Comparative case studies of environmental factors and vortex features in developing and non-developing tropical disturbances over the Bay of Bengal

Abstract

Pre-existing tropical disturbances often serve as a precursor to tropical cyclone (TC) formation. However, a very small percentage of these disturbances turn up as TCs, depending on the environmental conditions. Disturbances that remain non-developed throughout their lifespan have very similar characteristics to the disturbances that develop as a TC. Therefore, it is important to comprehend the environmental factors responsible for transforming tropical disturbances into tropical cyclones. In this study, we analyzed the environmental conditions surrounding the disturbance vortex, as well as the structural evolution of the vortices during the pre-genesis stage, for two non-developing and two developing disturbances (which later evolved into different categories) over the Bay of Bengal. The analysis was conducted using output data from the Advanced Research Weather Research and Forecasting model, blackbody temperature observations from INSAT-3D, and daily Optimum Interpolation Sea Surface Temperature analysis data. All the disturbances processed over the Indian Ocean warm pool region where the sea surface temperature (SST) was above the threshold of TC genesis. However, the SST anomaly was negative ahead of the non-developing disturbance and positive for the developing disturbances. The main characteristics of the developing disturbances that differed from the non-developing disturbance are the upright structure of the vortex with undistorted vortex core from the dry air intrusion, larger area coverage of cold cloud top with persistent cyclonically spiral cloud bands centred at the vortex core, and the extension of the strong ascending region over time. The intensity of low-level vorticity was unable to identify the developing potential of the disturbances. The synchronous incremental variation of the kinetic energy density of primary and secondary circulations and the vertical helicity in the developing disturbances was helpful for their sustainability.