一次秋季温带气旋的雨雪天气过程分析
A case study of the mixed rainfall-snowfall event associated with an extratropical cyclone in autumn
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摘要: 2012年11月3-5日,受强冷空气和气旋发展的影响,华北地区出现了秋冬之交首场雨雪过程.此次雨雪天气过程具有降水强度大、雨雪范围广、初雪时间早、降水相态多变以及雨雪降温大风并存等特点.着重分析了过程的多尺度相互作用及其成因机理等,得到以下结果:(1)通过对北极涛动指数变化的分析发现,此次过程前期由于北极涛动指数负位相出现的时间较早,并且强度大,维持时间长,对应当年秋冬之交首场雨雪天气出现亦时间早、强度大.(2)此次过程前期华北地区西风带上游长波波数趋于减少,导致后期位于华北地区受其引导的相应低压系统稳定少动.(3)东亚-西太平洋地区建立的“Ω”形阻塞形势阻挡了其上游西风带槽的东移,特别是后期此阻塞高压南部迅速生成的低值系统与阻塞高压的高值中心构成了十分稳定的对偶之势,阻断了其上游西风带槽的东移,导致相应地面气旋系统长时间稳定维持在北京及其邻近地区,从而促使该地区出现了历史上罕见的强降水事件.(4)该过程涡度平流和温度平流对锋前上升运动的形成和气旋的发展演变有重要的影响,这次天气过程的气旋发展机制与经典的第二类温带气旋发生、发展机制不尽相同.(5)此次过程中水汽条件极为有利,特别是在气旋发展后期,来自海上的水汽源源不断流入移动缓慢的气旋区域,为该区域的罕见强降水创造了极为有利的水汽条件.(6)在整体大气稳定层结环境下,锋面前沿的水汽输送和抬升在低层形成了大气不稳定层结,有利于低层上升运动的触发,锋面加强引起的风垂直切变加强促进了大气不稳定层结之上的条件性对称不稳定区向上扩展,并与高层的条件性对称不稳定区连通,从而为低层触发的上升运动向高层发展提供了有利条件.Abstract: A mixed rain-snow storm associated with a strong burst of cold air and development of an extratropical cyclone occurrcd over North China from 3 to 5 November 2012.This early snowfall event was characterized by a dramatic drop in temper aturc, strong winds,high precipitation intensity, broad spatial extent, and coexistence of multi-phase precipitating hydrometeors.This study investigates the multi-scale interactions between,the large-scale circulation background and the synoptic-scale weather systems.The results arc as follows.(1)The Arctic Oscillation(AO) had been in is negative phase long before the event leading to southward advection of cold air into North China in advance of the storm.(2)The large-scale atmospheric expc ricnecd a decreased number of long waves upstream of North China prior to the storm,resulting in reduced wave velocity and an almost stagnant low pressure system extratropical cyclone over North China.(3) An Ω-shaped blocking high over East Asia and the western Pacific obstructed the eastward movement of an upstream trough,allowing the corresponding surface cyclone to stahilize and persist over ticijing and its neighboring areas.This blocking high was a major factor in making this event a historically most severe precipitation event in autumn in Beijing for the past 60 year.(4) Baroclinic instability at lower levels gave rise to rapid development of the cyclone under the classical "second type" development mechanism for extratropical cyclones.(5) Moisture originated from the Yellow Sca entered the slowly-moving cyclone in a steady stream,creating fairly favorable water vapor supply for the heavy rainfall-snowfall,especially during the later stage of the cyclone development.(6) Moisture transport and frontal lifting triggered low-level instability and updrafts.Intensification of the front enhanced the vertical wind shear, causing conditional symmetric instability (CSI) to expand upward within the unstable lower troposphere,and to eventually gear into the CSI region of the upper troposphere,which facilitated the upward development of low-level updrafts.
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Keywords:
- Rainfall and snowfall /
- Extratropical cyclone /
- Causc analysis /
- Autumn
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