Role of large-scale and microphysical precipitation efficiency on rainfall characteristics of tropical cyclones over the Bay of Bengal

Accurate prediction of rainfall is one of the significant challenges in TC forecasting. In this study, the emphasis is to investigate precipitation efficiency (PE) and associated factors responsible for copious rainfall associated with tropical cyclones (TCs). Two pre-monsoon TCs, i.e., Fani and Yaas, that made landfall over the east coast of India and caused devastation, are considered for this study. Simulation of the TCs was performed using the Advanced Research Weather Research and Forecasting (ARW-WRF) model for up to 96 forecast hours. Results suggest Yass (VSCS), being relatively weaker, TC produced a much higher amount of rainfall compared to Fani (ESCS). The heavy rainfall in Yaas is robustly facilitated by large-scale environmental conditions such as intense low-level vertically integrated moisture flux transport, precipitable water, and low-level convergence. In addition, it is also found that both the large-scale precipitation efficiency (LSPE) and the cloud microphysics precipitation efficiency (CMPE) were significantly higher in the case of Yaas (VSCS), facilitating its intense rainfall characteristics compared to Fani (ESCS). The higher LSPE is regulated by the strong signatures of water vapor flux convergence, atmospheric drying, microphysical consumption of water vapor in the lower part of the troposphere, and gain of solid hydrometeors in the upper troposphere. Overall, the unprecedented intense large-scale moisture transport in Yaas set up a conducive environment for higher precipitation compared to Fani. Our results suggest that the interactions between large-scale environmental systems and local scale precipitation efficiency are key for accurately determining the rainfall and intensification of the TC.