Role of Moisture on Rapid Intensification of the Super Cyclone Amphan (2020) Under Different Ocean Surface Forcings Over the Bay of Bengal

Abstract

The present study examines the moisture regulation and its impact on the energetics and rapid intensification (RI) phases of a super cyclone (SuCS) Amphan (2020) over the Bay of Bengal. Surface ocean forcing in terms of sea surface temperature (SST) is utilized from two operational models (HYCOM and NEMO) to simulate the Amphan using the Weather Research and Forecasting (WRF) model during 0000 UTC 16–20 May 2020. Three numerical experiments (CNT, NEM, and HYC) were conducted with Global Forecast System (GFS) initial conditions at 9 km and 3 km resolutions. The results show that strong lower-level moisture supports enhanced vertical updrafts in CNT and NEM, leading to overestimated intensification of RI and SuCS phases compared to HYC. Further, moisture budget analysis reveals increased moisture advection and flux convergence resulting in vigorous precipitation in CNT and NEM than HYC. Further, energetics analysis depicts the steepest accumulation of latent and kinetic energy in CNT, followed by NEM and HYC. In contrast, the moderately moist HYC exhibits realistic energy regulation during the RI, SuCS, and post-SuCS phases compared to CNT and NEM. This is noted to be driven by moderate vertical updrafts in HYC that enable accurate moisture advection and its regulation during RI and SuCS phases. In totality, the realistic moisture regulation in HYC is linked to the mid-level overturning circulation (500–300 hPa) and diabatic heating specifically during the post-SuCS phase, which is not attained by CNT and NEM. These findings have direct implications for operational forecasting and disaster management, aiding readiness and policy formulation.