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基于天气分型的四川盆地西侧极端强降水事件环流异常配置与演变特征

Configuration of Large-Scale Circulation Anomalies and their Evolution for Extreme Heavy-Precipitation Events in the West of the Sichuan Basin Based on Classified Weather Patterns

  • 摘要: 四川盆地西侧是中国区域极端强降水事件发生频率最高的区域之一,然而,由于区域地形和天气系统复杂性,对强降水事件理解尚不充分。本文以发生在盆地西侧的100个极端强降水事件为研究对象,基于2001~2020年中国地面气象观测站网观测的降水数据、GPM-IMERG降水数据以及ERA5大气再分析资料,采用基于K-means聚类分析的天气分型方法,并结合大气动力—热力诊断方法,分析了不同类型降水事件环流异常配置与演变特征。研究结果表明,三种类型强降水发生期间,位势高度均表现为高层正异常(200 hPa)和低层负异常(850 hPa),大气垂直速度明显增大,温度异常呈现“上冷下暖”结构,并伴随着来自低纬海洋地区的水汽输送增强。但三种类型降水对应的南亚高原(SAH)、西太副高(WPSH)以及西风急流等大尺度环流因子在位置和强度上存在差异:发生频次最多的1型降水的水汽主要来自低纬孟加拉湾和中国南海;2型降水发生期间WPSH发展最强,其水汽主要来自东亚季风控制下的中国南海,而孟加拉湾水汽输送受到限制;而发生在SAH强度偏弱期间的3型降水发生频次最少,其主要受青藏高原东北侧的反气旋异常和东亚季风控制下的水汽输送影响。三种类型降水环流演变特征分析显示,强降水发生前,罗斯贝波作用量呈增强趋势、大气垂直速度增加以及温度异常更为显著,这些异常特征使得局地对流活动和高空辐散低层辐合过程增强,有利于极端降水事件形成。进一步分析表明,与SAH比较而言,WPSH位置和强度的变化对强降水形成预测具有更好的指示意义。

     

    Abstract: The western side of the Sichuan Basin is one of the regions in China that experiences the highest frequency of extreme heavy precipitation events (HPEs). However, due to terrain complexity and atmospheric circulation systems, the formation mechanism of HPEs remains elusive. Based on the gauged data by the China Meteorological Administration from 2001 to 2020, GPM-IMERG precipitation data, and ERA5 reanalysis data, we selected 100 extreme HPEs that occurred in the west of the Sichuan Basin and classified them into three categories using the K-means clustering method. Then, the anomalies of atmospheric circulation and their evolution with respect to different HPE categories were explored. The results show that during the precipitation period, the geopotential height is characterized by a positive anomaly at the upper level (200 hPa) and negative anomaly at the lower level (850 hPa), together with enhanced vertical wind speed, demonstrating a “top-cold and bottom-warm” atmospheric temperature structure and increased water vapor transport from the low-latitude ocean area. However, the atmospheric circulation configuration of anomalies, including the South Asian high (SAH), Western Pacific Subtropical high (WPSH), and upper-level westerly jet, shows an obvious difference and plays a dominant role in shaping the precipitation formation of different types and associated water vapor transport. For category 1 (HPEs with the highest frequency of occurrence), associated water vapor transport mainly comes from the Bay of Bengal and the South China Sea and is controlled by the Indian and East Asian monsoons. For category 2, HPEs occur with strengthened WPSH, and its water vapor mainly comes from the South China Sea, while the water vapor transport from the Bay of Bengal is restricted. For category 3, its water vapor transport is mainly controlled by the East Asian monsoon. Before the occurrence of HPEs, the Rossby wave action shows an increasing trend, atmospheric vertical wind speed increases significantly, and temperature anomaly is more significant. These features are conducive to the occurrence of extreme precipitation events. Moreover, the results indicate that changes in the WPSH position and intensity on synoptic time scales are better indicators for predicting heavy precipitation formation compared with SAH.

     

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