两次冰雹过程边界层气象要素变化特征

Characteristics of Two Hail Cases in Boundary Layer

基于常规气象观测、浙江省自动气象站、宁波多普勒雷达和凉帽山岛370 m高塔边界层资料,对宁波市2012年7月7日(个例1)和2013年3月22日(个例2)两次冰雹过程进行了比较分析,结果表明:两次过程发生在不同季节和天气背景下,个例1由强热对流单体所致,个例2则是具有高架雷暴特征的移动性强对流带所造成。地面流场都显示:雹暴前侧入流辐合带和后侧下沉气流辐散区相对雹暴中心位置变化不大,两次过程前部冷出流边界前沿距离降雹中心约8~10 km及其后约4 km处形成了下沉尾涡。雹暴前侧入流区边界层主要表现为气压下降、气流转向雹暴中心、风速及辐合加大等;而下沉辐散区则表现出天气要素的剧烈变化。个例1高塔处于冰雹前侧入流区,边界层气象要素变化低层先于高层,临近降雹时由于拖曳作用削弱了抽吸作用,近地层各气象要素均有短时间的反向变化。个例2高塔处于低层入流区时,风速增大也最先出现在塔层底部;雹暴过境时,冷高压出流造成高塔处边界层风向、风速剧烈变化,上下各层时间较一致。

Based on the data from the conventional meteorological observations,the automatic weather stations in Zhejiang province,the Ningbo Doppler radar and the 370 meters tower on the Liangmaoshan island etc.,detailed comparison analysis was carried out on two hail cases occurred on 7 July 2012( case 1) and 22 March2013( case 2) in Ningbo. The results showthat the two cases happened under different synoptic backgrounds,with case 1 initialized from a strong thermal convectional cell,while case 2 developed from a moving convective zone with characteristics of elevated thunderstorms. The 2 cases displayed similar surface flowpatterns,with the front inflowconvergence zones and the rear divergence areas both relatively stable to the storm centers. The front outflowedges were found 8 ~ 10 km away from the hail- hit areas in both cases with sinking trail vortices about4 km behind. The boundary layer of the front inflowzones primarily showed pressure dropping,air flowturning to the storm center,and increasing of both velocity and convergence intensity. Within the divergence areas,drastic meteorological element changes were observed. In case 1,the tower was in front of the hail storm inflowzone,the meteorological changes at the tower were observed at the lower levels prior to the higher levels. Also it showed a short time period of reversed changes of all elements right before hailfall,due to the downward air-flowdragging and weakening of air suction. In case 2,when the tower was in the lower inflowregion,velocities were observed strengthened at the lowlevels first as well. When the hail storm went by,the boundary layer elements at the tower showed simultaneously abrupt changes as the result of cold outflows.