Study of an Extremely Severe Cyclonic Storm “Fani” over Bay of Bengal using regional NCUM modeling system: A case study

Abstract

This study assesses the predictive capability of the high resolution National Centre for Medium Range Weather and Forecasting (NCMRWF) regional Unified Model (NCUM-R) for pre- and post-genesis characteristics associated with a TC. We have selected an Extremely Severe Cyclonic Storm (ESCS) “Fani”, which formed over Bay of Bengal (BoB) during 26 April–04 May, 2019 for the study. To examine the pre-genesis of the storm, we used the model analyses and forecasts of NCUM-R three days prior to the genesis (i.e 26th April 2019) of the TC. The results clearly show the establishment of a moisture conveyor belt (MCB) during the pre-genesis period, which connects the TC inner core with underlying oceanic region. The MCB plays an important role in the transport of large amounts of moisture towards the storm inner core from the oceanic regions and subsequently for the release of latent heat in the vicinity of the storm, resulting the intensification of TC. These processes were clearly predicted by the NCUM-R model during the pre-genesis period. The study also examined role of various dynamic and thermodynamic variables; intensity; track and its errors and various indexes at different stages of TC. The evolution of intensity based on mean sea level pressure (MSLP) and 10 m maximum sustainable wind (MSW) clearly shows the rapid intensification and dissipation of the storm in NCUM-R simulations, the pattern is reasonably well matched with the observations. The area averaged thermodynamical variables around the eyewall of the storm in the forecast based on different initial conditions (ICs) are well simulated by the model and the pattern is well matched with the ERA5 reanalyses. The model predicted track from different ICs are reasonably well matched with the observed track. The large scale environmental flow plays a vital role in the recurvature of the TC. The study clearly suggested that the NCUM-R model is able to predict the intensity, movement and structure of the storm during pre- and post-genesis period. However, the model results show intense storm during early stage as compared to observation.