelated noise. Consequently, an improved prediction is achieved in the POP-Net relative to others. The POP-Net shows a high-correlation skill for 17-month lead time prediction (correlation coefficients exteeding 0.5) during the 1994-2020 validation period. The POP-Net also alleviates the spring predictability barrier (SPB). Clearly, value-added artificial neural networks for improved ENSO predictions are demonstrated by including the process-oriented analyses to enhance signal representations.
Quantify spatial characteristics of the moisture transport on precipitation seasonality and recycling variability in Central Asia
Linhao Zhong, Lijuan Hua, Zhaohui GONG, Yao Yao, Lin MU
, Available online   , Manuscript accepted  03 December 2021, doi: 10.1007/s00376-021-1383-5
The moisture contribution and transport pathway are quantitatively examined for Central Asia (CA) to explain the precipitation seasonality and the moisture transport variation during 1979-2015 by the Langrangian water cycle model based on the reanalysis and observational data. The westerly-related (northwesterly and westerly) transport explains 42% CA precipitation and dominates the southwest CA, where the highest precipitation is reached in cold season. But the southeast CA including part of Northwest China attains the maximal precipitation in warm season and is solely dominated by the southerly transport, which totally explains about 48% CA precipitation. The rest 10% CA precipitation is explained by the northerly transport that stably impacts on the north CA with the maximal precipitation reached in warm season. Most CA areas are exposed to seasonality-varying moisture transport, except that the southeast and north CA are impacted by southerly and northerly transport all year round. In general, the midlatitude westerlies-driven transport and the Indian monsoon-driven southerly-related transport mainly explain the spatial difference of the precipitation seasonality in CA. Moreover, the contribution ratio of the local evaporation in CA to the precipitation features the significant interdecadal variability coupled with a meridionally tripolar anomalies of moisture transport. Since the early 2000s, CA has experienced a decade of anomalously low local contribution, which seems jointly determined by the weakened moisture contribution over midlatitudes (Atlantic, Europe and CA itself), and the enhanced contribution over the high latitudes (the West Siberia and the Arctic) and tropical area (South Asia and the Indian Ocean).
Intensity Evolution of Zonal Shear Line over the Tibetan Plateau in Summer: A Perspective of Divergent and Rotational Kinetic Energies
xiaohong BAO, Xiuping Yao
, Available online   , Manuscript accepted  02 December 2021, doi: 10.1007/s00376-021-1302-9
Based on the ERA5 reanalysis datasets during 1980–2019, a total of eleven zonal shear lines (ZSLs) that cause heavy precipitation and last more than 60 hours over the Tibetan Plateau in summer are selected for composite analysis. By decomposing the kinetic energy (Κ) near the ZSL into the divergent and rotational kinetic energies ( KD and ΚR) and the kinetic energy for the interaction between the divergent wind and the rotational wind (ΚRD), the influence of the rotational and divergent winds on the evolution of the ZSL intensity is investigated from the perspective of ΚD and ΚR. The main results were as follows. The ZSL is a comprehensive reflection of rotation and convergence. The intensity evolution of ZSL is basically synchronized with those of Κ, ΚR and ΚRD, but lags behind that of ΚD by about 3 hours. The enhancement of Κ is mainly contributed by that of ΚR, which is governed by the conversion from ΚD to ΚR. Furthermore, the increase of the conversion from ΚD to ΚR is controlled by that of the geostrophic effect term Af, which is determined by the joint enhancement of the zonal rotational and meridional divergent wind components (uR and vD). Therefore, the joint enhancement of uR and vD controls the increase of the ZSL intensity , which in turn leads to the increased precipitation.
The Roles of Barotropic Instability and Beta Effect in Eyewall Evolution of Tropical Cyclones
Jie Jiang, Yuqing Wang
, Available online   , Manuscript accepted  25 November 2021, doi: 10.1007/s00376-021-1209-5
Diabatic heating by convection in the eyewall often produces an annular region of high potential vorticity (PV) around the relatively low PV eye in a strong tropical cyclone (TC). Such a PV ring is barotropically unstable and can encourage exponentially growing PV waves. In this study, such instability and the subsequent nonlinear evolutions of three TC-like vortices with different degrees of hollowness PV ring on an f-plane are first examined using an unforced, inviscid shallow-water-equation model. Results show that the simulated eyewalls evolve similarly to those in the nondivergent barotropic model. It is also found that the polygonal eyewall structure can be decomposed into vortex Rossby waves (VRWs) of different wavenumbers with different amplitudes, allowing for wave-wave interaction to produce complicated behaviors of mesovortices in the TC eyewall. The same set of PV rings has been examined on a beta-plane. Although the beta effect has been rendered unimportant to the eyewall evolution due to the relatively small-scale of inner-core circulation, it is shown in this study that the beta effect may erode the coherent structure of mesovortices in the eyewall of an initially hollow PV-ring vortex. Mesovortices modeled on the beta-plane with greater beta parameter tends to experience an earlier breakdown along with the enhanced radial gradients of the basic-state (azimuthal mean) angular velocity, followed by wave-wave and wave-flow interactions, leading to earlier merger and axisymmetrization processes. This implies that the beta effect could be one of the forcings that shorten the lifetime of quasi-steady mesovortices in the eyewall of real TCs.
Impacts of New Implementing Strategies for Surface and Model Physics Perturbations in TREPS on Forecasts of Landfalling Tropical Cyclones
Xubin Zhang
, Available online   , Manuscript accepted  23 November 2021, doi: 10.1007/s00376-021-1222-8
To improve the ensemble prediction system of the tropical regional atmosphere model for the South China Sea (TREPS) in predicting landfalling tropical cyclones (TCs), the impacts of three new implementing strategies for surface and model physics perturbations in TREPS were evaluated for 19 TCs making landfall in China during 2014–2016. For sea surface temperature (SST) perturbations, spatially uncorrelated random perturbations were replaced with spatially correlated ones; the multiplier f, which is used to form perturbed tendency in the Stochastically Perturbed Parameterization Tendency (SPPT) scheme, was inflated in regions with evident convective activity (f-inflated SPPT); and the Stochastically Perturbed Parameterization (SPP) scheme with 14 perturbed parameters selected from the planetary boundary layer, surface layer, microphysics, and cumulus convection parameterizations was added. Overall, all of these methods improved forecasts more significantly for non-intensifying than intensifying TCs. Compared with f-inflated SPPT, the spatially correlated SST perturbations generally showed comparable performance but were more and less skillful for intensifying and non-intensifying TCs, respectively. The advantages of the spatially correlated SST perturbations and f-inflated SPPT were mainly present in the deterministic guidance for both track and wind and in the probabilistic guidance for reliability of wind. For intensifying TCs, adding SPP led to mixed impacts with significant improvements in probability-matched mean of modest wind and in probabilistic forecasts of rainfall; while for non-intensifying TCs, adding SPP led to frequently positive impacts on the deterministic guidance for track, intensity, strong wind and moderate rainfall and on the probabilistic guidance for wind and discrimination of rainfall.
Application of a Neural Network to Store and Compute the Optical Properties of Non-Spherical Particles
Jinhe YU, Lei BI, Wei HAN, Xiaoye ZHANG
, Available online   , Manuscript accepted  23 November 2021, doi: 10.1007/s00376-021-1375-5
Radiative transfer simulations and remote sensing studies fundamentally require accurate and efficient computation of the optical properties of non-spherical particles. This paper proposes a deep learning (DL) scheme in conjunction with an optical property database to achieve this goal. Deep neural network (DNN) architectures were obtained from a dataset of the optical properties of super-spheroids with extensive shape parameters, size parameters, and refractive indices. The dataset was computed through the invariant imbedding T-matrix method. Four separate DNN architectures were created to compute the extinction efficiency factor, single-scattering albedo, asymmetry factor, and phase matrix. The criterion for designing these neural networks was the achievement of the highest prediction accuracy with minimal DNN parameters. The numerical results demonstrate that the determination coefficients are greater than 0.999 between the prediction values from the neural networks and the truth values from the database, which indicates that the DNN can reproduce the optical properties in the dataset with high accuracy. In addition, the DNN model can robustly predict the optical properties of particles with high accuracy for shape parameters or refractive indices that are unavailable in the database. Importantly, the ratio of the database size (~127 GB) to that of the DNN parameters (~20 MB) is approximately 6810, implying that the DNN model can be treated as a highly compressed database that can be used as an alternative to the original database for real-time computing of the optical properties of non-spherical particles in radiative transfer and atmospheric models.
Structure and evolution of decadal spiciness variability in the North Pacific during 2004-2020, revealed from Argo observations
Guanghui Zhou, Rong-Hua Zhang
, Available online   , Manuscript accepted  19 November 2021, doi: 10.1007/s00376-021-1358-6
Ocean spiciness is referred to as density-compensated temperature and salinity variations with warm (cool) and salty (fresh) waters having high (low) spiciness, respectively. The structure and evolution of density-compensated (warm/salty or cool/fresh) spiciness anomalies are investigated in the North Pacific thermocline using Argo observations during the period 2004-2020. Two well-organized decadal spiciness events are identified through isopycnal surface analyses. One warm/salty spiciness anomaly of about 0.15 °C and 0.05 g kg-1 temperature and salinity perturbations on 25 kg m-3 isopycnal appears in the eastern subtropical North Pacific at 18°N-30°N, 120°W-150°W in 2007, which then migrates southwestward along the mean circulation and arrives in the western tropics at 145°E–175°W, ~15°N in 2012-2013, with the reduced salinity perturbation of about 0.043 g kg-1. Another cool/fresh spiciness anomaly of about -0.2 °C and -0.07 g kg-1 temperature and salinity perturbations originates from the eastern subtropics at 120°W-150°W, ~30°N in 2014 and follows the similar advection pathway during the period from 2014-2015 to 2019-2020. The subduction pathway can be adequately determined by the mean Montgomery stream function on the isopycnal surface; the propagation direction and speed are in good agreement with the expectation for the role played by advection due to the mean geostrophic current. Moreover, the subducted decadal spiciness anomalies can have negative feedback on sea surface temperature (SST) variability in the western tropical Pacific through the diapycnal processes. The identifications of these density-compensated spiciness anomalies and their propagation pathways provide a clear illustration of the oceanic extratropics-tropics interactions in the North Pacific Ocean.
Comparison of PM2.5 and CO2 in large cities of China during the COVID-19 lockdown
Chuwei Liu, Zhongwei Huang, Jianping Huang, Chunsheng LIANG, Lei Ding, Xinbo LIAN, Xiaoyue Liu, Li Zhang, Danfeng WANG
, Available online   , Manuscript accepted  18 November 2021, doi: 10.1007/s00376-021-1281-x
Estimation of human activities’ impact on PM2.5 pollution and CO2 emissions in different regions of city group is of great importance for the development of efficient policies. In early 2020, China implemented a lockdown policy to contain the spread of COVID-19, resulting in a significant reduction in human activities. This is an opportunity to study the impact of human activities such as transportation and industry on air pollution. We study the variation in air quality in megacities of China by combining in-situ environmental and meteorological datasets, Suomi-NPP/VIIRS fire radiative power (FRP) and Carbon Monitor project CO2 emissions. Our study shows that PM2.5 concentrations in the spring of 2020 decreased by 41.97% in the Yangtze River Delta (YRD) and 43.30% in the Pearl River Delta (PRD) respectively, due to traffic and manufacturing industries were significantly shut down. Nevertheless, PM2.5 concentrations in the Beijing-Tianjin-Hebei (BTH) region only decreased by 2.01% because the energy and steel industries could not be fully paused. In addition, unfavorable weather conditions also lead to heavy PM2.5 pollution. Moreover, CO2 concentrations did not change much in China during short-term emission reduction, although the CO2 emissions dropped by 19.52% compared to the same period in 2019. This suggests that concerted efforts from different emission sectors and long-term strategies to reduce emissions are necessary, along with improved energy use efficiency, and the use of cleaner energy to control air pollution and CO2 emissions.
Evaluation of Arctic Sea Ice Drift and its Relationship with Near-surface Wind and Ocean Current in Nine CMIP6 Models from China
Xiaoyong YU, Chengyan LIU, Xiaocun WANG, Jian CAO, Jihai DONG, Yu LIU
, Available online   , Manuscript accepted  15 November 2021, doi: 10.1007/s00376-021-1153-4
The simulated Arctic sea ice drift and its relationship with the near-surface wind and surface ocean current during 1979–2014 in nine models from China that participated in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) are examined by comparison with observational and reanalysis datasets. Most of the models reasonably represent the Beaufort Gyre (BG) and Transpolar Drift Stream (TDS) in the spatial patterns of their long-term mean sea ice drift, while the detailed location, extent, and strength of the BG and TDS vary among the models. About two-thirds of the models agree with the observation/reanalysis in the sense that the sea ice drift pattern is consistent with the near-surface wind pattern. About the same proportion of models shows that the sea ice drift pattern is consistent with the surface ocean current pattern. In the observation/reanalysis, however, the sea ice drift pattern does not match well with the surface ocean current pattern. All nine models missed the observational widespread sea ice drift speed acceleration across the Arctic. For the Arctic basin-wide spatial average, five of the nine models overestimate the Arctic long-term (1979–2014) mean sea ice drift speed in all months. Only FGOALS-g3 captures a significant sea ice drift speed increase from 1979 to 2014 both in spring and autumn. The increases are weaker than those in the observation. This evaluation helps assess the performance of the Arctic sea ice drift simulations in these CMIP6 models from China.
Identification of Convective and Stratiform Clouds Based on the Improved DBSCAN Clustering Algorithm
Yuanyuan ZUO, Hu Zhiqun, Shujie Yuan, jiafeng zheng, Xiaoyan YIN, Boyong LI
, Available online   , Manuscript accepted  09 November 2021, doi: 10.1007/s00376-021-1223-7
A convective and stratiform cloud classification method for weather radar is proposed based on the density-based spatial clustering of applications with noise (DBSCAN) algorithm. To identify convective and stratiform clouds in different developmental phases, two-dimensional (2D) and three-dimensional (3D) models are proposed by applying reflectivity factors at 0.5°, and at 0.5°, 1.5° and 2.4° elevation angles, respectively. According to the thresholds of the algorithm, which include the echo intensity, the echo top height of 35 dBZ (ET), the density threshold, and the ε neighborhood, cloud clusters can be marked into four types: deep-convective cloud (DCC), shallow-convective cloud (SCC), mixed convective-stratiform cloud (MCS), and stratiform cloud (SFC) types. Each cloud cluster type is further identified as a core area and boundary area, which can provide more abundant cloud structure information. The algorithm is verified using two volume scan data observed with new-generation S-band weather radars in Nanjing and Xuzhou. The results show that the classification of clouds is intuitive, and the core and boundary areas of each type are obvious. Because density thresholds are different and multiple elevations are utilized in 3D modeling, the identified echo types and areas are dissimilar between the 2D and 3D models. The majority of cloud clusters are marked as SCCs by the 2D model but as SFCs by the 3D model. In addition, the 3D model can avoid the influence of the melting layer and better suggest convective clouds in the developmental stage.
An Isentropic Mass Circulation View on the Extreme Cold Events in the 2020/21 Winter
Yueyue YU, Yafei LI, Rongcai REN, Ming CAI, Zhaoyong GUAN, Wei HUANG
, Available online   , Manuscript accepted  04 November 2021, doi: 10.1007/s00376-021-1289-2
Three extreme cold events successively occurred across East Asia and North America in the 2020/21 winter. This study investigates the underlying mechanisms of these record-breaking persistent cold events from the isentropic mass circulation (IMC) perspective. Results show that the midlatitude cold surface temperature anomalies always co-occurred with the high-latitude warm anomalies, and this was closely related to the strengthening of the low-level equatorward cold air branch of the IMC, particularly along the climatological cold air routes over East Asia and North America. Specifically, the two cold surges over East Asia in early winter were results of intensification of cold air transport there, influenced by the Arctic sea ice loss in autumn. The weakened cold air transport over North America associated with warmer northeastern Pacific sea surface temperatures (SSTs) explained the concurrent anomalous warmth there. This enhanced a wavenumber-1 pattern and upward wave propagation, inducing a simultaneous and long-lasting stronger poleward warm air branch (WB) of the IMC in the stratosphere and hence a displacement-type Stratospheric Sudden Warming (SSW) event on 4 January. The WB-induced increase in the air mass transported into the polar stratosphere was followed by intensification of the equatorward cold branch, hence promoting the occurrence of two extreme cold events respectively over East Asia in the beginning of January and over North America in February. Results do not yield a robust direct linkage from La Niña to the SSW event, IMC changes, and cold events, though the extratropical warm SSTs are found to contribute to the February cold surge in North America.
Variations in Amplitudes and Wave Energy along the Energy Dispersion Paths for Rossby Waves in the Quasigeostrophic Barotropic Model
Yaokun LI, Jiping CHAO, Yanyan KANG
, Available online   , Manuscript accepted  03 November 2021, doi: 10.1007/s00376-021-1244-2
Variations in wave energy and amplitude for Rossby waves are investigated by solving the wave energy equation for the quasigeostrophic barotropic potential vorticity model. The results suggest that compared with rays in the nondivergent barotropic model, rays in the divergent model can have enhanced meridional and zonal propagation, accompanied by a more dramatic variability in both wave energy and amplitude, which is caused by introducing the divergence effect of the free surface in the quasigeostrophic model. For rays propagating in a region enclosed by a turning latitude and a critical latitude, the wave energy approaches the maximum value inside the region, while the amplitude approaches the maximum at the turning latitude. Waves can develop when both the wave energy and amplitude increase. For rays propagating in a region enclosed by two turning latitudes, the wave energy approaches the minimum value at one turning latitude and the maximum value at the other latitude, while the total wavenumber approaches the maximum value inside the region. The resulting amplitude increases if the total wavenumber decreases or the wave energy increases more significantly and decreases if the total wavenumber increases or the wave energy decreases more significantly. The matched roles of the energy from the basic flow and the divergence of the group velocity contribute to the slightly oscillating wave energy, which causes a slightly oscillating amplitude as well as the slightly oscillating total wavenumber.
Contrasts between the Interannual Variations of Extreme Rainfall over Western and Eastern Sichuan in Mid-summer
Mengyu DENG, Riyu LU, Chaofan LI
, Available online   , Manuscript accepted  20 October 2021, doi: 10.1007/s00376-021-1219-3
Rainfall amount in mid-summer (July and August) is much greater over eastern than western Sichuan, which are characterized by basin and plateau, respectively. It is shown that the interannual variations of extreme rainfall over these two regions are roughly independent, and they correspond to distinct anomalies of both large-scale circulation and sea surface temperature (SST). The enhanced extreme rainfall over western Sichuan is associated with a southward shift of the Asian westerly jet, while the enhanced extreme rainfall over eastern Sichuan is associated with an anticyclonic anomaly in the upper troposphere over China. At low levels, on the other hand, the enhanced extreme rainfall over western Sichuan is related to two components of wind anomalies, namely southwesterly over southwestern Sichuan and northeasterly over northeastern Sichuan, which favor more rainfall under the effects of the topography. Relatively speaking, the enhanced extreme rainfall over eastern Sichuan corresponds to the low-level southerly anomalies to the east of Sichuan, which curve into northeasterly anomalies over the basin when they encounter the mountains to the north of the basin. Therefore, it can be concluded that the topography in and around Sichuan plays a crucial role in inducing extreme rainfall both over western and eastern Sichuan. Finally, the enhanced extreme rainfall in western and eastern Sichuan is related to warmer SSTs in the Maritime Continent and cooler SSTs in the equatorial central Pacific, respectively.
The Weakening Relationship between ENSO and the South China Sea Summer Monsoon Onset in Recent Decades
Peng HU, Wen CHEN, Shangfeng CHEN, Lin WANG, Yuyun LIU
, Available online   , Manuscript accepted  19 October 2021, doi: 10.1007/s00376-021-1208-6
The El Niño-Southern Oscillation (ENSO) is traditionally regarded as the most important factor modulating the interannual variation of the South China Sea summer monsoon (SCSSM) onset. A preceding El Niño (La Niña) usually tends to be followed by a delayed (an advanced) monsoon onset. However, the close relationship between ENSO and SCSSM onset breaks down after the early-2000s, making seasonal prediction very difficult in recent years. Three possible perspectives have been proposed to explain the weakening linkage between ENSO and SCSSM onset, including interdecadal change of the ENSO teleconnection (i.e., the Walker circulation), interferences of other interannual variability (i.e., the Victoria mode), and disturbances on intraseasonal time scales (i.e., the quasi-biweekly oscillation). By comparing the epochs of 1979–2001 and 2002–19, it is found that the anomalous tropical Walker circulation generated by ENSO is much weaker in the latter epoch and thus cannot deliver the ENSO signal to the SCSSM onset. Besides, in recent years, the SCSSM onset is more closely linked to extratropical factors like the Victoria mode, and thus its linkage with ENSO becomes weaker. In addition to these interannual variabilities, the intraseasonal oscillations like the quasi-biweekly oscillation can disrupt the slow-varying seasonal march modulated by ENSO. Thus, the amplified quasi-biweekly oscillation may also contribute to the weakening relationship after the early-2000s. Given the broken relationship between ENSO and SCSSM onset, the extratropical factors should be considered in order to make skillful seasonal predictions of SCSSM onset, and more attention should be paid to the extended-range forecast based on intraseasonal oscillations.
The Synergism between Methanogens and Methanotrophs and the Nature of their Contributions to the Seasonal Variation of Methane Fluxes in a Wetland: The Case of Dajiuhu Subalpine Peatland
Luwen WANG, Jiwen GE, Liang FENG, Yaoyao LIU, Yu LI, Jiumei WANG, Xiang XIAO, Zhiqi ZHANG
, Available online   , Manuscript accepted  19 October 2021, doi: 10.1007/s00376-021-1255-z
Wetland ecosystems are the most important natural methane (CH4) sources, whose fluxes periodically fluctuate. Methanogens (methane producers) and methanotrophs (methane consumers) are considered key factors affecting CH4 fluxes in wetlands. However, the symbiotic relationship between methanogens and methanotrophs remains unclear. To help close this research gap, we collected and analyzed samples from four soil depths in the Dajiuhu subalpine peatland in January, April, July, and October 2019 and acquired seasonal methane flux data from an eddy covariance (EC) system, and investigated relationships. A phylogenetic molecular ecological networks (pMENs) analysis was used to identify keystone species and the seasonal variations of the co-occurrence patterns of methanogenic and methanotrophic communities. The results indicate that the seasonal variations of the interactions between methanogenic and methanotrophic communities contributed to CH4 emissions in wetlands. The keystone species discerned by the network analysis also showed their importance in mediating CH4 fluxes. Methane (CH4) emissions in wetlands were lowest in spring; during this period, the most complex interactions between microbes were observed, with intense competition among methanogens while methanotrophs demonstrated better cooperation. Reverse patterns manifested themselves in summer when the highest CH4 flux was observed. Methanoregula formicica was negatively correlated with CH4 fluxes and occupied the largest ecological niches in the spring network. In contrast, both Methanocella arvoryzae and Methylocystaceae demonstrated positive correlations with CH4 fluxes and were better adapted to the microbial community in the summer. In addition, soil temperature and nitrogen were regarded as significant environmental factors to CH4 fluxes. This study was successful in explaining the seasonal patterns and microbial driving mechanisms of CH4 emissions in wetlands.
Analysis of the Winter Cloud-to-Ground Lightning Activity and Its Synoptic Background in China during 2010–20
Manman MA, Xiaogang HUANG, Jianfang FEI, Chi ZHANG, Chao LI, Xiaoping CHENG
, Available online   , Manuscript accepted  19 October 2021, doi: 10.1007/s00376-021-1260-2
Cloud-to-ground (CG) lightning data and the ECMWF ERA-Interim reanalysis dataset are analyzed to gain insight into the spatiotemporal distribution and synoptic background of winter-season CG flashes between December 2010 and February 2020 in China. We identify three Winter Lightning Frequent Areas (WLAs): the southwest side of the Yunnan-Guizhou Plateau (WLA1), the east side of the Yunnan-Guizhou Plateau (WLA2), and the Poyang Lake Plain (WLA3). The CG lightning flashes most frequently occur at local midnight and have a monthly peak in February. The CG lightning in WLA1 is mostly generated in non-frontal weather; however, the lightning in WLA2 and WLA3 mostly occurs in frontal systems. The frontal circulation situation is divided into four typical types: transversal trough after high pressure, low vortex, confrontational convergence, and asymptotic convergence. In all typical weather patterns, the lightning occurs downstream of a 500 hPa trough and is accompanied by a southwesterly low-level jet. The convective parameters of winter thunderstorms differ greatly from those of summer thunderstorms. The maximum convective available potential energy (MCAPE) and K-index (KI) are more useful metrics than convective available potential energy (CAPE) and Showalter index (SI) during winter. This study further deepens the understanding of the distribution characteristics of winter CG lightning in China, which motivates further research to improve the ability of winter thunderstorm prediction.
Ice Nucleation of Cirrus Clouds Related to the Transported Dust Layer Observed by Ground-Based Lidars over Wuhan, China
Yun HE, Fan YI, Fuchao LIU, Zhenping YIN, Jun ZHOU
, Available online   , Manuscript accepted  14 October 2021, doi: 10.1007/s00376-021-1192-x
Cirrus clouds related to transported dust layers were identified on 22 occasions with ground-based polarization lidar from December 2012 to February 2018 over Wuhan (30.5°N, 114.4°E), China. All the events occurred in spring and winter. Cirrus clouds were mostly located above 7.6 km on top of the aloft dust layers. In-cloud relative humidity with respect to ice (RHi) derived from water vapor Raman lidar as well as from ERA5 reanalysis data were used as criteria to determine the possible ice nucleation regimes. Corresponding to the two typical cases shown, the observed events can be classified into two categories: (1) category A (3 cases), in-cloud peak RHi ≥ 150%, indicating competition between heterogeneous nucleation and homogeneous nucleation; and (2) category B (19 cases), in-cloud peak RHi < 150%, revealing that only heterogeneous nucleation was involved. Heterogeneous nucleation generally took place during instances of cirrus cloud formation in the upper troposphere when advected dust particles were present. Although accompanying cloud-top temperatures ranged from –51.9°C to –30.4°C, dust-related heterogeneous nucleation contributed to primary ice nucleation in cirrus clouds by providing ice nucleating particle concentrations on the order of 10-3 L-1 to 102 L-1. Heterogeneous nucleation and subsequent crystal growth reduced the ambient RHi to be less than 150% by consuming water vapor and thus completely inhibited homogeneous nucleation.
One-Dimensional Variational Retrieval of Temperature and Humidity Profiles from the FY4A GIIRS
Qiumeng XUE, Li GUAN, Xiaoning SHI
, Available online   , Manuscript accepted  11 October 2021, doi: 10.1007/s00376-021-1032-z
A physical retrieval approach based on the one-dimensional variational (1D-Var) algorithm is applied in this paper to simultaneously retrieve atmospheric temperature and humidity profiles under both clear-sky and partly cloudy conditions from FY-4A GIIRS (geostationary interferometric infrared sounder) observations. Radiosonde observations from upper-air stations in China and level-2 operational products from the Chinese National Satellite Meteorological Center (NSMC) during the periods from December 2019 to January 2020 (winter) and from July 2020 to August 2020 (summer) are used to validate the accuracies of the retrieved temperature and humidity profiles. Comparing the 1D-Var-retrieved profiles to radiosonde data, the accuracy of the temperature retrievals at each vertical level of the troposphere is characterized by a root mean square error (RMSE) within 2 K, except for at the bottom level of the atmosphere under clear conditions. The RMSE increases slightly for the higher atmospheric layers, owing to the lack of temperature sounding channels there. Under partly cloudy conditions, the temperature at each vertical level can be obtained, while the level-2 operational products obtain values only at altitudes above the cloud top. In addition, the accuracy of the retrieved temperature profiles is greatly improved compared with the accuracies of the operational products. For the humidity retrievals, the mean RMSEs in the troposphere in winter and summer are both within 2 g kg–1. Moreover, the retrievals performed better compared with the ERA5 reanalysis data between 800 hPa and 300 hPa both in summer and winter in terms of RMSE.
Comparative Analysis of the Characteristics of Rainy Season Raindrop Size Distributions in Two Typical Regions of the Tibetan Plateau
Gaili WANG, Ran LI, Jisong SUN, Xiangde XU, Renran ZHOU, Liping LIU
, Available online   , Manuscript accepted  09 October 2021, doi: 10.1007/s00376-021-1135-6
Mêdog and Nagqu are two typical regions of the Tibetan Plateau with different geographical locations and climate regimes. These differences may lead to discrepancies in the raindrop size distributions (DSDs) and precipitation microphysical processes between the two regions. This paper investigates discrepancies in the DSDs using disdrometer data obtained during the rainy season in Mêdog and Nagqu. The DSD characteristics are studied under five different rainfall rate categories and two precipitation types (stratiform and convective). For the total datasets, the number concentrations of drops with diameters D > 0.6 (D < 0.6) mm are higher (lower) in Nagqu than in Mêdog. The fitted normalized gamma distributions of the averaged DSDs for the five rainfall rate categories show that Nagqu has a larger (lower) mass-weighted mean diameter Dm (normalized intercept parameter, lgNw) than does Mêdog. The difference in Dm between Nagqu and Mêdog increases with the rainfall rate. Convective clusters in Nagqu could be identified as continental-like, while convective precipitation in Mêdog could be classified as maritime-like. The relationships between the shape factor μ and slope parameter Λ of the gamma distribution model, the radar reflectivity Z, and the rainfall rate R are also derived. Furthermore, the possible causative mechanism for the notable DSD variation between the two regions during the rainy season is illustrated using reanalysis data and automated weather station observations. Cold rain processes are mainly responsible for the low concentrations of large drops observed in Nagqu, whereas warm rain prevails in Mêdog, producing abundant small drops.
A Survey of Statistical Relationships between Tropical Cyclone Genesis and Convectively Coupled Equatorial Rossby Waves
Shuguang WANG, Juan FANG, Xiaodong TANG, Zhe-Min TAN
, Available online   , Manuscript accepted  09 October 2021, doi: 10.1007/s00376-021-1089-8
Convectively coupled equatorial Rossby waves (ERW) modulate tropical cyclone activities over tropical oceans. This study presents a survey of the statistical relationship between intraseasonal ERWs and tropical cyclone genesis (TCG) over major global TC basins using four-decade-long outgoing longwave radiation (OLR) and TC best-track datasets. Intraseasonal ERWs are identified from the OLR anomalies using an empirical orthogonal function (EOF) analysis method without imposing equatorial symmetry. We find that westward-propagating ERWs are most significant in four tropical ocean basins over the summer hemisphere and that ERWs exhibit similar northeast-southwest (southeast-northwest) tilted phase lines in the northern (southern) hemisphere, with an appreciable poleward advance of wave energy in most TC basins. The EOF-based ERW indices quantitatively show that ERWs significantly modulate TC genesis. The convectively active (suppressed) phases of ERWs coincide with increased (reduced) TCG occurrences. The TCG modulation by ERWs is at a maximum where the ERWs propagate through the climatological TCG hotspots. As a result, the total number of TCG occurrences in the TC basins varies significantly according to the ERW phase. The ERW-TCG relationship is significant over the northwestern Pacific Ocean, northeastern Pacific Ocean, and the northern Indian Ocean during the northern summer seasons. In the southern summer season, the ERW-TCG relationship is significant over the southern Indian Ocean, Indonesian-Australia basin, and the southwestern Pacific Ocean. However, ERW activities are weak in the main TC development region of the Atlantic Ocean; and the impact on Atlantic TCG appears to be insignificant.
The Predictability of Ocean Environments that Contributed to the 2020/21 Extreme Cold Events in China: 2020/21 La Niña and 2020 Arctic Sea Ice Loss
Fei ZHENG, Ji-Ping LIU, Xiang-Hui FANG, Mi-Rong SONG, Chao-Yuan YANG, Yuan YUAN, Ke-Xin LI, Ji WANG, Jiang ZHU
, Available online   , Manuscript accepted  08 October 2021, doi: 10.1007/s00376-021-1130-y
Several consecutive extreme cold events impacted China during the first half of winter 2020/21, breaking the low-temperature records in many cities. How to make accurate climate predictions of extreme cold events is still an urgent issue. The synergistic effect of the warm Arctic and cold tropical Pacific has been demonstrated to intensify the intrusions of cold air from polar regions into middle-high latitudes, further influencing the cold conditions in China. However, climate models failed to predict these two ocean environments at expected lead times. Most seasonal climate forecasts only predicted the 2020/21 La Niña after the signal had already become apparent and significantly underestimated the observed Arctic sea ice loss in autumn 2020 with a 1–2 month advancement. In this work, the corresponding physical factors that may help improve the accuracy of seasonal climate predictions are further explored. For the 2020/21 La Niña prediction, through sensitivity experiments involving different atmospheric–oceanic initial conditions, the predominant southeasterly wind anomalies over the equatorial Pacific in spring of 2020 are diagnosed to play an irreplaceable role in triggering this cold event. A reasonable inclusion of atmospheric surface winds into the initialization will help the model predict La Niña development from the early spring of 2020. For predicting the Arctic sea ice loss in autumn 2020, an anomalously cyclonic circulation from the central Arctic Ocean predicted by the model, which swept abnormally hot air over Siberia into the Arctic Ocean, is recognized as an important contributor to successfully predicting the minimum Arctic sea ice extent.
A New Index Developed for Fast Diagnosis of Meteorological Roles in Ground-Level Ozone Variations
Weihua CHEN, Weiwen WANG, Shiguo JIA, Jingying MAO, Fenghua YAN, Lianming ZHENG, Yongkang WU, Xingteng ZHANG, Yutong DONG, Lingbin KONG, Buqing ZHONG, Ming CHANG, Min SHAO, Xuemei WANG
, Available online   , Manuscript accepted  08 October 2021, doi: 10.1007/s00376-021-1257-x
China experienced worsening ground-level ozone (O3) pollution from 2013 to 2019. In this study, meteorological parameters, including surface temperature (T2), solar radiation (SW), and wind speed (WS), were classified into two aspects, (1) Photochemical Reaction Condition (PRC = T2 × SW) and (2) Physical Dispersion Capacity (PDC = WS). In this way, a Meteorology Synthetic Index (MSI = PRC/PDC) was developed for the quantification of meteorology-induced ground-level O3 pollution. The positive linear relationship between the 90th percentile of MDA8 (maximum daily 8-h average) O3 concentration and MSI determined that the contribution of meteorological changes to ground-level O­3 varied on a latitudinal gradient, decreasing from ~40% in southern China to 10%–20% in northern China. Favorable photochemical reaction conditions were more important for ground-level O3 pollution. This study proposes a universally applicable index for fast diagnosis of meteorological roles in ground-level O3 variability, which enables the assessment of the observed effects of precursor emissions reductions that can be used for designing future control policies.
The Variability of Air-sea O2 Flux in CMIP6: Implications for Estimating Terrestrial and Oceanic Carbon Sinks
Changyu LI, Jianping HUANG, Lei DING, Yu REN, Linli AN, Xiaoyue LIU, Jiping HUANG
, Available online   , Manuscript accepted  08 October 2021, doi: 10.1007/s00376-021-1273-x
The measurement of atmospheric O2 concentrations and related oxygen budget have been used to estimate terrestrial and oceanic carbon uptake. However, a discrepancy remains in assessments of O2 exchange between ocean and atmosphere (i.e. air-sea O2 flux), which is one of the major contributors to uncertainties in the O2-based estimations of the carbon uptake. Here, we explore the variability of air-sea O2 flux with the use of outputs from Coupled Model Intercomparison Project phase 6 (CMIP6). The simulated air-sea O2 flux exhibits an obvious warming-induced upward trend (~1.49 Tmol yr−2) since the mid-1980s, accompanied by a strong decadal variability dominated by oceanic climate modes. We subsequently revise the O2-based carbon uptakes in response to this changing air-sea O2 flux. Our results show that, for the 1990--2000 period, the averaged net ocean and land sinks are 2.10±0.43 and 1.14±0.52 GtC yr−1 respectively, overall consistent with estimates derived by the Global Carbon Project (GCP). An enhanced carbon uptake is found in both land and ocean after year 2000, reflecting the modification of carbon cycle under human activities. Results derived from CMIP5 simulations also investigated in the study allow for comparisons from which we can see the vital importance of oxygen dataset on carbon uptake estimations.
Detection and Attribution of Changes in Thermal Discomfort over China during 1961−2014 and Future Projections
Wanling LI, Xin HAO, Li WANG, Yuqing LI, Jiandong LI, Huixin LI, Tingting HAN
, Available online   , Manuscript accepted  26 September 2021, doi: 10.1007/s00376-021-1168-x
In this study, variation in the frequency of thermal discomfort days over China during the period of 1961−2014, including heat discomfort days (HDDs) and cold discomfort days (CDDs), and the influence of external forcings on it are discussed. HDDs are the conditions of overheating and overhumidity (represented by humidity index), and CDDs reflect the dangers from cold temperatures and winds (represented by wind chill index). Observations show significant increases (decreases) in the frequency of HDDs (CDDs) over China from 1961 to 2014, with clear regional distinctions. The historical ALL and greenhouse gas (GHG) simulations can sufficiently reproduce the spatial patterns of the observational trend in the frequency of both HDDs and CDDs over China. Further, the impacts of GHG and anthropogenic forcings on the HDDs (CDDs) are detectable over China, except for central and eastern China, based on the optimal fingerprinting method. GHG forcing is identified as a dominant factor for the observational changes in the frequency of HDDs over southern China; GHG and anthropogenic forcings have dominant effects on the variation in the frequency of CDDs over southwestern China. Although trends in the frequency of HDDs and CDDs in historical aerosol forcing simulations seems to be opposite to observations, an aerosol signal fails to be detected. Natural forcing contributes to the observational variation in the frequency of HDDs over northwestern China. In addition, the future projections of thermal discomfort days indicate that Chinese residents will face more threats of heat discomfort and fewer threats of cold discomfort in the future under global warming.
Seasonal Prediction of Summer Precipitation over East Africa using NUIST-CFS1.0
Temesgen Gebremariam ASFAW, Jing-Jia LUO
, Available online   , Manuscript accepted  22 September 2021, doi: 10.1007/s00376-021-1180-1
East Africa is particularly vulnerable to precipitation variability, as the livelihood of much of the population depends on rainfed agriculture. Seasonal forecasts of the precipitation anomalies, when skillful, can therefore improve implementation of coping mechanisms with respect to food security and water management. This study assesses the performance of Nanjing University of Information Science and Technology Climate Forecast System version 1.0 (NUIST-CFS1.0) on forecasting June–September (JJAS) seasonal precipitation anomalies over East Africa. The skill in predicting the JJAS mean precipitation initiated from 1 May for the period of 1982–2019 is evaluated using both deterministic and probabilistic verification metrics on grid cell and over six distinct clusters. The results show that NUIST-CFS1.0 captures the spatial pattern of observed seasonal precipitation climatology, albeit with dry and wet biases in a few parts of the region. The model has positive skill across a majority of Ethiopia, Kenya, Uganda, and Tanzania, whereas it doesn’t exceed the skill of climatological forecasts in parts of Sudan and southeastern Ethiopia. Positive forecast skill is found over regions where the model shows better performance in reproducing teleconnections related to oceanic SST. The prediction performance of NUIST-CFS1.0 is found to be on a level that is potentially useful over a majority of East Africa.
The Linkage between Midwinter Suppression of the 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 a strengthened East Asian winter monsoon. The intensified East Asian trough and atmospheric blocking in the North Pacific as well as the significantly negative low-level air temperature anomalies, lying upstream of the MNPST, are expected to be distinctly associated with the 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, weakened 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 mechanism associated with the MWS.
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 the terrestrial vegetation distribution and carbon fluxes to global warming by using the new dynamic global vegetation model in the second version of the Chinese Academy of Sciences (CAS) Earth System Model (CAS-ESM2). We conducted two sets of simulations, a present-day simulation and a future simulation, which were forced by the present-day climate during 1981–2000 and the future climate during 2081–2100, respectively, as derived from RCP8.5 outputs in CMIP5. CO2 concentration is kept constant in all simulations to isolate CO2-fertilization effects. The results show an overall increase in vegetation coverage in response to global warming, which is the net result of the greening in the mid-high latitudes and the browning in the tropics. The results also show an enhancement in carbon fluxes in response to global warming, including gross primary productivity, net primary productivity, and autotrophic respiration. We found that the changes in vegetation coverage were significantly correlated with changes in surface air temperature, reflecting the dominant role of temperature, while the changes in carbon fluxes were caused by the combined effects of leaf area index, temperature, and precipitation. This study applies the CAS-ESM2 to investigate the response of terrestrial ecosystems to climate warming. Even though the interpretation of the results is limited by isolating CO2-fertilization effects, this application is still beneficial for adding to our understanding of vegetation processes and to further improve upon model parameterizations.
Calculating the Climatology and Anomalies of Surface Cloud Radiative Effect Using Cloud Property Histograms and Cloud Radiative Kernels
Chen ZHOU, Yincheng LIU, Quan WANG
, Available online   , Manuscript accepted  08 September 2021, doi: 10.1007/s00376-021-1166-z
Cloud radiative kernels (CRK) built with radiative transfer models have been widely used to analyze the cloud radiative effect on top of atmosphere (TOA) fluxes, and it is expected that the CRKs would also be useful in the analyses of surface radiative fluxes, which determines the regional surface temperature change and variability. In this study, CRKs at the surface and TOA were built using the Rapid Radiative Transfer Model (RRTM). Longwave cloud radiative effect (CRE) at the surface is primarily driven by cloud base properties, while TOA CRE is primarily decided by cloud top properties. For this reason, the standard version of surface CRK is a function of latitude, longitude, month, cloud optical thickness (τ) and cloud base pressure (CBP), and the TOA CRK is a function of latitude, longitude, month, τ and cloud top pressure (CTP). Considering that the cloud property histograms provided by climate models are functions of CTP instead of CBP at present, the surface CRKs on CBP-τ histograms were converted to CTP-τ fields using the statistical relationship between CTP, CBP and τ obtained from collocated CloudSat and MODIS observations. For both climate model outputs and satellites observations, the climatology of surface CRE and cloud-induced surface radiative anomalies calculated with the surface CRKs and cloud property histograms are well correlated with those calculated from surface radiative fluxes. The cloud-induced surface radiative anomalies reproduced by surface CRKs and MODIS cloud property histograms are not affected by spurious trends that appear in Clouds and the Earth's Radiant Energy System (CERES) surface irradiances products.
Multiscale Combined Action and Disturbance Characteristics of Pre-summer Extreme Precipitation Events over South China
Hongbo LIU, Ruojing YAN, Bin WANG, Guanghua CHEN, Jian LING, Shenming FU
, Available online   , Manuscript accepted  08 September 2021, doi: 10.1007/s00376-021-1172-1
The dominant frequency modes of pre-summer extreme precipitation events (EPEs) over South China (SC) between 1998 and 2018 were investigated. The 67 identified EPEs were all characterized by the 3–8-d (synoptic) frequency band. However, multiscale combined modes of the synoptic and three low-frequency bands (10 20-d (quasi-biweekly, QBW); 15–40-d (quasi-monthly, QM); and 20–60-d (intraseasonal)) accounted for the majority (63%) of the EPEs, and the precipitation intensity on the peak wet day was larger than that of the single synoptic mode. It was found that EPEs form within strong southwesterly anomalous flows characterized by either lower-level cyclonic circulation over SC or a deep trough over eastern China. Bandpass-filtered disturbances revealed the direct precipitating systems and their life cycles. Synoptic-scale disturbances are dominated by mid–high latitude troughs, and the cyclonic anomalies originate from downstream of the Tibetan Plateau (TP). Given the warm and moist climate state, synoptic-scale northeasterly flows can even induce EPEs. At the QBW and QM scales, the disturbances originate from the tropical Pacific, downstream of the TP, or mid–high latitudes (QBW only). Each is characterized by cyclonic–anticyclonic wave trains and intense southwesterly flows between them within a region of large horizontal pressure gradient. The intraseasonal disturbances are confined to tropical regions and influence SC by marginal southwesterly flows. It is concluded that low-frequency disturbances provide favorable background conditions for EPEs over SC and synoptic-scale disturbances ultimately induce EPEs on the peak wet days. Both should be simultaneously considered for EPE predictions over SC.
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 temperatures. In this study, the distinct features of these two supercold waves are analyzed 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 and was not confined only to the Ural Mountains as it was for the earlier cold waves; (2) the collocation of the Asian blocking pattern and the polar vortex deflection towards East Asia preferred the hyperpolar path of cold air accumulation and the subsequent southward outburst; and (3) both high- and low-frequency processes worked in concert, leading to the very intense cold waves. The cold air advance along the northwest path, which coincides with the southeastward intrusion of the Siberian High (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 the 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 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 Zangbo River Basin since the Last Glacial Maximum
Shuang LIU, Kaiheng HU, Weiming LIU, Paul A. CARLING
, Available online   , Manuscript accepted  01 September 2021, doi: 10.1007/s00376-021-1150-7
Global 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 caused by climate changes over a geological timescale due to the impossibility of direct observations. In this study, we examine the hydro-climatic variation of the Yarlung Zangbo River Basin in the Tibet Plateau since the Last Glacial Maximum (LGM) by combining δ18O proxy records in Indian and Omani caves with the simulated Indian summer monsoon, surface temperature, precipitation, evapotranspiration and runoff via the Community Climate System Model and the reconstructed glacier coverage via the Parallel Ice Sheet Model. The mean river runoff was kept at 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 such a small contribution of precipitation to runoff and the 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 area 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 have accelerated widespread ice-dam breaches and led to extreme outburst flood events. Combining local geological proxy records and regional simulations could be a useful approach for the study of paleo-hydrologic variations in big river basins.
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 (2 km-and-10 min) data from the Advanced Himawari Imager onboard the Himawari-8 satellite, this study documents the fine-scale characteristics of daytime cloud regimes (CRs) over coastal South China during the pre-summer rainy season (April–June). Six CRs (CR1–CR6) are identified based on the joint frequency distribution of cloud top brightness temperature and cloud optical thickness, namely, the optically thin-to-moderate cloud mixture, optically thin warm clouds with cirrus, optically thick warm clouds, weak convective cloud mixture, strong convective clouds, and extreme, deep convective clouds. The optically thick warm clouds are the major CR during April and May, with higher frequencies over land, especially along the urban agglomeration, rather than the offshore which may be an indicator of the higher aerosol concentrations being a contributing factor over the cities. The CRs with weak convective cloud mixtures and strong convective clouds appear more frequently over the land, while the two CRs with optically thinner clouds occur mainly offshore. Synoptic flow patterns (SPs) are objectively identified and examined focusing on those favoring the two major rain-producing CRs (CR5 and CR6) and the highly reflective CR with optically thick warm clouds (CR3). The two SPs favoring CR5 and CR6 are characterized by abundant moisture with low-level jets after monsoon onset, and a northwest high-southeast low pattern with strong dynamic convergence along the coastline, respectively. The non-convective CR3 with high reflectance is related to a SP that features the western North Pacific subtropical high extending more westward, leading to a moderate moisture supply and a wide range of convective available potential energy, but also, large convective inhibition.
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 southern 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 a 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 experiment 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 a 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 adaptive mesh refinement (AMR) algorithm. The 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 the 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, the MCV scheme is implemented on a height-based, terrain-following grid with variable resolution to solve the nonhydrostatic governing equations of atmospheric dynamics. The AMR grid of Berger-Oliger consists of several groups of blocks with different resolutions, where the MCV model developed on a 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 for improving computational efficiency without losing accuracy.
Diagnosing SST Error Growth during ENSO Developing Phase in the BCC_CSM1.1(m) Prediction System
Ben TIAN, Hong-Li 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 perspective 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 to the decay phase. During 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 skill 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 evolutionary process similar to that of El Niño decay and the transition to the La Niña growth phase. Meanwhile, for short lead-time ENSO predictions, the initial wind errors begin to play an increasing role, particularly in linking with the subsurface heat content errors in the central-western Pacific. By comparing the multiple samples of initial fields in the model, it is clearly found that poor SST predictions of the Niño-3.4 region are largely due to contributions of the initial errors in certain specific locations in the tropical Pacific. This demonstrates that those sensitive areas for initial fields in ENSO prediction are fairly consistent in both previous ideal experiments and our operational predictions, indicating the need for targeted observations to further improve operational forecasts of ENSO.
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 years of ERA5 and CMORPH rainfall data, two main CMBLJs are found, one on each side of Hainan Island (named BLJ-WEST and BLJ-EAST), which are always strengthened jointly. Both CMBLJs often occur in the pre-summer rainy season and exhibit an 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 convergence at the terminus of each CMBLJ, terrain-induced lifting, and relevant atmospheric stratification. Coastal rainfall downstream of the BLJ-WEST is much weaker than that downstream of the BLJ-EAST because of higher CIN over the Beibu Gulf, which is caused by lower temperature lapse rates and adiabatic heating in the lee of the Annamite Range. The inland rainfall increases along with CMBLJ 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 cold and dry air from the Indochina Peninsula. Therefore, anomalous dynamic lifting, moisture flux convergence, and CAPE/CIN associated with CMBLJ 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 are less than 2 s for the same detection point in the FDA. The SAS can maintain DTD < 2 s for an expanded seven-front-end AWR. With the smallest DTD, gridded wind fields are derived from AWR data, by sampling of the interpolated grid, onto a rectangular grid of 100 m×100 m×100 m at a 12 s temporal resolution in the FDA. The first X-band single-polarized three-front-end AWR was deployed in field experiments in 2018 at Huanghua International Airport, China. Having completed the data synthesis and processing, the preliminary observation results of the first AWR are described herein.
The Impact of Moist Physics on the Sensitive Area Identification for Heavy Rainfall Associated Weather Systems
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 areas 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.
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.
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.
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.
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.
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.
Meeting Summary
The IAMAS-CNC Early Career Scientists Nobel Prize Online Interpretation Workshop
Jing Li, Zhongjing Jiang, Yueming Dong, Lu Zhang, Tong Ying, Zhenyu Zhang, Mu Mu
, Available online   , Manuscript accepted  22 December 2021, doi: 10.1007/s00376-021-1455-6
The 16th Workshop on Antarctic Meteorology and Climate and 6th Year of Polar Prediction in the Southern Hemisphere Meeting
David H. BROMWICH, Matthew A. LAZZARA, Arthur M. CAYETTE, Jordan G. POWERS, Kirstin WERNER, John J. CASSANO, Steven R. COLWELL, Scott CARPENTIER, Xun ZOU
, Available online   , Manuscript accepted  15 November 2021, doi: 10.1007/s00376-021-1384-4
The Chinese Carbon-Neutral Goal: Challenges and Prospects
Ning ZENG, Kejun JIANG, Pengfei HAN, Zeke HAUSFATHER, Junji CAO, Daniel KIRK-DAVIDOFF, Shaukat ALI, Sheng ZHOU
, Available online   , Manuscript accepted  21 December 2021, doi: 10.1007/s00376-021-1313-6
On 22 September 2020, within the backdrop of the COVID-19 global pandemic, China announced its climate goal for peak carbon emissions before 2030 and to reach carbon neutrality before 2060. This carbon-neutral goal is generally considered to cover all anthropogenic greenhouse gases. The planning effort is now in full swing in China, but the pathway to decarbonization is unclear. The needed transition towards non-fossil fuel energy and its impact on China and the world may be more profound than its reform and development over the past 40 years, but the challenges are enormous. Analysis of four representative scenarios shows significant differences in achieving the carbon-neutral goal, particularly the contribution of non-fossil fuel energy sources. The high target values for nuclear, wind, and bioenergy have approached their corresponding resource limitations, with solar energy being the exception, suggesting solar's critical role. We also found that the near-term policies that allow for a gradual transition, followed by more drastic changes after 2030, can eventually reach the carbon-neutral goal and lead to less of a reduction in cumulative emissions, thus inconsistent with the IPCC 1.5°C scenario. The challenges and prospects are discussed in the historical context of China's socio-economic reform, globalization, international collaboration, and development.
Urbanization Impact on Regional Climate and Extreme Weather: Current Understanding, Uncertainties, and Future Research Directions
Yun QIAN, TC CHAKRABORTY, Jianfeng LI, Dan LI, Cenlin HE, Chandan SARANGI, Fei CHEN, Xuchao YANG, L. Ruby LEUNG
, Available online   , Manuscript accepted  06 December 2021, doi: 10.1007/s00376-021-1371-9
Urban environments lie at the confluence of social, cultural, and economic activities and have unique biophysical characteristics due to continued infrastructure development that generally replaces natural landscapes with built-up structures. The vast majority of studies on urban perturbation of local weather and climate have been centered on the urban heat island (UHI) effect, referring to the higher temperature in cities compared to their natural surroundings. Besides the UHI effect and heat waves, urbanization also impacts atmospheric moisture, wind, boundary layer structure, cloud formation, dispersion of air pollutants, precipitation, and storms. In this review article, we first introduce the datasets and methods used in studying urban areas and their impacts through both observation and modeling and then summarize the scientific insights on the impact of urbanization on various aspects of regional climate and extreme weather based on more than 500 studies. We also highlight the major research gaps and challenges in our understanding of the impacts of urbanization and provide our perspective and recommendations for future research priorities and directions.
A Concise Overview on Solar Resource Assessment and Forecasting
Dazhi YANG, Wenting WANG, Xiang'ao XIA
, Available online   , Manuscript accepted  18 November 2021, doi: 10.1007/s00376-021-1372-8
China’s recently announced directive on tackling climate change, namely, to reach carbon peak by 2030 and to achieve carbon neutrality by 2060, has led to an unprecedented nationwide response among the academia and industry. Under such a directive, a rapid increase in the grid penetration rate of solar in the near future can be fully anticipated. Although solar radiation is an atmospheric process, its utilization, as to produce electricity, has hitherto been handled by engineers. In that, it is thought important to bridge the two fields, atmospheric sciences and solar engineering, for the common good of carbon neutrality. In this überreview, all major aspects pertaining to solar resource assessment and forecasting are discussed in brief. Given the size of the topic at hand, instead of presenting technical details, which would be overly lengthy and repetitive, the overarching goal of this review is to comprehensively compile a catalog of some recent, and some not so recent, review papers, so that the interested readers can explore the details on their own.
News & Views (invited)
Contributions of Fuqing Zhang to Predictability, Data Assimilation, and Dynamics of High Impact Weather: A Tribute
Zhiyong Meng, Eugene Clothiaux
, Available online   , Manuscript accepted  26 November 2021, doi: 10.1007/s00376-021-1362-x
This article reviews Fuqing Zhang’s contributions to mesoscale atmospheric science from research to mentoring to academic service over his 20-year career. His fundamental scientific contributions are highlighted on predictability, data assimilation, and dynamics of high impact weather, especially gravity wave and tropical cyclones. His extremely generous efforts in efficiently transmitting to the community the new scientific knowledge and ideas, through mentoring, interacting, workshop organizing, and reviewing are summarized. Special appreciations were given to his tremendous contributions to the development of mesoscale meteorology in China and the education of Chinese graduate students and young scientists.
Letter and Notes
Strengthened Regulation of the Northwest Indian Ocean Warming on the South China Sea Summer Monsoon Onset in the Past Decade
Yang Ai, Ning Jiang, Weihong Qian, Jeremy Cheuk-Hin LEUNG, Yanying Chen
, Available online   , Manuscript accepted  24 November 2021, doi: 10.1007/s00376-021-1364-8
Traditionally, a late (early) onset of the South China Sea (SCS) summer monsoon (SCSSM) is mostly following a warm (cold) El Niño-Southern Oscillation (ENSO) event in winter, supporting the high seasonal predictability of SCSSM onset. However, the empirical seasonal forecast skill of SCSSM onset based on ENSO has decreased since 2010. Meanwhile, unexpected postponed SCSSM onset is also observed in the past decade. We attribute these changes to the Northwest Indian Ocean (NWIO) warming. The NWIO warming has (1) suppressed the seasonal convection over the SCS, which weakens the impacts of ENSO on SCSSM onset and postponed the start of SCSSM; and (2) favored more high frequency convection activities, which enlarges its seasonal prediction uncertainties. Our present results propose a possible clue for the impacts of global warming on the seasonal forecast via uneven ocean warming.
Notes & Letters
Observations of Dynamic Turbulence in the Lower Stratosphere over Inner Mongolia Using a High-resolution Balloon Sensor Constant Temperature Anemometer
Xiaoyu REN, Yi LIU, Zhaonan CAI, Yuli ZHANG
, Available online   , Manuscript accepted  26 October 2021, doi: 10.1007/s00376-021-1233-5
We present characterizations of the dynamic turbulence in the lower stratosphere measured by a new balloon-based system designed for detecting finer scale dynamic turbulence. The balloon-based system included a constant temperature anemometer (CTA) operating at a sampling rate of 2 kHz at an ascent speed of 5 m s−1 (corresponding to a vertical resolution of 2.5 mm), an industrial personal computer, batteries, sensors for ambient temperature and humidity, an A/D converter, and others. The system was successfully launched to 24 km altitude over Bayannur City, Inner Mongolia Province. Results show the spatial intermittence of the turbulence layers, with clear boundaries between turbulent and non-turbulent regions. This is the first time that the dynamic turbulence spectrum down to the viscous sub-range has been obtained throughout the lower stratosphere over China. With that, the energy dissipation rates of dynamic turbulence could be calculated with high precision. The profile of the dissipation rates varied from 7.37 × 10−7 to 4.23 W kg−1 and increased with altitude in the stratosphere.
A New Method of Significance Testing for Correlation-Coefficient Fields and Its Application
Xiaojuan SUN, Siyan LI, Julian X. L WANG, Panxing WANG, Dong GUO
, 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 is based 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.
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.