Predicting the rapid intensification and dynamics of pre-monsoon extremely severe cyclonic storm ‘Fani’ (2019) over the Bay of Bengal in a 12-km global model

Abstract

The present study investigates into genesis, intensification, dynamical behavior and the prediction of Extremely Severe Cyclonic Storm (ESCS) ‘Fani' over the Bay of Bengal (BoB) in the National Centre for Medium-Range Weather Forecasting (NCMRWF) global numerical weather prediction (NWP) modeling system ‘NCUM.’ The global model is adopted from the UK Met office's Unified Model (UM). The ESCS ‘Fani' formed from 26 April–04 May 2019, over the warm waters of BoB. In the criterion of landfalling tropical cyclones (TCs), which crossed the Orissa coast, it was the most intense cyclonic storm during pre-monsoon season since 1965. The TC exhibited a markedly different characteristic having genesis very near to the equator (near 2.7°N and 88.7°E) and possessing one of the longest tracks (~3030-km) over the BoB region. The NCUM global model was operationally run during the occurrence of the TC. The diagnosis of the TC's genesis and rapid intensification (RI) in the model is carried out using various metrics such as Genesis potential parameter (GPP), winds at 850-hPa, vertically integrated moisture flux, potential vorticity (PV) at isentropic level 315 K, Severe Weather Threat (SWEAT) index and daily averaged latent heat flux, etc. It is found that the early genesis, structure, RI, and movement of the TC were well captured by the model in advance. The Model predicted TC tracks for a total of seven initial conditions (from 27 April to 03 May 2019, at 24-h interval) are found closely co-related with the observed best track of the TC provided by India Meteorological Department (IMD). The lower values of Direct Position Errors (DPEs), along-track errors (ATEs), and cross-track errors (CTEs) indicate the skillful prediction of the TC by the model. The structure, amplitude, and location (SAL) simulated precipitation of the TC over the BoB and along the central-east coast of India are in agreement with the observed systems. Our detailed diagnostic analysis suggests that the TC formation, development, and intensification was mostly controlled by the warmer BoB sea surface temperatures.