Large-scale characteristics of rapidly intensifying tropical cyclones over the Bay of Bengal and a rapid intensification (RI) index

Abstract

A rapid intensification index (RII) is developed for tropical cyclones over the Bay of Bengal. The RII uses large-scale characteristics of tropical cyclones to estimate the probability of rapid intensification (RI) over the subsequent 24 hrs. The RI is defined as an increase of intensity 30 kt (15.4 ms-1) during 24 hrs, which represents approximately the 93rd percentile of 24 hrs intensity changes of tropical cyclones that developed over the Bay of Bengal during 1981-2010. It is found that 32% of all very severe cyclonic storms (VSCS) and all super cyclonic storms (SUCS) underwent RI phase at least once during their lifetime. Various large-scale variables associated with the RI cases are compared to those of non-RI cases. These comparisons show that the RI cases generally occur at higher latitude and are intensifying at a faster rate during the previous 12 hrs than the non-RI cases. The statistical analysis also shows that the RI cases are embedded in regions where the upper-level divergence, lower-level relative vorticity and relative humidity are more and vertical winds shear is less than certain threshold values of the respective variables. Finally, the initial wind speed of RI cases is higher and tends to move with a faster translational speed than the non-RI cases. The RII technique is developed by combining threshold (index) values of the eight variables for which statistically significant differences are found between the RI and non-RI cases. The probability of RI is found to be increases from 0% to 100% when the total number of indices satisfied increases from zero to eight.