“菲特”减弱时浙江大暴雨过程成因分析

Cause Analysis on the Severe Rainfall in Zhejiang During the Weakening of Fitow

文章利用常规观测和自动站加密、凉帽山高塔、雷达站及NCEP再分析等资料,分析了2013年10月7-9日1323号"菲特"台风登陆减弱后浙江异常强暴雨事件,得到:这次大暴雨过程是"菲特"减弱后,由于北侧的弱冷空气从近地层渗透流入,导致垂直涡度加强,上升气流增强,高层出流强,低层能量锋区堆积,辐合增强,诱生中小尺度系统而产生的;由于东侧"丹娜丝"的活动,使得浙江北部地区有持续强盛的偏东气流,提供了充沛的水汽和能量,近地层偏东气流和东北气流的辐合是强降雨的动力机制。偏东风明显增大比雨量增大有2 h左右的提前,水汽增加和减少与雨量增大和减小有6 h的提前;呈喇叭口状西高南高的杭州湾地形有迎风坡作用和地形辐合,对偏东、东北气流参与造成的降雨过程有增幅作用。由此,秋季台风雨量预报要考虑冷空气的加入可能引起的强降雨时间延长;盛行偏东风时,杭州湾海域风力明显偏大,可能引起周围地区的降雨增幅;用当地多源资料、边界层观测资料做暴雨预报局地性临近修正有很好的指导意义。

The abnormal severe rainfall resulting from depression circulation of Typhoon Fitow (1323 )is analyzed using routine observation data,AWS data,Liangmao Mountain wind tower data,radar data,sat-ellite data and NECP reanalysis data.The results show that the heavy rainstorm occurs during the weake-ning of Typhoon Fitow.When the weak cold air to the north of Fitow invades into the air column from low layers,it causes the development of the vertical vorticity,the ascending current and the upper outflow and the accumulation of energy frontal zone as well as the convergence expanding,producing the meso-and mi-cro-scale weather system in this process.The persistently strong easterlies in the north of Zhejiang are re-sponsible for the movement of Typhoon Danas to its east,which provides abundant water-vapor and heat energy,making the low-level convergence stronger.The convergence of easterly flow and northeast flow in the surface layer is the dynamic mechanism for this extremely heavy rainfall.The strength of the easter-lies increases 2 hours earlier than the increase of the rainfall.The water-vapor increases and decreases 6 hours earlier than the increase and decrease of the rainfall.The bell-shaped Hangzhou Bay which has higher terrain in its west and south has precipitation enhancement effect by the windward slope and topo-graphic convergence.Therefore the predicted rainfall time should be prolonged when cold air intrudes into the typhoon in autumn,and the wind in the Hangzhou Bay is obviously stronger when the prevailing wind directs from east,for it may increase the precipitation in surrounding areas.Improving the usage of the lo-cal and PBL observation data of local and PBL has significant effects on the nowcasting of rainstorm.