梅雨期经大别山两侧暴雨中尺度低涡对比分析

Comparison between Two Rainstorm Meso-scale Vortices Circumambulated Respectively from Two Sides of Mountain Dabieshan during Meiyu Season

通过统计分析2007-2011年梅雨期间江淮流域暴雨日数和低涡过程,结果表明低涡暴雨占41%,且绝大多数为浅薄低涡(700 hPa以下),此类低涡易受大别山地形影响。在地形和高空引导气流的共同作用下,经大别山南侧沿长江流域及经山脉北侧沿淮河流域的浅薄低涡遇大别山绕行、爬坡同时存在,并且北部低涡增强大于南侧,进而影响到低涡暴雨形成沿淮河流域和长江流域的两条雨带。环境高低空急流的风切变配置状态不仅有利于浅薄低涡的气旋式增强,并且指示低涡东移路径与低涡位置。而势力较弱的低空急流受大别山南部地形的影响,也表现出有绕行和减弱的阶段,进而可影响到山北淮河流域低涡的强度增幅和伴随的暴雨强度比山南长江流域低涡强一些。绕行山脉南北两侧的低涡暴雨带的湿位涡特征表明,垂直剖面上湿位涡正斜压分量垂向梯度带的配置,且其强度与对应的降水强度成正比,沿淮河的北路低涡湿位涡因环境风场垂直切变大,其强度更强。数值试验结果表明,大别山地形对低涡路径的南北绕行、低涡强度的山前减弱山后加强以及水汽辐合的强弱有直接影响。山脉南部迎风坡的强辐合抬升以及山脉北部弧形背风处对气流的拉伸辐合汇聚,成为大别山地形有利于水汽辐合上升,增强低涡暴雨量的两个重要部位。由于大别山南段的主体部分范围高大,所以对绕行山南部的低涡影响更为显著。

A statistic analysis of rainstorm vortices during Meiyu seasons from 2007 to 2011 at Yangtze —Huaihe River basin shows that the low vortex rainstorm is up to 41%in all rainstorm day during the statistical period,and most of the rainstorm vortices are shallow vortices(under 700 hPa),so they are easy to be impacted by the Dabieshan mountain(about 1500 m height).Two rainstorm vortices are analyzed when they circumambulate and climb the Dabieshan mountain from south along Yangtze River Basin and from north along Huaihe River Basin,respectively under both influences of mountain and steering current of trough foreside at high level.The shallow vortices go round the mountain from two sides with north stronger than south one,and cause their rain zone positions difference along Huaihe River basin and along Yangtze River basin respectively.Meantime the wind shear structure of high level jet and the low level jet together increase the cyclone vorticity of the shallow vortices and indicate the eastward paths and positions of low vortices.Meanwhile,the low level jet also responses to the mountain hindering,the weak low level jet is weaken and shows a circumambulated state to the mountain by blocking of the south part of Mountain Dabieshan.The circulation state causes a weak growing of both vortex and rainstorm intensities at south end than strong growing at north end of Mountain Dabieshan.The distribution of the moisture potential vorticity of vortex rainstorm shows that at vertical-longitudinal cross-section the baroclinic and barotropy gradient areas overlap each other and make a strong rainfall environment,the moisture potential vorticity is proportional to the intensity of rainfall,and the Huaihe River Basin vortex has stronger moisture potential vorticity as there is stronger vertical wind shear at north part of Mountain Dabieshan.The numerical simulation shows that the Dabieshan mountain topography remarkably impacts on the low vortex circumambulated paths from south or north,the low vortex intensity increases at leeward or decrease at windward,and moisture flux convergence strong or weak with vortex intensity.The topography effect causes the center position of low vortex,moisture convergence and rainfall easily close or overlap at two key areas(face-wind side of the mountain south part and the lee-ward arc area of mountain north part),where the air vertical stretched,moisture convergence strengthen and the rainfall of the vortex rainstorm are special heavy.Due to the south part of the mountain is larger and higher than the north part,the topography impact on the south vortex along Yangtze River Basin is more obvious.