南亚高压北缘的高空气流发散与梅雨锋暴雨发展的关系

从1981、1982和1984三年中6月历史天气图发现,长江流域附近24小时大于100毫米以上的降雨区位于高空200hPa南亚高压北缘的气流发散区中。这种气流发散有两种类型:一类是槽后发散型;另一类是槽前发散型。分析得出与这种高空气流发散相联的高空辐散是梅雨锋上雨带形成和暴雨发展的促发因素。诊断判明200hPa的辐散主要由惯性平流风的散度所决定,而850hPa的辐合主要是对流风散度起支配作用。由于惯性平流作用是地转平衡破坏的因素,而垂直运动是流场与质量场调整到地转适应的因素,由此证明,暴雨区中低层的辐合是对高层辐散的响应,也表明暴雨发展的促发因素是高层的辐散。作者也发现,暴雨出现前,高层等压面的高度有明显下降,这是由高空辐散引起空气柱质量的减少所致。本文提议,梅雨发生发展的预报应着眼于梅雨锋云带上空由高空气流发散引起的辐散。

Relationship Between the Upper Flow Divergent over the Northern Flank of Southern Asiatic High and the Development of Heavy Rain over Mei-Front

It has been discovered that the heavy rain associated with Mei-Front is related to the upper flow divergent over the northern flank of Southern Asiatic High (SAH). Two types of the flow divergent have been identified, one of which is divergent between two northwesterlies associated with the SAH circulation and the rear of westerly trough, the other is divergent between the southwesterlies in front of westerly trough and the northwesterlies of SAH. A case study for the heavy-rain during 27-28 June 1981 has been taken. The result indicates that the upper divergence caused by the upper flow divergent over Mei-Front cloud band is the trigger for the heavy rain development. The divergences caused by three ageostrophic wind components have been estimated at 200 and 850 hPa. The results show that the divergence responsible for the heavy rain development is caused by inertial advecton wind at 200 hPa and by convection wind at 850 hPa. This fact means that the lower convergence in heavy rain regionis responsive to the upper divergence. The time change of isobaric surface height at the station with heavy reavy rain also shows a large drop at upper isobaric surface before the occurrence of heavy rain. These two phnomena illustrate the role of upper divergence over Mei-Front cloud band in the heavy rain development.