2020年长江中下游梅汛期干冷空气演变特征及其作用分析

Evolution characteristics and effect on precipitation of dry cold air in the middle and lower reaches of the Yangtze River during 2020 Meiyu period

利用常规观测资料、NCEP/NCAR再分析资料、风廓线雷达等加密观测资料,分析了2020年梅雨期干冷空气的演变和结构特征及其在梅汛期暴雨中的作用,得到以下结论:(1)2020年梅雨期降雨在“单阻”环流型下开始增强,在“双阻”环流型下达到最强时期,在“双阻”形势崩溃后逐渐趋于结束。极涡减弱,冷空气在80°N以南地区的堆积并随着中高纬度扰动南侵,是本次梅汛期降水加强的有利形势。(2)梅汛期干冷空气的活动在对流层中层相较于对流层低层更为明显,500 hPa上北风加强的时段与降水过程有较好的对应。(3)梅雨锋两侧正、反环流及次级环流上升支的叠加,对流层高层南北风的辐散及中低层辐合区共同加强了梅雨锋上空的动力抬升作用。(4)风廓线雷达观测及干湿气团的混合演变表明,中高层干冷空气侵入及中低层暖湿气流的发展,在雨区上空形成了“湿-干-湿”的湿度层结,增强了对流不稳定层结。干冷空气侵入后由干冷的下沉气流与暖湿上升气流造成辐合的旋转上升,有利于低涡的生成及降水的增强。

Based on conventional observation data,NCEP/NCAR reanalysis data,wind profiler radar and other intensive observation data,the evolution and structural characteristics of dry and cold air during Meiyu period in 2020 and its role in heavy rain are analyzed.The following conclusions are obtained.1)The rainfall in Meiyu period begins to strengthen under the"single resistance"circulation pattern,and reaches its maximum under the"double resistance"circulation pattern.After the collapse of the"double resistance"situation,it gradually comes to an end.The weakening of the polar vortex,the accumulation of cold air in the south of 80°N,and the southward invasion with the disturbance of middle and high latitudes are the favorable conditions for the enhancement of precipitation.2)During the Meiyu flood season,the activity of dry cold air in the middle troposphere is more obvious than that in the lower troposphere.The period of strengthening northerly wind at 500hPa corresponds well to the precipitation process.3)The superposition of positive and negative circulation on both sides of the Meiyu front and the ascending branch of the secondary circulation,the divergence of the north-south wind in the upper troposphere and the convergence area in the middle and low-level jointly strengthen the dynamic uplift over the Meiyu front.4)The wind profile radar observation and the mixed evolution of dry and wet air masses show that the invasion of dry and cold air in the middle and upper levels and the development of warm and wet air flow in the middle and low layers form a"wet dry wet"humidity stratification over the rain area,which enhances the convective instability stratification.After the invasion of dry and cold air,the dry and cold downdraft and the warm and wet updraft cause the convergence and rotation to rise,which is conducive to the formation of cyclones and the enhancement of precipitation.