Analysis Of Water Vapor Characteristics And Formation And Evolution Mechanism Of High-altitude "dry" Process In The Yangtze River Delta From 2015 To 2019

By using the data of soundings,ERA5,GDAS as well as the backward trajectory model HYSPLIT,we analyzed the distribution characteristics water vapor characteristics of high-altitude "dry" process occurring in the Yangtze River Delta and selected a typical case in the winter of 2019 to analyze its formation and evolution.The results are as follows:This high-altitude "dry" process covered a wide area in the horizontal direction and appeared at a height between 900-550 h Pa.In the Yangtze River Delta region,the characteristics of the inter-annual change of the high-altitude "dry" process are not obvious,but there are obvious seasonal changes.There are 17,20,and 20 occurrences in spring,autumn and winter.In 5 years,there is no high-altitude "dry" process in summer.The atmospheric water vapor content during the 800-300 h Pa of high-altitude "dry" process in the Yangtze River Delta from 2015 to 2019 was less than the seasonal average water vapor content.The water vapor content in the autumn high-altitude "dry" process was the highest among the three seasons,about 5mm,and the water vapor content in spring and winter close to 0,and the distribution of water vapor content increases from northeast to southwest.There were 5 main weather types during the 52 high-altitude "dry" processes that occurred in the Yangtze River Delta in 5 years: low pressure trough,high pressure ridge,low pressure,high pressure and pressure sharing field.Among them,the high pressure ridge and low pressure trough appeared most frequently,accounting for 67.3% of the total.In different weather conditions,when the northwest direction of dry and cold air flow appears,it will lead to the occurrence and development of "dry" process.The occurrence of this "dry" process is caused by the sink of a high-altitude west-northwest airflow,which is then transported to East China,leading to a significant divergence of the water vapor in the middle and the lower troposphere.During the maintenance period of the process,all the air masses arriving in East China were found as high-latitude continental dry air masses,which possess a relative humidity less than 10%.Moreover,the replenish of the water vapor along the vertical direction is negligible;During the re-humidification period of the process,it was found that a humid westward airflow was transported to East China,resulting in a convergence of a large amount and the strongest convergence of water vapor in the middle and lower troposphere occurs at the height corresponding to 650 h Pa.It was also found that during this time period,the relative humidity largely increased,from 2% to 92%.