The Impact Of Predecessor Rain Events Associated With Bay Of Bengal Tropical Cyclones On Southwest China

The tropical cyclones over the Bay of Bengal(BOB TCs)is one of the important weather systems which can cause heavy rain in southwest China,and it can cause predecessor rain events(PREs)in southwest China before the circulation of main body landing.In this paper,by using the NCEP reanalysis data,the ERA-Interim reanalysis data,the TBB data,TRMM satellite rainfall data and the JTWC data,the predecessor rain events associated with the Bay of Bengal tropical cyclones were identified objectively by using Eulerian-Lagrangian method,the climatological characteristics of the predecessor rain events were examined by using composite analysis on this basis,and the evolution of the predecessor rain events was illustrated by using statistic and diagnostic analysis.The main conclusions show that:A total of 21 PREs that occurred over southwest China during 1981–2012 were identified as being associated with 21 BOB TCs.The PREs occur more frequently in the pre-monsoon season of April-May and the post-monsoon season September-November.with peaks in May and October respectively.The PREs occur frequently in southwest China,especially in the southwest of Yunnna Province,when TCs track northward or northeastward.The median PRE lifetime is two days.BOB TCs can produce remote PREs in southwest China with the median maximum daily rainfall of 73 mm.The median distance between the PREs and parent TCs is 1266 km.The key configurations favorable for PRE occurrence over southwest China are the westward extension of the western North Pacific subtropical high dominating the South China Sea,a trough that is partially a signature of the TC circulation presented in the northern BOB,and the East Asia long-wave trough located in the east coast of China,about 120°E.The the upper-level East Asia subtropical jet(EASJ)is located on30°N,PRE regions are situated beneath the equatorial-ward entrance of the EASJ.The southwesterly flow appears associated with strong pressure gradient between the subtropical high and the BOB trough,which is beneficial for steering TCs conveying abundant water vapor from the BOB to the PRE regions.PREs associated with BOB TCs is a product of the tropical cyclone,subtropical western Pacific high,EASJ,the south trough and the east Asian trough.BOB TCs provided abundant moisture and strong moisture convergence for the PRE regions,pressure gradient between the TCs,the south branch trough and the subtropical high strengthened the transport of moisture;PRE regions are situated beneath the equatorial-ward entrance of the EASJ,and the secondary circulation strengthen ascent and instability over PRE regions.The interaction of cold air and warm air is beneficial to produce frontogenesis and baroclinicity precipitation.Case analysis shows that the upper-level diabatically driven outflow from the TC can affect the intensity and form of the EASJ,provide favorable conditions for the PRE maintain,which provided favorable conditions for precipitation.