Tropical cyclogenesis identification using eddy detection technique for the Bay of Bengal Basin

Abstract

The study deals with formative stages and lead detection time of tropical cyclogenesis using Okubo-Weiss Parameter, a proven robust eddy detection technique used for tropical cyclones that developed over the north Indian Ocean basin. Specifically, it aims to identify the initial traces of upper atmospheric genesis and understand the spatio-temporal evolution of pre-cyclonic eddy vortices in the atmospheric column using high-resolution WRF model outputs simulated for different cyclones. A coarser resolution grid of 27 km was used for identification purpose and a finer resolution of 9 km used to evaluate the characteristics of eddy vortices. Case studies were performed with four post-monsoon severe cyclones viz.; Phailin(2013), Vardah(2013), Gaja(2018), Madi(2013) and two pre-monsoon cyclones Mora(2017) and Aila(2009) that developed over this region. Study reveals a good skill in genesis prediction with a minimum of four days (~ 90 h) lead time in all cases irrespective of pre- and post-monsoon seasons. Factors that favor cyclogenesis during pre-monsoon season (except relative humidity) contribute to early detection of pre-cyclonic vortices in the atmosphere. Initiation mechanism of tropical cyclogenesis occurs at upper atmospheric levels and also detected at higher lead time for pre-monsoon cases unlike the post-monsoon cases. A lower relative vorticity at 850 hPa in pre-monsoon seasons is a major factor that initiates pre-cyclonic vortices at relatively higher levels compared to post-monsoon cyclones. Study also investigated the behavior of eddies in the atmospheric column for a non-developing case and compared these findings against developing cases. The study signifies that this technique is quite promising and has potential for early detection of tropical cyclogenesis in the atmospheric column prior to satellite detection over ocean surface.