Prediction of rapid intensification for land-falling extremely severe cyclonic storms in the Bay of Bengal

During the past 30 years (1990–2020), all extremely severe cyclonic storms (ESCS) that formed over the Bay of Bengal (BoB) region had undergone rapid intensification (RI) attributed due to climate change. A detailed evaluation is therefore necessary to forecast and ascertain the rapid changes in intensity especially the RI over BoB that has direct socio-economic consequences in coastal regions. This study performed a detailed evaluation on the intensity and RI of ESCS over the BoB region using Weather Research and Forecasting (WRF) model. Observations were assimilated using conventional and satellite radiances to improve the initial conditions of four ESCSs (Mala, 2006; Sidr, 2007; Phailin, 2013 and Hudhud, 2014). Numerical experiments used a double-nested domain of 27 and 9 km horizontal resolution with 73 vertical levels. The forecasted intensity, structure, and storm tracks were in good agreement with the available observations. Mean Absolute Error (MAE) of Maximum Surface Winds (MSW) were 5 m s⁻¹, 6 m s⁻¹, 5 m s⁻¹, and 5 m s⁻¹ from Day 1 to Day 4, respectively, with mean initial intensity error of about 4.5 m s⁻¹. Results indicate that RI of ESCS was well apprehended in WRF model showing significant differences in the forecast for different cases. In most of the cases, the RI was under-predicted. The overall performance of WRF model was reasonably good in forecasting RI, and the estimated statistics for Probability of Detection (POD) and False Alarm Rate (FAR) was about 68% and 21%, respectively, based on the four cases considered in this study. Forecasted reflectivity and vertical profiles of heating rate, divergence, hydrometeors, vertical winds, and temperature anomaly are also presented in this study. Mean track error of storms varied between 75 and 135 km, at 24 to 96 h, respectively, while the mean landfall time and positional errors were about 3 h and 69 km, respectively. This research has also highlighted the need for a re-evaluation of the WRF model’s performance in forecasting the track, structure, and intensity of ESCS over BOB during the RI.