Impact Factor: 3.158

Dec.  2021

Article Contents

# Magnitude, Scale, and Dynamics of the 2020 Mei-yu Rains and Floods over China

Fund Project:

AV, MM, RS, AGT and NPK were supported by the COSMIC project through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund, contract number P106301. NPK was supported by a Natural Environmental Research Council (NERC) Independent Research Fellowship (NE/L010976/1) and by the ACREW programme of the National Centre for Atmospheric Science. We thank Omar V. MÜLLER for help with GloFAS-ERA5

• Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/1999. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with a halted northward progression of the EASMF and prolonged mei-yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12−16 June and 4−8 July 2020) are selected from this period for dynamical characterization, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualizes heavy precipitation in Asia in summer 2020 and showcases several analysis tools developed by the authors for the study of such events.
摘要: 2020年初夏，东亚和南亚许多地区均受到暴雨和洪水的强烈影响。本研究给出此次影响长江流域强降水和洪水的统计学和动力学特征。通过对亚洲各大流域日降水和月降水资料的分析表明，长江流域是受此次强降水影响最为明显的区域之一。2020年6-7月长江流域降水为过去20年间最强，且强降水覆盖了流域内绝大多数地区。长江流量为1998/1999年以来最大。利用一种可以自动识别日尺度东亚夏季风锋的方法，分析东亚夏季风锋与强降水之间的关系，结果表明长江流域降水异常偏多与东亚夏季风锋北抬过程停滞、长江流域上空持续超过一个月的梅雨环流背景相关。选取两个持续5天的强降水过程（2020年6月12-16日和7月4-8日），利用拉格朗日轨迹追踪法，分析降水中动力过程，重点关注与东亚夏季风锋期气流辐合相关的动力特征。在两段降水过程中，季风气流和副热带气流的辐合均与强降水相伴出现；但高层环流型差异明显，这极大影响了东亚夏季风锋附近水平平流的平衡。本研究以2020年夏季亚洲强降水为例，展示了作者发展的一系列分析方法在此类强降水事件中的应用方法和前景。
• Figure 1.  Basin-average Asian precipitation anomalies for June and July 2020, for basin areas in the interval 20 000–200 000 km2. Anomalies computed relative to the 2000–19 mean from IMERG. The YRB outline is shown in red.

Figure 2.  (a) and (b) show June and July IMERG precipitation over the YRB, and (c) shows the June and July ERA5 precipitation. (a) and (c) show monthly mean total YRB precipitation and (b) shows number of days any of the 32 YRB sub-basins (shown in Fig. 4) are in the 99th percentile. In (a) and (c), the mean (solid black) and ±σ (dashed black) of June–July precipitation are also shown. (d) shows the intraseasonal variability of the YRB daily IMERG precipitation for 2020, and 5-day moving means of total YRB precipitation (dashed black), upper YRB precipitation (sub-basins west of 108°E, yellow line) and lower YRB precipitation (sub-basins east of 108°E, blue line). Also shown are the two event windows (black vertical lines).

Figure 3.  ERA5-GloFAS (Harrigan et al., 2020) Yangtze River discharge. (a) shows the monthly mean discharge for JJA, (b) and (c) show the discharge and cumulative discharge over each year respectively, and (d) shows the flow duration curve for each year.

Figure 4.  June and July 2020 monthly precipitation anomalies (a and b) and return times (c and d) for 32 YRB sub-basins. The main course of the Yangtze River (thick black line) and the largest tributaries (thin black lines) are shown.

Figure 5.  Dekadal (10-day) means of June–July 2020 IMERG daily precipitation (shading, mm). The magenta line indicates the ERA5 mean dekadal EASMF location at 850 hPa in 2020, while the yellow line refers to the 1979−2018 ERA5 dekadal EASMF climatology.

Figure 6.  Maps of (a) total event precipitation (shading, mm), (b) geopotential height at 500 hPa (shading, dam), wind vectors at 850 hPa (arrows, m s−1), potential vorticity at 250 hPa (blue contour, 2 PVU) and mean sea level pressure (black contour, 1015 hPa), (c) 500−1000 hPa IVT (shading, kg m−1 s−1) and IWV (brown, red and orange lines indicating 52, 58 and 64 kg m−2, respectively), wind vectors at 850 hPa (arrows, m s−1) and wind speed at 200 hPa (green contours, every 10 m s−1 from 30 m s−1), (d) anomalies at 850 hPa of θe (shading, K) and wind vectors (arrows, m s−1). The magenta line indicates the mean 5-day location of the EASMF at 850 hPa. All quantities in panels (b)−(d) are averaged over Event 1 (12−16 June 2020) and the anomalies in panel (d) are evaluated against 1979−2018 climatology for the same days. All data from ERA5, apart from precipitation (IMERG).

Figure 7.  Cross-sections, transect AB in Fig. 6, of moisture flux (shading, kg kg−1 m s−1), wind speed (green contours, every 7.5 m s−1 up from 25 m s−1), equivalent potential temperature (red contours, every 5 K), wind vectors (arrows, m s−1, computed using the horizontal wind parallel to the section as horizontal component and the vertical velocity multiplied by 200 as vertical component, consistent with the aspect ratio), relative humidity (black dashed contour, 80%), potential vorticity (blue dashed contour, 2 PVU) and daily precipitation (cyan bars, mm). The magenta “x” indicates the location of the EASMF at 850 hPa according to the detection algorithm. All quantities are averaged over Event 1 (12−16 June 2020). All data from ERA5, apart from precipitation (IMERG).

Figure 8.  As in Fig. 6 but for Event 2 (4−8 July 2020).

Figure 9.  As in Fig. 7 but for Event 2 (4−8 July 2020).

Figure 10.  (a, b): Lagrangian trajectories, starting at 00 UTC on (a) 15 June 2020, (b) 7 July 2020 and computed backward for 144 hours. The trajectories’ starting points are selected by taking the 25 max and min θe points (red and blue dots, respectively) between 700−850 hPa in the selection domain (black box) at start time.The selection domain is chosen as 32°−34.5°N, 117.5°−122.5°E for Event 1 and 29.5°−32°N,115°−120°E for Event 2, consistent with EASMF location on the respective dates. Colour shading indicates the equivalent potential temperature of trajectories at each position. (c, d): Time profiles of pressure for the trajectories of (c) Event 1 and (d) Event 2, as shown in (a) and (b), respectively. Colour shading indicates specific humidity. Data from ERA5, regridded to a 0.5°-horizontal grid.

•  [1] ZHANG Yuanchun, SUN Jianhua*, and FU Shenming, 2014: Impacts of Diurnal Variation of Mountain-plain Solenoid Circulations on Precipitation and Vortices East of the Tibetan Plateau during the Mei-yu Season, ADVANCES IN ATMOSPHERIC SCIENCES, 31, 139-153.  doi: 10.1007/s00376-013-2052-0 [2] Ting WANG, Ke WEI, Jiao MA, 2021: Atmospheric Rivers and Mei-yu Rainfall in China: A Case Study of Summer 2020, ADVANCES IN ATMOSPHERIC SCIENCES, 38, 2137-2152.  doi: 10.1007/s00376-021-1096-9 [3] Xinyu LI, Riyu LU, 2018: Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the Tropical Western North Pacific during Summer, ADVANCES IN ATMOSPHERIC SCIENCES, 35, 1231-1242.  doi: 10.1007/s00376-018-7304-6 [4] Robin T. CLARK, Peili WU, Lixia ZHANG, Chaofan LI, 2021: The Anomalous Mei-yu Rainfall of Summer 2020 from a Circulation Clustering Perspective: Current and Possible Future Prevalence, ADVANCES IN ATMOSPHERIC SCIENCES, 38, 2010-2022.  doi: 10.1007/s00376-021-1086-y [5] Gill M. MARTIN, Nick J. DUNSTONE, Adam A. SCAIFE, Philip E. BETT, 2020: Predicting June Mean Rainfall in the Middle/Lower Yangtze River Basin, ADVANCES IN ATMOSPHERIC SCIENCES, 37, 29-41.  doi: 10.1007/s00376-019-9051-8 [6] Philip E. BETT, Gill M. MARTIN, Nick DUNSTONE, Adam A. SCAIFE, Hazel E. THORNTON, Chaofan LI, 2021: Seasonal Rainfall Forecasts for the Yangtze River Basin in the Extreme Summer of 2020, ADVANCES IN ATMOSPHERIC SCIENCES, 38, 2212-2220.  doi: 10.1007/s00376-021-1087-x [7] Xinyu LI, Riyu LU, 2021: Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s, ADVANCES IN ATMOSPHERIC SCIENCES, 38, 1823-1834.  doi: 10.1007/s00376-021-1051-9 [8] Kelvin S. NG, Gregor C. LECKEBUSCH, Kevin I. HODGES, 2022: A Causality-guided Statistical Approach for Modeling Extreme Mei-yu Rainfall Based on Known Large-scale Modes—A Pilot Study, ADVANCES IN ATMOSPHERIC SCIENCES.  doi: 10.1007/s00376-022-1348-3 [9] LIU Run, LIU Shaw Chen, Ralph J. CICERONE, SHIU Chein-Jung, LI Jun, WANG Jingli, ZHANG Yuanhang, 2015: Trends of Extreme Precipitation in Eastern China and Their Possible Causes, ADVANCES IN ATMOSPHERIC SCIENCES, 32, 1027-1037.  doi: 10.1007/s00376-015- 5002-1 [10] Dan WANG, Aihui WANG, Lianlian XU, Xianghui KONG, 2020: The Linkage between Two Types of El Niño Events and Summer Streamflow over the Yellow and Yangtze River Basins, ADVANCES IN ATMOSPHERIC SCIENCES, 37, 160-172.  doi: 10.1007/s00376-019-9049-2 [11] NING Liang, QIAN Yongfu, 2009: Interdecadal Change in Extreme Precipitation over South China and Its Mechanism, ADVANCES IN ATMOSPHERIC SCIENCES, 26, 109-118.  doi: 10.1007/s00376-009-0109-x [12] WANG Yi, YAN Zhongwei, 2011: Changes of Frequency of Summer Precipitation Extremes over the Yangtze River in Association with Large-scale Oceanic-atmospheric Conditions, ADVANCES IN ATMOSPHERIC SCIENCES, 28, 1118-1128.  doi: 10.1007/s00376-010-0128-7 [13] DING Yuguo, CHENG Bingyan, JIANG Zhihong, 2008: A Newly-Discovered GPD-GEV Relationship Together with Comparing Their Models of Extreme Precipitation in Summer, ADVANCES IN ATMOSPHERIC SCIENCES, 25, 507-516.  doi: 10.1007/s00376-008-0507-5 [14] Yongguang ZHENG, Yanduo GONG, Jiong CHEN, Fuyou TIAN, 2019: Warm-Season Diurnal Variations of Total, Stratiform, Convective, and Extreme Hourly Precipitation over Central and Eastern China, ADVANCES IN ATMOSPHERIC SCIENCES, 36, 143-159.  doi: 10.1007/s00376-018-7307-3 [15] FENG Jinming, WEI Ting, DONG Wenjie, WU Qizhong, and WANG Yongli, 2014: CMIP5/AMIP GCM Simulations of East Asian Summer Monsoon, ADVANCES IN ATMOSPHERIC SCIENCES, 31, 836-850.  doi: 10.1007/s00376-013-3131-y [16] LI Hongmei, FENG Lei, ZHOU Tianjun, 2011: Multi-model Projection of July--August Climate Extreme Changes over China under CO$_{2}$ Doubling. Part I: Precipitation, ADVANCES IN ATMOSPHERIC SCIENCES, 28, 433-447.  doi: 10.1007/s00376-010-0013-4 [17] SU Tonghua, XUE Feng*, ZHANG He, 2014: Simulating the Intraseasonal Variation of the East Asian Summer Monsoon by IAP AGCM4.0, ADVANCES IN ATMOSPHERIC SCIENCES, 31, 570-580.  doi: 10.1007/s00376-013-3029-8 [18] Yuan WANG, 2015: Air Pollution or Global Warming: Attribution of Extreme Precipitation Changes in Eastern China——Comments on "Trends of Extreme Precipitation in Eastern China and Their Possible Causes", ADVANCES IN ATMOSPHERIC SCIENCES, 32, 1444-1446.  doi: 10.1007/s00376-015-5109-4 [19] FENG Juan*, CHEN Wen, 2014: Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases, ADVANCES IN ATMOSPHERIC SCIENCES, 31, 344-354.  doi: 10.1007/s00376-013-3118-8 [20] MAN Wenmin, and ZHOU Tianjun, 2014: Regional-scale Surface Air Temperature and East Asian Summer Monsoon Changes during the Last Millennium Simulated by the FGOALS-gl Climate System Model, ADVANCES IN ATMOSPHERIC SCIENCES, 31, 765-778.  doi: 10.1007/s00376-013-3123-y

Export:

## Manuscript History

Manuscript revised: 30 June 2021
Manuscript accepted: 13 July 2021
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Magnitude, Scale, and Dynamics of the 2020 Mei-yu Rains and Floods over China

###### Corresponding author: Ambrogio VOLONTÉ, a.volonte@reading.ac.uk;
• 1. Department of Meteorology, University of Reading, Reading, RG6 6ES, UK
• 2. National Centre for Atmospheric Science, University of Reading, Reading, RG6 6ES, UK

Abstract: Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/1999. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with a halted northward progression of the EASMF and prolonged mei-yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12−16 June and 4−8 July 2020) are selected from this period for dynamical characterization, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualizes heavy precipitation in Asia in summer 2020 and showcases several analysis tools developed by the authors for the study of such events.

Reference

/