低层偏东气流对贵州梵净山东侧强降水的作用

利用常规观测资料、地面加密自动站资料、雷达探测资料与NCEP 1°×1°再分析资料等,对低层偏东气流影响下贵州铜仁梵净山东侧4次强降水天气过程进行了分析,重点探讨了在低层偏东气流与地形共同作用下的强降水形成机制,并归纳低层偏东气流影响下的梵净山东侧强降水概念模型。结果表明:(1)高空槽、低层切变线、地面中尺度辐合线是影响梵净山东侧强降水的主要天气系统;(2)低层浅薄偏东气流对梵净山东侧强降水起着关键作用,当低空气流u分量随高度减小时,地形迎风坡气流辐合上升,而气流v分量随高度增加时,地形迎风坡会产生与山脉垂直的水平涡管,在地形抬升作用下涡管向上凸起形成两个涡管环流圈,涡度垂直分量使山脚附近上升气流加强而有利于山脚产生强降水;(3)梵净山东侧强降水区的形成存在三种机制,即迎风坡山脚多次触发对流形成雨量叠加效应、地面中尺度辐合线自身触发组织对流、回波沿地面中尺度辐合线东移形成“列车效应”,三种机制产生的降水带与地面中尺度辐合线走向一致。

Effect of low-level eastward airflow on formation of severe precipitation on the east side of Fanjing Mountain

Using conventional observation data,intensive automatic weather station observation data,radar products,NCEP 1°×1°reanalysis data and other data,we have analyzed the four severe precipitation events on the east side of Fanjing Mountain in Tongren,Guizhou.These events were under the influence of low-level eastward airflow.Our analysis focuses on the formation mechanism of severe precipitation under the combined effect of low-level eastward airflow and topography,and established a conceptual model of severe precipitation on the east side of Fanjing Mountain under the influence of low-level eastward airflow.The main results are as follow.(1)The upper-level trough,the low-level shear line and the surface mesoscale convergence line are the key weather systems affecting the severe precipitation on the east side of Fanjing Mountain.(2)The shallow low-level eastward airflow plays a key role in the severe precipitation on the east side of Fanjing Mountain.When the u component of low-level airflow decreases with height,the ascending airflow at the windward slope terrain converges.When the v component of the airflow increases with height,a horizontal vortex tube being perpendicular to the orientation of mountains is generated at the windward slope terrain,where two vortex loops form due to the effect of topographic force.The vertical component of vortici ty strengthens the updraft near the foot of the mountain in which is conducive to the generation of severe precipitation.(3)There are three mechanisms for the formation of the severe precipitation zone on the east side of Fanjing Mountain,that is,rainfall superposition effect formed for the convection that is trigged many times at the foot of the windward slope,organizational convection triggered by surface meso scale convergence line itself,and"train effect"formed with the eastward motion of echo along surface mesoscale convergence line.The se vere precipitation zones generated by the three mechanisms mentioned above are consistent with the orientation of surface mesoscale conver gence line.