暴雨过程中对流云合并现象的观测与分析

利用静止/极轨气象卫星、新一代多普勒天气雷达、地面观测和NCEP再分析资料, 对2008年7月22日淮河流域一次暴雨过程中的对流云合并现象进行观测分析。综合观测显示, 这是一次在低层显著气压梯度作用下发生的对流云合并现象, 是一次多尺度、多合并方式的典型过程, 不仅有对流单体之间的合并, 还存在着对流云核(强中心)之间的合并。根据合并的进程, 可以划分为三个主要阶段:单体发展、云桥形成以及系统合并。卫星云图显示, 对流云核合并后云团结构更加紧密、边缘更加光滑;在雷达回波上, 合并后回波顶高和垂直积分液态含水量有显著的增加。对流云核合并完成后, 区域内最高云顶开始回落, 垂直积分液态含水量的最大值开始减少, 并在地面产生强降水。另外, 对流单体之间的合并不仅导致地面降水范围有所扩大, 而且还使降水持续了较长的时间。对合并过程可能存在的机制分析表明, 存在着三个方面的动力因素:(1)大尺度环境场中垂直运动存在的水平不均匀性, 是促成对流云团合并的环境因素。(2)对流系统间存在的低压中心及其引起的显著地面气压梯度, 是对流系统间合并的主要原因。(3)一个云核的下沉气流加强了另一个云核的上升气流, 是对流云核合并的动力学原因。

Study of Convective Cloud Merger in Heavy Rain Using Multi-Observation Data

Using NCEP re-analysis data and multi-observation data from meteorological satellites, weather radar, and a surface observation net, we analyze a rainstorm convective cloud merger case on 22 July 2008 in the Huaihe River basin. This cumulus merging event was a typical multi-scale (including cell merger and cloud core merger) and multi-formation merging process, and was caused by a low-layer atmospheric pressure gradient in the unstable region. The entire merging process could be classified into three stages: cell development, cloud bridge formation, and system merging. As a result of the merger, large, complex convective systems were produced and became more intense, and heavy rains were recorded at the surface. After cloud cores merged, the cloud area increased. Satellite cloud images revealed that the cloud boundary became smoother, and increases in the echo top and vertical integer liquid could be observed in the radar echoes. After the merger was complete, the highest cloud top was descended[H4], and the vertical integer liquids were in a downdraft, which caused heavy, expanded ranges and long-duration precipitation on the ground. Three dynamic factors caused the clouds to merge: First, the horizontal asymmetry of vertical movement in the large-scale environment field was an environmental factor. Second, the surface pressure gradient force of the atmosphere was one of the dominant factors for cloud bridge generation and cloud merging. Third, the main reason the cloud cores merged was the coupling of vertical motion between one core’s downdrafts and another’s updrafts.