江淮地区两次罕见冻雨过程与热带海-气系统和平流层极涡异常的联系

Association of two rare freezing rain events over Jianghuai region with anomalous tropical air-sea conditions and stratospheric polar vortex

  • 摘要: 基于1981—2024年ERA5再分析大气环流资料、美国国家海洋和大气管理局(NOAA)气候预测中心(CPC)逐日降水资料和NOAA拓展重建的全球月平均海表温度资料,对比分析了江淮地区2010和2024年初两次罕见冻雨过程(简称过程1和过程2)的实况、环流形势和温度层结差异,并探究了热带海-气信号和平流层极涡异常的可能影响和机制。结果表明:(1)过程2冻雨影响范围和持续时间强于过程1,过程1中冻雨出现在2010年2月10日白天,主要分布在淮北地区,而过程2中冻雨则主要出现在2024年2月20—21日,范围南扩至沿江一带。两次冻雨过程高低层系统配置具有一定的相似性,均发生在北方冷空气南下与南支槽前和西太平洋副热带高压外围西南气流输送的暖湿空气相互对峙的环流形势下,稳定而持续的暖湿水汽输送是两次过程中融化层和逆温层建立并维持的关键。(2)同期热带太平洋厄尔尼诺事件有利于江淮地区上空出现中层冷、低层暖的垂直温度配置,同时低层西北太平洋反气旋性环流可增强水汽输送,为两次冻雨过程提供有利的大尺度气候背景条件。(3)与此同时,西太平洋地区上空活跃的MJO(Madden-Julian Oscillation)相关深对流在西北太平洋地区激发出强大的反气旋性环流,为江淮地区冻雨过程的发生提供了充沛的水汽,使得低层融化层和逆温层建立并维持。(4)两次冻雨过程前期,平流层爆发性增温(SSW)事件引起的平流层极涡形变和向亚洲地区偏移通过调控中高纬度地区对流层环流演变引导冷空气爆发南下,与暖湿气流强烈交汇于江淮地区,由此形成稳定持续的逆温结构、强烈低层水平风场辐合及上升运动,两次冻雨过程得以发生。总体而言,在江淮地区两次历史罕见的冻雨过程中,热带海-气信号和平流层极涡形变的相互配合对这两次冻雨过程的发生起着重要作用。因此,在江淮地区冻雨过程的短期预报中需考虑MJO活动和SSW事件的影响。

     

    Abstract: Based on the ERA5 atmospheric reanalysis dataset and daily precipitation data from the Climate Prediction Center (CPC) along with the extended reconstructed global monthly mean sea surface temperature (SST) from the National Oceanic and Atmospheric Administration (NOAA) during 1981—2024, two rare freezing rain events that occurred over Jianghuai region in early 2010 and 2024 (hereafter Event 1 and Event 2) are analyzed and compared. The analysis is focused on differences in observed characteristics, atmospheric circulation patterns and temperature stratification associated with the two freezing rain events. The association of the two events with tropical ocean-atmosphere forcings and stratospheric polar vortex anomalies as well as their possible effects are further explored. Results are as follows: (1) The range and duration of Event 2 are respectively larger and longer than that of Event 1. In Event 1, freezing rain mainly occurred over Huaibei region during the daytime of 10 February 2010, whereas in Event 2, it occurred on 20—21 February 2024 and extended southward, reaching the Yangtze River Basin. These two freezing rain events occurred under similar circulation patterns, which are characterized by the convergence of cold air from north and warm moist air transported by southwesterlies in front of the trough and the periphery of the Western Pacific subtropical high. The stable and sustained transport of warm moist air is crucial for the establishment and maintenance of the melting layer and the inversion layer during both events. (2) The concurrent El Niño event in the tropical Pacific is favorable for the establishment of a vertical temperature configuration with cooling in the middle levels and warming in the lower levels over the Jianghuai region. Meanwhile, the intensified western North Pacific anticyclone anomalies in the lower levels enhanced moisture transport, providing a large-scale background favorable for the two freezing rain events. (3) The deep convection associated with the active Madden-Julian Oscillation (MJO) over the Western Pacific triggered strong anomalous anticyclonic circulations over the Northwestern Pacific, providing abundant moisture transport to Jianghuai region for the occurrences of the two freezing rain events. This facilitated the establishment and maintenance of the melting layer and inversion layer in the lower levels. (4) In the early stages of the two freezing rain events, the Sudden Stratospheric Warming (SSW) events resulted in the deformation and southward displacement of the stratospheric polar vortex to the Asia. This led to cold air outbreaks by modulating tropospheric circulations in the mid-high latitudes. The cold air strongly converged with warm and moist air over Jianghuai region. As a result, a stable and sustained inversion structure was established, which, accompanied by intense convergence of low-level horizontal wind fields and upward motion, facilitated the occurrence of the two freezing rain events. In general, during the two unexpectable freezing rain events over Jianghuai region, the joint effects of tropical ocean-atmosphere forcing and the deformation of the stratospheric polar vortex played a significant role in the occurrence of these events. Therefore, MJO and SSW events should be considered for short-term predictions of freezing rain processes in the Jianghuai region.

     

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