四川盆地干湿西南涡个例大气环境效应对比研究

Comparative Study on Atmospheric Environment Effects of Cases of Dry and Wet Southwest Vortices in Sichuan Basin

针对关注较少的西南涡大气环境效应问题,使用2014-2016年常规地面气象观测资料、空气污染监测数据、气象探空数据及NCEP的CFSv2再分析资料,统计了2014-2016年冬春季的西南涡及其对应时段的空气质量,发现在冬春季由于水汽缺乏,西南涡多属于不引发降水的干性低涡,其活动期间易造成四川盆地重空气污染过程。在此基础上又选取了四川盆地2015年两次干、湿西南涡活动过程进行了对比研究。首先对2015年12月29日至2016年1月3日重污染过程的探析发现,12月30日盆地上空形成干西南涡,其强度弱、厚度薄,其前部的弱下沉运动所产生的"焚风效应"使得成都上空形成脱地强低空逆温,产生极不利于污染物扩散的"锅盖效应",从而导致最大混合层厚度和近地面风速非常小,静稳型天气特征凸显,造成代表城市成都的细颗粒物PM10、PM2. 5的日间浓度均值分别上升为398μg·m^-3和268μg·m^-3,达到此次重空气污染过程中的峰值。与之相对比,研究发现2015年8月16-18日的湿西南涡活动过程中,强烈的垂直上升运动及其引发的强降水,产生了极有利的大气扩散和湿清除效应,迅速有效地降低了污染物浓度,与干西南涡加重大气污染的环境效应截然相反。

For the rarely researches that the atmospheric environment effects of Southwest Vortex(SWV),routinely observed meteorological data,air pollution monitoring data,atmospheric sounding data and Climate Forecast System Version 2(CFSv2) products have been used to counted the number of SWV in winter and spring2014-2016 and the synchronous air quality grade with diagnostic and statistical methods in this study.It is found that SWVs developed in spring and winter are mostly dry low vortices due to the lack of water vapor.This type of SWV rarely induces precipitation and can easily cause air pollution weather in the Sichuan Basin.Based on those results,a dry and a wet SWV in Sichuan Basin in 2015 were selected for comparative study.The analysis of a severe air pollution over the Sichuan Basin from 29 December 2015 to 3 January 2016 suggests that a dry SWV was generated over Sichuan Basin on 30 December with weak intensity and small thickness.The foehn effect arising from the weak descending motion at the rear of dry SWV caused an intensive untouched-ground low-level inversion over Chengdu,generated a ’pot cover’ effect that suppressed the diffusion of air pollutants,brought about an quite small maximum mixing depth and surface wind speed which presented an obvious stagnate synoptic status,gave rise to the concentrations of PM10 and PM2.5 of representative cities in basin were 398 and 268μg·m^-3,respectively,which were the peak values during this heavy air pollution process.In contrast,the study found that,during August 16-17,2015,the strong upward movement and heavy precipitation resulted from the wet SWV produced the extremely favorable effects of atmospheric diffusion and wet deposition,rapidly and effectively reduced the concentration of pollutants,which was opposed to the environmental effect of the dry SWV that aggravating atmospheric pollution.