Analysis on the characteristics and causes of intraseasonal differences of the continuous rainfall in Hanjiang River Basin during the summer and autumn in 2021

doi:10.11755/j.issn.1006-7639(2024)-04-0563

Abstract

Abstract:

Hanjiang River Basin is an important water source area in China, and studying its precipitation characteristics is of great significance for flood prevention and drought resistance. Based on the daily precipitation data of 62 stations in the Hanjiang River and NCEP/NCAR reanalysis data, the intraseasonal variation characteristics of the precipitation over the Hanjiang River from August to October in 2021 and its relationship with atmospheric circulation and sea surface temperature were studied by using percentile, correlation methods and T-N wave activity flux. The results show that the record-breaking precipitation in the Hanjiang River during this period occurred in the upper reaches of the basin, characterized by extreme intensity and large total precipitation. Precipitation was above normal in both the summer and autumn periods, but the rainy regions in autumn were positioned further to the north. In summer, the energy of Rossby waves dispersing eastward from the North Atlantic through Siberia maintained a “two-troughs-two-ridges” pattern over Eurasia, bringing strong cold air. Affected by the strengthening and westward extension of the subtropical high in the western Pacific, moist water vapors were transported to the north through the southwest and eastward water vapor channels. The old and warm air confronted and converged on the south side of the upper-level jet stream, resulting in abnormally high precipitation. In autumn, the energy from Rossby waves dispersing from the North Pacific maintained a “two-troughs-one-ridge” circulation pattern over Eurasia, with relatively weaker cold air. The breaking of the subtropical high in the western Pacific led to southern water vapor channel. The northward movement of the high-altitude jet stream caused the convergence of cold and warm air to rise northward, resulting in the northward movement of above-average precipitation. The abnormal precipitation during summer in 2021 in the Hanjiang River Basin was influenced by the positive anomalies in sea surface temperatures in the tropical eastern Atlantic, while the autumn was influenced by the cold sea surface temperatures in the central equatorial Pacific.

Key words: summer and autumn flood, intraseasonal differences, T-N wave activity flux, tropical sea surface temperature

摘要：

汉江流域是中国重要的调水水源区，研究其降水特征对防涝抗旱具有重要意义。基于汉江流域62个国家气象站降水资料及美国国家环境预报中心/国家大气研究中心（National Center for Environmental Prediction/National Center for Atmospheric Research，NCEP/NCAR）再分析资料，通过百分位数、相关分析和T-N波作用通量，探讨了2021年伏秋（8—10月）连汛期间汉江降水的季内差异特征及其与大气环流和海温的关系。结果表明： 2021年伏秋期间，汉江上游流域出现破纪录降水，极端性强、总量大。降水在伏夏和秋季两个时段均偏多，但秋季的多雨区位置更偏北。伏夏期间，北大西洋经西伯利亚向东频散的Rossby波使得欧亚上空维持“两槽两脊”，冷空气较强，同时西太平洋副热带高压（简称“副高”）强势西伸，通过西南和偏东两支通道向北输送暖湿水汽；冷暖空气在高空急流南侧对峙并辐合上升，导致降水异常偏多。秋季，北太平洋频散的Rossby波使得欧亚上空维持“两槽一脊”，冷空气较弱；副高断裂导致水汽通道偏南，高空急流北抬使冷暖空气辐合上升位置偏北，造成雨区偏北。2021年汉江流域伏夏降水异常受热带东大西洋海温正异常影响，秋季受赤道中太平洋冷海温影响。

关键词: 伏秋连汛, 季内差异, T-N波作用通量, 热带海温

CLC Number:

P467

XIAO Ying, GAO Yaqi, DU Liangmin, REN Yongjian. Analysis on the characteristics and causes of intraseasonal differences of the continuous rainfall in Hanjiang River Basin during the summer and autumn in 2021[J]. Journal of Arid Meteorology, 2024, 42(4): 563-575.

肖莺, 高雅琦, 杜良敏, 任永建. 汉江流域2021年伏秋连汛降水季内差异特征及成因分析[J]. 干旱气象, 2024, 42(4): 563-575.

Figures/Tables 15

Fig.1 The spatial distribution of representative meteorological stations in the Hanjiang River Basin （Thick solid line represents Hanjiang， the black triangles represent Danjiangkou and Zhongxiang from left to right， respectively）

Fig.2 The spatial distributions of precipitation anomaly percentage in the Hanjiang River Basin from August to October in 2021 （Unit：%）

Fig.3 The box diagram of accumulated precipitation (a) and accumulated number of extreme precipitation stations (b) in the upper reaches of the Hanjiang River from August to October during1961-2021 (The upper dashed line represents 1 standard deviation, and the lower represents -1 standard deviation)

Fig.4 Spatial distance similarity coefficients of two adjacent pentad precipitation in the upper reaches of the Hanjiang River from August to October in 2021 （The horizontal axis value of 44 represents the similarity coefficient between 44th and 43rd candidates， and so on）

Fig.5 The precipitation anomaly percentage during P1 (a) and P2 (b) in the upper reaches of the Hanjiang River in 2021 (Unit: %)

Fig.6 The 500 hPa geopotential height field (black isolines) and anomaly (the color shaded) during P1 (a) and P2 (b) period in 2021 (Unit: gpm) (The red lines indicate the climatic state greater than or equal to 5 880 gpm)

Fig.7 The spatial distribution of T-N wave activity flux (vectors, Unit: m2·s-2) and related stream function anomalies (the color shaded, Unit: 106 m2·s-1) during P1 (a) and P2 (b) period in 2021 (The red line indicates the upper reaches of the Hanjiang River. The same as below)

Fig.8 The distribution of vertically integrated water vapor flux (vectors, Unit: kg·m-1·s-1), water vapor flux divergence (color shaded, Unit: 10-5 kg·s-1·m-2) during P1 (a), P2 (b) period in 2021

Fig.9 The distribution of correlation coefficients between areal rainfall in the upper reaches of the Hanjiang River and the zonal wind at 200 hPa during P1 (a) and P2 (b) period, 1961-2021 (Shaded areas with climatological wind speed greater than or equal to 30 m·s-1, the dotted area passed the significance test of α=0.05)

Fig.10 Latitude-time cross sections of zonal wind at 200 hPa averaged over 80°E—140°E from August 1 to October 31， 2021 （Unit： m·s-1）（Red dotted lines represent climatological westerly jet axes）

Fig.11 Monthly variation of TEAI (a) and EMPI (b) from January, 2020 to October, 2021

Fig.12 The lead-lag correlation coefficients between TEAI (a), EMPI (b) with area rainfall during P1 and P2 period (The dash-dotted and long dashed lines indicate the significance test of α=0.05 and α=0.01, respectively)

Fig.13 The distribution of correlation coefficients between the average June TEAI and 500 hPa geopotential height field in P1 period during 1961-2021, the 500 hPa geopotential height field (black isolines) and its anomaly (color shaded) during P1period in 2021 (b, Unit: gpm) (The dotted area passed the significance test of α=0.05, the red lines represent the climate state greater than or equal to 5 880 gpm)

Fig.14 The correlation fields between the average July EMPI and 200 hPa zonal winds (a), 500 hPa geopotential heights (b) from 1961 to 2021, the correlation fields between geopotential height field in the key area with 500 hPa geopotential heights during P2 period in the North Pacific (color shaded) (c), 500 hPa height field (black isolines) and its anomalies (color shaded) from September 11 to October 31, 2021 (Unit: gpm) (d) (The dotted area passed the significance test of α=0.05, the red lines indicate the climatic state greater than or equal to 5 880 gpm)

Fig.15 Schematic diagram of atmosphere-ocean system conditions for the formation of summer (a) and autumn (b) flood of the Hanjiang River in 2021

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