Diagnosing And Simulating The Maintenance And Intensification Of The Landfall Tropical Storm "Bilis" In Association With The Vapor Transports In Lower Latitudes

The landing tropical storm Bilis(coded as0604) is a typical example that vapor transports in lower latitudes impact landing tropical storms. The influence of moisture transportation from lower latitudes, esspically the cross-equatorial flows on Bilis’sustention and precipitation are performed with the WRF(weather research and forecasting) model and the HYSPLIT v4.8model, based on the dataset of NCEP/NCAR reanalysis, JMA Tropical Cyclone Best Tracks. The study builds theoretical basis for tropical storms torrential rains predication. The conclusions are as follows:First of all, the cross-equatorial flows, esspically at Somali and Bay of Bengal were active, then the monsoon surge outbreak after Bilis coming ashore and led to stronger water vapor flux into Bilis, which was helpful to the maintainance and intensification of the tropical storm. After Bilis coming ashore, rain distribution show asymmetry obviously, completing the shift of the rainfall center from the north to south side of the Bilis and rainfall is significantly increased before the landfall. The water vapor for rainstorm increase mainly comes from strong southwest monsoons and the cross-equatorial flow transportation.Secondly, Somali jet and cross-equatorial flows at80～100。E was in favor to the cyclonic circulation’s sustention, cutting down the moisture supply from lower latitudes can remarkably weaken the rainstorm intensity and range from Bilis. By using the WRF mesoscale numerical model, the features of Bilis moisture transport are simulated. The result suggests that the decrease of water vapor from low latitude was not in favorable for the maintenance of the tropical storm "Bilis" and the increase in rainfall. The cross-equatorial flows at80～100°E and40～60°E was in favor to the cyclonic circulation’s sustention. Cutting down the moisture supply from Somali jet stream can remarkably weaken the precipitation from Bilis. The decrease of water vapor from low latitude can remarkably weaken the corresponding areas’ rainfall in terms of weakening convective activities which play a major role in typhoon sustention.Thirdly, on the whole life cycle of Bilis has four major water channels, respectively, Somalia, Bay of Bengal, cross-equatorial flow at120°E and the eastern Pacific water vapor. The water vapor transport of the Bay of Bengal play an important role on the rainfall intensification from Bilis. By using HYSPLITv4.8modle, the vaporinflow corridors of the water vapor transportation and the contribution of vapor for major passages analyzed during the activity of Bilis. The results show that southwest water vapor transport from Somalia and the Bay of Bengal is most important, while cross-equatorial flow channel at120°E and water vapor from the eastern Pacific from northeast of Bilis into the area. Compared the water vapor contribution rate of each channel while before with after the rainstorm increase, southwest water vapor transport from the Bay of Bengal considerably enhance after the increase in downpour from Bilis. Therefore, low latitude water vapor from the Bay of Bengal increase plays a significant role on the formation and development of the increase in torrential rains.