晋北地区飑线下山变化特征对比分析OA

This article utilizes North China regional radar composite reflectivity factor mosaic products,single-site radar data from northern Shanxi,ERA5 reanalysis data,and surface observation data collected in the warm season(May-September)of 2021-2023,and conducts statistical analyses on the changing characteristics of squall lines descending mountains in this region.The results show that a total of 29 squall lines are identified,and following the moving directions of the squall lines,they are classified into four types:west-moving,northwest-moving,north-moving,and basin-originating.Then,based on their intensity changes when descending mountains,they are classified into three types:intensifying,weak-ening,and maintaining types,of which the weakening type is the most common,accounting for 67％of the total.All west-moving squall lines belong to the weakening type upon descent,while all north-moving types of squall lines are of the intensifying type.The northwest-moving type includes squall lines that in-tensify,weaken or maintain the intensity upon descent.Analysis of the environmental background ahead of the descending path for intensifying and weakening types within the northwest-moving squall lines re-veals that,compared to weakening squall lines,the intensifying ones exhibit slightly stronger dynamic conditions,while conditions related to moisture,convective available potential energy and vertical wind shear are comparable or slightly poorer.Therefore,it's difficult to accurately forecast whether a squall line will intensify or weaken upon descent based solely on the environmental conditions ahead of its path.For short-time nowcasting,radar data can be used to forecast whether a squall line will intensify or weaken upon descent.The intensifying squall lines upon descent typically exhibit stronger echo intensity,with moving speed around 17 m·s-1.Large gradient zone of reflectivity factor is concentrated in the front sec-tion of a squall line,with an overall bow-shaped squall line accompanied by a gust front.Radial velocity cross-section shows a distinct organized structure with forward inflow ascending slantwise along the rear inflow.In contrast,the weakening squall lines upon descent typically have weak or moderate echo intensi-ty,and their moving speeds are generally below 10 m·s-1.Large gradient zone of reflectivity factor ap-pears at the rear part of a squall line,which has overall a straighter shape,not accompanied by gust front.On the radial velocity cross-section,there is no distinct organized structure with forward inflow ascending slantwise along the rear inflow.