Environmental Thresholds of Tropical Cyclone Rapid Intensification in the South China Sea

Tropical cyclones (TCs) in the South China Sea (SCS) would cause tremendous disasters because the TCs can undergo rapid intensification (RI) very shortly before landfall. Accurate prediction of RI of the TCs in the SCS thus poses considerable challenges to forecasters issuing associated warnings and to government agencies making effective disaster prevention and mitigation.

In this study, the thresholds of a set of atmospheric and oceanic variables which are possibly related to the occurrence of RI of the TCs in the SCS are examined using a new method. Compared with methods in previous studies, the environmental thresholds used in the current study have better ability to distinguish RI cases from non-RI cases based on the probability of RI (PRI). Vertical wind shear, low-level convergence, and low-level moisture flux convergence are the critical environmental variables associated with the RI in the SCS. The PRI reach the highest when there are 6 (for Tropical Storm) or 5 (for Severe Tropical Storm, Typhoon & Severe Typhoon) variables meet the thresholds. The specific variable combinations with the highest PRI for different TCs intense categories are also given in this article.

An independent test for the 2013 and 2014 typhoon seasons indicates that the PRI is less than 6% for the majority (68.88%) of non-RI cases and the lowest PRI for the RI cases is 7.69%, suggesting that the environmental thresholds proposed in this study have promising skill in intensity forecasting of the TCs in the SCS.