Observational Analysis for a Torrential Rain Caused by a Landfall Typhoon

By the landfall Typhoon Haitang and propagating eastward frontal systems,torrential rains appear near Handan and its adjacent area from the evening of July 21 to the morning of July 23,2005.By analyzing the synoptic features,characteristics of water vapor transportation,evolution of FY2-C infrared satellite images and stratification features,the cause of the torrential rain is discussed.The results show that,interactions among the tropical low,stable subtropical high and troughs on 500 hPa leads to the torrential rain.Cold air masses propagate southwards from middle to high latitudes and meet with the warm and moist air masses transported by the tropical low,then the latter are lifted by its cold counterpart and the Taihang Mountains and mesoscale convections generate in the vicinity of Handan.The analysis of wind vectors from automatic surface observations shows that the mesoscale convergence line between the northerlies and the easterlies in lower troposphere is very important to the torrential rain,which determines the locations and timings of the torrential rain.The water vapor supply of the torrential rain in the western part of Handan is mainly from South China Sea and East China Sea,while the supply in the eastern part of the city is mainly from South China Sea.At last,simple stratification analysis shows that convective instability and conditional symmetric instability may coexist above the torrential rain zones,which provide instable energy for the torrential rains.