Synoptic Analysis of an Extreme Autumn Rainstorm Process in Yunnan,China
Precipitation patterns in several regions of the globe are progressively evolving towards extremes.Governed by the convergence of East Asian monsoons and South Asian monsoons,as well as locally modulated by complex mountainous terrain in the Yun-Gui Plateau,extreme precipitation in Yunnan,China,exhibits high spat-temporal heterogeneity,with great differences in precipitation evolution across different regions.In previous research on extreme precipitation in Yunnan,meteorologists had paid more attention to extreme rainstorms in the context of large-scale with associated influence system,hardly explored the fine evolution patterns and their causal differences.This study employed hourly field observation data from regional automatic meteorological stations and high-resolution ERA-Interim(gridded at 0.125° x0.125° and a temporal resolution of 6 hours interval)reanalysis data to meticulously analyze a historic heavy rainstorm event occurred in Yunnan,China,on October 9,2015.This event etched in history as the most severe and far-reaching episode of extreme precipitation in Yunnan since records began in 1951.It conducted a diagnostic analysis of the intricate dynamics of extreme precipitation weather process,meteorological system evolution,the convergence of lower tropospheric moisture,the activities within front zones and shear lines,and topographic climate effect,to elucidate the differences in the causes of extreme rainstorm evolution at different stages and in different regions.(1)The extreme rainstorm on October 9,2015 was the result of the interaction between Bay of Bengal Low and cold front shear.The main body of water vapor transport was the southwest jet stream of Bay of Bengal,followed by the southerly airflow from Indochina Peninsula.The strong convergence of low-level water vapor in the western part of the cold front shear reached-3.0 × 10-7 g·cm-2·hPa-1·s-1;the meridional gradient in the western section of the frontal zone was-21 × 10-5 K·m-1;the interface of east-west wind was stably maintained at 99°E-100.5°E,and the north-south wind was confronted at 25°N-26°N.Compared with the January 9,2015 rainstorm process,which was the 2nd most similar influence system since 2000,the intensity of low-level water vapor transport and convergence,the intensity of warm moist air and the frontal zone of this extreme rainfall were significantly stronger,and the east-west winds and north-south winds confronted each other for a longer period of time.(2)Extreme rainstorms were moving rainy zone with a northwest-southeast orientation and a wide range of impacts.The rain zone was maintained for a longer period of time,which was formed by the strong convergence of cold and warm airflow in the western section of the shear,the intensification of water vapor transport from the southwestern jet to southwestern Yunnan,and the enhancement of the confluence of southwestern and southerly airflow affecting southern Yunnan.The role of each factor in the initial,developmental,and maintenance phases of heavy precipitation varied.(3)The causes of precipitation evolution in heavy rainstorm centers at different stages and different regions were also very different.It was the result of interaction of many factors,such as the convergence of low-level meridional and zonal winds,the rise of frontal convergence,the forced uplift of meridional and zonal airflow by topography,and the convergence of low-level water vapor.This study provides a scientific basis for improving the forecasting capability of extreme precipitation events and disaster prevention and mitigation in Yunnan.
extreme rainfallBay of Bengal Lowactivity within front zone and shear linetopographic climate effectYunnan
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