Rapid Intensification and Associated Large-scale Circulation of Super Typhoon Rammasun in 2014

Most offshore tropical cyclones (TCs) would be weaken because of energy decay due to land surface friction, while a small part of them intensify instead. There is an average of 0.8 TCs per year which intensifies abruptly in offshore waters of South China. Statistical, diagnostic and numerical work reveal that sea surface temperature (SST), TC inner structure and characteristics of atmospheric circulation are the most important factors. And for the latter, strength or strengthening of a low-level jet linking with TC and weak environmental wind vertical shear are significant, and widely applied in operation to judge the TC intensity change trend. The mechanism of TC abrupt intensification in offshore waters is still not fully revealed, and therefore it is still very difficult to give an accurate forecast for these phenomena in the route operation. Wrong judge and forecast for TC abrupt intensification in offshore waters would lead to underestimation and insufficient defense of TC disasters. Super typhoon Rammasun is intensifying continuously for over 24 h before it makes landfall at Wenchang of Hainan at 0730 UTC 18 July 2014, with 40 m·s^-1 of maximal central wind speed, 960 hPa of minimal central sea level pressure at central part of South China Sea at 0000 UTC 17 Jul 2014, 72 m·s^-1 888 hPa at 0600 BT 18 Jul 2014. And Rammasun becomes the strongest landfalling TC on national record.Based on CMA TC track data, satellite and radar images, NCEP reanalysis data, NOAA high-resolution blended analysis of daily optimum interpolation SST and synoptic analysis and kinetic energy budget, study is carried out on the characteristic of abrupt intensity change of Rammasun and its cause.Synoptic analysis show that the continuous abrupt intensification of Rammasun is concerned with the favorable atmospheric background circulation, the anomalous warm SST in central and northern part of South China Sea during the middle ten days of July 2014, intensification of the southwestern low level jet and cross-equatorial flow linking with Rammasun, and the maintenance of upper level outflow especially the unstable atmosphere in the downstream region. Its moving into the unstable circumstance is advantageous to the convective activities and the efficiency of convective condensation latent heat release within the cyclone circulation, and leads to the maintenance or intensification of the TC.Kinetic energy budget output reveals, main kinetic energy source at low level is from the wind crossing through the isobar, and this is connected with vertical circulation of ascending in the central area and descending in the outer region forced by convective condensation latent head. To consider in view of convective activities and condensation latent heat release is helpful for understanding of TC intensity change and forecast in operation.