THE STATISTICAL CHARACTERISTICS OF IMPACT OF THE WESTWARD-GOING TROPICAL CYCLONES ON RAINFALL IN YUNNAN PLATEAU

Using data available from the Retrieval System Based on Yearbooks of Tropical Cyclones over the Western North Pacific,NCEP/NCAR reanalysis daily data and observed precipitation data for 1959 to 2007 in Yunnan,a province located in a low-latitude plateau,this work analyzes the climatic characteristics and the corresponding large-scale circulation patterns related to the western North Pacific westward moving TCs(WMTCs).Its impacts on the rainfall in the Yunnan Plateau are studied.Results show that WMTCs happen almost every year,mainly from July to September.It shows a downward trend in decadal variation.Nearly the entire Yunnan area is affected by them but the eastern part experiences the most severe influences.Most of the WMTCs migrate from the South China Sea,primarily make landfall in Hainan and Guangdong and enter the Northern Bay.The tracks of these typhoons can be classified into five categories,in which the most significant impact results from those making landfall in Guangdong.All categories of the tropical cyclones can induce province-wide heavy rainfall in Yunnan.Super typhoons bring about the heaviest and most extensive rainfall over the low-latitude plateau while the associated circulation pattern is marked with a dominant 500 hPa meridional circulation at middle latitudes,an active monsoon depression and Intertropical Convection Zone(ITCZ) at low latitudes and a westward-located South Asia High at 100 hPa,which is favorable for tropical cyclones to travel westward.WMTCs tend to go westward into the interior part of China if the subtropical high extends its westernmost ridge point to the northeast of Yunnan,or expands its periphery anti-cyclonic circulation to the Tibetan Plateau,or merges with the Qinghai-Tibetan high.

Genetic diagnosis for heavy typhoon rainfall attenuated by Fujian landfall

This study used the dynamic synthetic analysis method to analyze the causes of attenuated heavy rainfall from a westward moving typhoon after landfall over Fujian by focusing on the genetic diagrgnosis of the strongest 12 h rainstorms based on typhoon data obtained from the Shanghai Typhoon Institute,precipitation data from Fujian Province,and NCEP reanalysis data from the United States.The results showed that:(1)the environmental field of the westward moving typhoon benefits the long-term maintenance of convergence in coastal areas,which provides synoptic scale forcing for rainstorm intensification along the southeastern coast;(2)the southwest jet in the boundary layer transports warm water vapor from low latitudes into the eastern circulation of typhoon;the water vapor peak occurs 6 h before the strongest rainstorm and can be used as a reference index to predict heavy rainstorms;(3)the high altitude strong divergence center is located at 100-150 hPa,and the strong convergence center is located near 925-950 hPa in the boundary layer,which is higher(lower)than the 200 hPa divergence layer(850 hPa convergence layer)commonly used in professional work;(4)warm and wet advection in the boundary layer transports unstable energy and weak cold air southward,strengthens the baroclinic pressure,increases the latent heat flux on the sea surface,and plays a significant role in triggering and developing mesoscale convective clouds along the southeast coast.