Rapid Intensification of Tropical Cyclones and Vertical Wind Shear over the Northwest-Pacific

In this paper the statistical characteristics of per 24h intensity change for tropical cyclone (TC) are analyzed, the thresholds of tropical cyclone rapid intensification (RI) are defined by using the tropical cyclone best track data of Joint Typhoon Warning Center (JTWC) and the NCEP/ NCAR reanalysis data from 1990 to 2009, and with comprehensive comparison and analysis of initial intensity, the seasonal variation and spatial distribution characteristics of TC rapid intensification are given in different thresholds of tropical cyclone rapid intensification. According to the calculation, the paper defines the thresholds of tropical cyclone rapid intensification as 25KT, 30KT, 35KT, 40KT, which respectively represent the 88th, 90th, 93th, 96th percentiles of 24-h overwater intensity changes of tropical cyclones in the Northwest-Pacific area. The initial intensity of tropical cyclone rapid intensification will increase when the threshold increases. About 60% of tropical cyclones undergo RI during the period from August to October. And, most of the tropical cyclones undergo RI in the region of 125°E-150°E,10°N-25°N. It is found that the relationship between the vertical wind shear at different levels in troposphere and tropical cyclone rapid intensification is different. When the threshold of tropical cyclone rapid intensification is 40KT in 200-500hPa, 500-850hPa and 200-850hPa, only 9%, 5%, 7% of tropical cyclones can undergo RI. The correlation coefficient between tropical cyclone rapid intensification and the vertical wind shear at different levels are -0.15, 0, -0.04 respectively. So, it is obvious that the vertical wind shear in 200-500hPa is less favorable for TC to undergo RI. The vertical wind shear can be divided into east vertical shear and west vertical shear. When the threshold is 40KT in 200-850hPa, about 57% of the TC can undergo RI under the circumstance of east vertical shear.