Comparative Analysis of Two Heavy Rain Cases Caused by Southeasterly Low-Level Jet

The comparative analysis on the extra heavy rain case during 24—26 July of 2010 and the heavy rain case on 22 September 2010 at the middle and southern coast area of Zhejiang were studied by using NCEP reanalysis data,satellite images and observations of automatic weather station.The conclusions are drawn that two heavy rain cases given above are all followed by southeasterly low-level jet.The extra heavy rain(excessive heavy rain in some areas) occurred in the middle and southern coast area of Zhejiang is triggered by meso-β-scale convective clusters which originated locally,and there are obvious supergeostrophic features in low-level in the case.The heavy rain occurred in the above coast area is related to the weak precipitation nephsystem brought by landed typhoon,and there is not obvious supergeostrophic wind in boundary layer in the case.Extra(or excessive) heavy rain is generated easily when the wind speed in boundary layer increases rapidly and the supergeostrophic wind becomes the strongest.When the supergeostrophic wind appeared,the joint function of topographic forcing effect and frictional effect in the boundary layer leads to the obvious reinforcement of vertical wind shear in low-level and sea-land breeze shear.It is unfavorable to the formation of extra heavy rain that the wind speed is in geostrophic balance in boundary layer.