梅雨锋急流暴雨日变化的地理原因研究

包括急流暴雨在内的梅雨降水存在显著日变化。过去曾从低空急流日变化等一些梅雨系统的内部联系寻求解释，但通过实例分析和气候比较认为：梅雨降水日变化的根本原因是东亚低层大气温度场日变化的区域性差异。造成这种差异的基本原因又包括两个方面，一是东亚特殊的地形构造，二是梅雨期特殊的云区分布。由于梅雨云带的存在，梅雨锋低槽内温度日变化很微弱；又由于高大地形的存在，梅雨锋低槽南北两侧温度日变化非常明显，使副高和中纬度变性高压在夜间增强，梅雨锋低槽也在夜间变深变窄，改变了早、晚时刻梅雨系统的相对强度和降水功能，导致降水日变化。低空急流系统的日变化是梅雨气压场形势出现日变化后发生的众多中间变化之一。而低层温度场日变化的区域性差异是东亚梅雨季节经常存在的一种气候现象。

Geographic Causes of the Daily Variation of Rainstorm in the Mei-Yu Frontal Jet

Mei-yu frontal rainfall including jet-related rainstorms experiences pronounced daily variation (DV).Scientists tried to seek explanations from the DV of lower-level jet as a member of the mei-yu precipitation system.Based on case study and climate analysis,this work arrives at the central cause of DV of the mei-yu rainfall,which lies in the regional difference in DV of the lower-level temperature field of east Asia,and the cause relates essentially to 1)distinct topography in this region and 2) a particular cloud pattern during the wet season.Because of the existence of mei-yu precipitation cloud band the temperature DV is faint inside the mei-yu trough and on account of large-scale prominent terrain available,significant temperature DV occurs on both sides of the trough,leading to the reinforcement of the subtropical high and midlatitude transformed high,and the deepening and narrowing of the trough in the night with the result that the relative intensity and precipitation function of the mei-yu frontal rainfall system are changed at day and night,resulting in precipitation DV.And the DV of lower-level systems,e.g.,jet stream,is among the intermediate changes happening after the DV is displayed in the mei-yu pressure field.Regional difference in lower-level temperature DV is a climatic phenomenon that often emerges in this wet season of east Asia.