Environmental Influences On The Rapidly Intensifying Tropical Cyclones In The South China Sea

Based on the CMA-STI Tropical Cyclone(TC) Best Track Data, the spatiotemporal characteristics of rapidly intensifying TCs in the South China Sea(SCS) from 1979 to 2012 are investigated in this study. The results show that rapid intensification mainly appeared to the southeast of Hainan Island and in the center of the SCS. Since 1990 s, the frequencies of TCs and intensification events in the SCS have decreased. Rapid intensification tended to happen from July to November with the highest frequency in the transition from summer to autumn. Additionaly, rapid intensification favoured occurring in the daytime.Based on the ERA-Interim reanalysis data(4 times daily), we propose a new threshold calculation method to analyze the environmental roles in rapid intensification. The environmental factors of interest include vertical wind shear, high- and low-level divergence, low-level vorticity, low-level moisture flux divergence, mid-level relative humidity, sea surface temperature, and ocean heat content. It is shown that vertical wind shear, low-level convergence, and low-level moisture flux convergence are the key factors governing the rapid intensification. For TCs weaker than severe typhoons, the stronger the intensity, the higher the probability of the occurrence of rapid intensification is. Without the consideration of ocean heat content, we found that the probability of the occurrence of rapid intensification is highest as 6(for tropical storms) or 5(for severe tropical storms, typhoons, and severe Typhoons) among the foregoing 7 factors meet the thresholds.The tracks, location of intensification and intensity before intensification of Super Typhoon Rammasun(1409) and Typhoon Kalmaegi(1415) were similar, but Rammasun underwent four rapid intensifications while Kalmaegi did not. The first two rapid intensifications of Rammasun were associated with the abundant low-level water vapor transport due to the strong Somali cross-equatorial flow and the subtropical high. The third and forth rapid intensification s of Rammasun were because of the high sea surface temperature and ocean heat content which were caused by a warm ocean eddy in the north of the SCS.In contrast, weaker low-level water vapor transport and ocean heat content related to Kalmaegi were unfavourable to rapid intensification.