A STUDY OF RAPID DECAY TYPHOONS IN OFFSHORE WATERS OF CHINA

On the basis of NCEP/NCAR reanalysis data and yearbooks of CMA tropical cyclones, statistical analysis is performed for 1949—2013 offshore typhoons subjected to rapid decay(RD). This analysis indicates that RD typhoons are small-probability events, making up about 2.2% of the total offshore typhoons during this period. The RD events experience a decadal variation, mostly in the 1960 s and 1970 s(maximal in the 1970 s), rapidly decrease in the 1980 s and 1990 s and quickly increase from 2000. Also, RD typhoons show remarkable seasonal differences: they arise mainly in April and July-December, with the prime stage being in October-November. The offshore RD typhoons occur mostly in the South China Sea(SCS) and to a lesser extent in the East China Sea(ECS); however, none are observed over the Huang Sea and Bo Sea.Composite analysis and dynamic diagnosis of the RD typhoon-related large-scale circulations are performed.Physical quantities of the composite analysis consist of 500-h Pa height and temperature fields, vapor transfer, vertical wind shear(VWS), density of core convection(DCC), and high-level jet and upper-air outflow of the typhoon. The results suggest that(1) at the 500-h Pa height field, the typhoon is ahead of a westerly trough and under the effects of its passing trough;(2) at the temperature field, the typhoon is ahead of a temperature trough, with an invading cold tongue present;(3) at the vapor transfer field, water transfer into the RD typhoon is cut off; and(4) at higher levels, the related jet weakens and the outbreak of convection becomes attenuated in the typhoon core. In addition, VWS bears a relation to the RD typhoon; in particular, strong VWS favors RD occurrence.The differences in RD events between the SCS and ECS show that for the RD, the VWS of the ECS environmental winds is markedly stronger in comparison with its SCS counterpart. The cold advection invading into the typhoons is more intense in the SCS than in the ECS, and the low-level vapor transfer and high-level outflow are weaker in the SCS RD typhoons.Data analysis shows that sea surface temperature(SST), VWS, and DCC can be employed as efficient factors to predict RD occurrence. With appropriate SST, VWS, and DCC, a warning of RD occurrence can be given 36, 30-36,and 30 h, respectively, in advance. These values suggest that atmospheric SST responses lag. Owing to this time lag,the prediction of RD typhoons is possible.