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

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.