一次路径偏北南折的秋季江淮气旋成因分析

Analysis of an autumn Changjiang-Huaihe cyclone with a northeastward-to-southeastward track

2015年11月5—8日发生的一次江淮气旋具有与以往过程不同的特征,发生时间晚、路径偏北东移、途经山东半岛后又南折,导致山东半岛连续降水36 h,降水时段内累积降水量最大接近50 mm。为揭示上述特点的成因,利用FNL(Final Analysis Data of Global Forecast System)再分析数据进行了天气形势分析,结果表明:副热带高压位置偏北且强度较强是导致气旋过程发生时间偏晚、路径偏北东移的重要原因,而副热带高压的异常变化与2015年强厄尔尼诺事件有很强的关联性。借助WRF(Weather Research and Forecasting)数值试验探究了气旋过程路径偏北又南折的原因,发现:1)黄海的高海温有利于海面水汽通量向上输送,积云对流加强,潜热释放促进江淮气旋增强发展;高空西南气流对增强气旋的控制较弱,导致气旋没有一直向北移动,中途发生南折。2)黄海南部海温较北部偏高约4℃,低空大气南、北部之间形成的能量锋区对江淮气旋的南折有指向作用;同时对流层低层偏北风的存在以及副热带高压的短时南撤为气旋的南折提供了动力环境。

A Changjiang-Huaihe cyclone,which occurred during 5-8 November 2015,is characterized by its later occurrence,northern location and northeastward-to-southeastward turning track. This cyclone led to heavy rains for 36 hours over Shandong Peninsula with maximum cumulative precipitation near 50 mm. Synoptic analysis of the cyclone is carried out in this study using the Final Analysis Data of Global Forecast System（ FNL）. It is revealed that the western Pacific subtropical high（ WPSH）,which is abnormally stronger and northern located, accounts for the cyclone＇s late occurrence and northern location,and that the anomalies of the WPSH are closely related to the strong El Nino event in 2015. To reveal the mechanism of the northeastward-to-southeastward track turning, a series of numerical experiments were conducted using the WRF（ Weather Research and Forecasting） model. It is found that：1） High sea surface temperature（ SST） of the Yellow Sea（ YS） is favorite for upward transportation ofwater vapor flux,which enhances the cumulus convections,and intensifies cyclone by latent heat release.The southwesterly steering flow has little influence on the track of the intensified cyclone,which keep moving northward and turns southward midway. 2） Another important factor contributing to the track turning is the existence of the energy front over the YS,which is generated by the large SST gradient（ SST in the southern YS is about 4 degrees higher than that in the northern YS）. In addition,the north winds in the low and middle troposphere and the withdrawing of the WPSH also help the cyclone turn southward.