Environmental Flow Field Vertical Shear Preliminary Analysis Of The Impact Of Tropical Cyclone Intensity Change

Using tropical cyclone (TC) best track and intensity data of Western North Pacific from the Joint Typhoon Warning Center (JTWC) of U.S.A. and the NCEP/NCAR Reanalysis data for the period of 1992—2002, the effects of vertical wind shear on TC intensity are examined. The samples are limited to westward or north-westward moving TC between 5° N to 20° N in order to minimize thermodynamic effects. It is found that:(1) The effects of vertical wind shear between 200hPa and 500hPa on TC intensity change is larger than that of the shear between 500hPa and 850hPa, while similar to that of the shear between 200hPa and 850hPa.(2) There does exit a threshold value of vertical wind shear for TC development while it decrease for stronger TC.(3) With weakening developing trend of TC, the average shear increases, implying that large shear tends to inhibit the development of TC or even cause it to weaken. The less the shear is, the better it is for the development of TC.(4) The developing time spent increases with the shear weakens of the same kind of TCs.(5) By the "Relative Time" analysis, the average shear of TC which can develop into TY (TD or TS) is below 7m/s (above 8m/s).We also use model WRF to model two different TCs in statistical analysis to find out the differences in the development periods. The results showed that favourable large-scale environment can also make TC grow strong with large vertical wind shear.