The Impacts Of Cyclonic Storm Activities Over The Bay Of Bengal On Rainfall Events In China

The tropical cyclones in both western North Pacific and the Bay of Bengal in the Northern Indian Ocean can impact China precipitation. However, researches on the Bay of Bengal storms’influence mechanism are far from enough compared to the one of typhoons. In this study, statistical features of the cyclone activities over the Bay of Bengal and their impacts on China precipitation are studied firstly. Then synoptic analysis is performed to investigate the causes of a heavy rainfall event in Southwest China related to storm Akash (0701). Finally, numerical simulation and sensitivity experiments are implemented to understand the impact of0202storm over the Bay of Bengal on heavy rainfall event in Southwest China and the mid and lower reaches of the Yangtze River. The main conclusions are summarized as follows:Statistical and composite analyses show that there are two peaks of the Bay of Bengal storms activities appearing from April to June and September to December respectively. And they mainly affect the precipitation in Southwest China and the mid and lower reaches of the Yangtze River. The key situation of a storm to exert an impact is that a southwest low-level jet (LLJ) between the storm and the western North Pacific subtropical high (the Subtropical High) can be built up to transport water vapor from the Bay of Bengal to rainfall regions in China. Moisture forward tracking indicates that the water vapor of storms enters Yunnan via the Indochina peninsula and then is transported into the mid and lower reaches of the Yangtze River. On the other hand, the moisture backward tracking of precipitation areas also shows that the Bay of Bengal is one of the significant moisture source regions.The vast majority of the Bay of Bengal storms (about98%) cannot move to China as far as its center position. However, some cloud clusters split from the storms can climb up northwardly to the Qinghai-Tibet Plateau or the Yunnan-Guizhou Plateau, which is one of the important ways that the storms over the Bay of Bengal lead to China precipitation based on statistical results from2001to2011. Numerical simulation on the0202storm over the Bay of Bengal reveals that its influences on China precipitation are closely related to its split-cloud-clusters climbing up northwardly.The ability of the storm to split cloud clusters and to transport water vapor to China precipitation region is closely related to the storm structure change. The storm structure changes distinctly when it splits cloud clusters climbing up northwardly, from symmetrical vortex to an oval one with splitting centers. Sensitivity experiments show that the cloud system of an enhanced storm is too tight to split cloud clusters easily. However, a weakened storm is difficult to provide enough cloud clusters and water vapor to China rainfall region with a dissipated cloud system and weak circulation.Diagnostic analysis on storm Akash (0701) also indicates that the cloud clusters split from the storm climb up northwardly to the Qinghai-Tibet Plateau to strengthen the clouds in front of the east-moving trough, resulting in the heavy rainfall in Southwest China. It is found that net inflow of water vapor, growth of moist baroclinicity and strong conditional symmetric instability are all favorable for this heavy rainfall event.