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

lkj & Cakxky dh (kkM+h esa m".kdfVca/kh; pOokrksa ds fy, ,d nqzrxfr rhozhdj.k lwpdkad ¼vkj-vkbZ- vkbZ-½ fodflr fd;k x;kA vkj- vkj- vkbZ- 24 ? kaVksa vkSj mlds ckn ds nzqrxfr rhozhdj.k ¼vkj- vkbZ-½ dh laHkkouk dk vkdyu djus ds fy, m".kdfVca/kh; pOokrksa ds vfHky{k.kksa dk cM+s iSekus ij mi;ksx djrk gSA 24 ? kaVksa ds nkSjku vkj- vkbZ- dks 30 ukWV ¼15-4 ,e-,l- -1 ½ ij rhozrk dh of) ds :Ik esa ifjHkkf"kr fd;k x;k gS tks 1981&2010 ds nkSjku caxky dh (kkM+h esa cus m".kdfVca/kh; pOokrksa ds 24 ? kaVs dh rhozrk ds ifjorZuksa ds 93osa 'kred dks n'kkZrk gSA blesa ;g ns(kk x;k gS fd 32 izfr'kr vfr izpaM lHkh pOokrh rwQku ¼oh- ,l- lh- ,l-½ vkSj lHkh egkpOokrh rwQku ¼,l- ;w- lh- ,l-½ viusvksa ls lac) cM+s iSekus ij fofHkUu ifjorZuksa dh xSj vkj- vkbZ- fLFkfr;ksa ds lkFk rqyuk dh xbZ gSA bu rqyukvksa ls ;g irk pyk gS fd vkj- vkbZ- dh fLFkfr;ki lkekU;r% mPp v{kka'k ij ? kfVr gksrh gS vkSj xSj vkj- vkbZ- fLFkfr;ksa dh vis{kk 12 ? kaVs igys vR;ar rsth ls rhoz gks tkrh gSaA lkaf(;dh; fo'ys"k.k ls Hkh ;g irk pyrk gS fd vkj- vkbZ- dh fLFkfr;ki mu {ks=ksa ls tqM+h gqbZ gksrh gS tgki ij mifjru Lrj ds vilj.k) fupys Lrj dh lkisf{kd Hkzfeyrk vkSj lkisf{kd vknzZrk vf/kd gksrh gS rFkk ftlesa viuhvksa dh vis{kk rhoz xfr ls LFkkukarfjr gksrh gqbZ vkxs c<+rh gSA vkj- vkbZ- vkbZ- rduhd dks vkB ifjofrZrkvksa ds la;qDr vkjafHkd ¼lwpdkad½ ekuksa }kjk fodflr fd;k x;k gSA ftuds fy, vkj- vkbZ- vkSj xSj vkj- vkbZ- fLFkfr;ksa ds e/; lkaf(;dh; :i ls mYys(kuh; fHkUurkvksa dk irk pyk gSA ftl le; v{kka'k dh dqy la(;k 'kwU; ls vkB rd igiqprh gS ml le; vkj- vkbZ- dh laHkkO;rk esa 0 izfr'kr ls 100 izfr'kr rd dh of) ns(kh xbZ gSA 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 rd 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.