Analysis on cause of rapid intensification of asymmetrical Typhoon Hato (1713) over the offshore of China

Using the 6-hourly reanalysis data of European Center ERA-Interim with horizontal resolution of 0.125°×0.125°, the daily sea surface temperature (SST) data of NOAA with horizontal resolution of 0.25°×0.25°, and the hourly black body temperature (TBB) data of Japan HMW8 satellite with horizontal resolution of 0.05°×0.05°, the analysis on cause of rapid intensification (RI) of asymmetrical Typhoon Hato (No.1713) over the offshore of China is carried out. The results are as follows. (1) Hato has developed under the precondition that the disturbance of convection in Hato was asymmetrical and Hato had kept to be asymmetrical during its RI. The amplitude of one wave asymmetry of TBB decreased gradually and the degree of asymmetry weakened when Hato intensified. (2) Abnormally high SST of 28.5~30℃ in the northern South China Sea favorable to the development of typhoon was one of the factors causing RI of Hato. (3) The intensity change of Hato was in proportion to that of the South Asia high and the subtropical high. RI of Hato over the offshore occurred under the favorable condition that the South Asia high was strengthening and moving eastward at 200 hPa while the subtropical high was strengthening and moving westward at 500 hPa, and the low-level southwesterly monsoon flow was increasing. The enhancement of the southwestward outflow over Hato was caused by anti-cyclonic circulation of the South Asia high on 200 hPa. Remarkable strengthening of upper-level divergence and low-level convergence in the south of Hato and the resulting development of asymmetrical strong convection were important causes of RI of Hato. Strengthening and eastward moving of the South Asia high at 200 hPa and the simultaneous increasing of the low-level southwesterly monsoon flow led to a significant enhancement of the environmental vertical wind shear (VWS) which played an important role in the distribution of convection and intensity of Hato. The environmental VWS below the threshold (12.5 m·s^-1) is a key condition for typhoon RI. (4) RI of Hato was closely related to the asymmetric structure circulation induced by the strengthening and westward moving of the subtropical high and the strengthening of the southwesterly monsoon flow. Increasing and upward stretching to the middle and upper troposphere of positive vorticity near typhoon center resulting from the asymmetrical distribution of horizontal wind and the deepening of such distribution also were important causes of RI of Hato.