Characteristics of dynamical and thermo-dynamical variables during heavy rainfall events over the Indian region

Abstract

There has been an increase in the frequency and intensity of heavy rainfall events (HREs) over the Indian region in recent years, and it is a huge challenge for the forecaster to accurately predict HREs with adequate lead time. In this study, four HREs, i.e., Kerala (KRA), Mumbai (MUM), Chennai (CHE), and Uttarakhand (UTK), occurred over different geographical regions of India considered. Total of five days (2 days before and after the event day-D0) of the HREs are extensively investigated. The ECMWF Fifth Generation Reanalysis (ERA5) and India Meteorological Department (IMD) data are used to examine rainfall and associated variables in these HREs. ERA5 is able to capture rainfall reasonably well in most of the HREs; however, there is a distinct underestimation found in UTK compared to IMD. Overall, the rainfall skills of ERA are mediocre for higher rainfall thresholds (>100 mm/hr). In general, higher convective available potential energy (CAPE) and lower convective inhibition (CIN) is distinctly noted for HREs. The strong monsoon flow facilitating intense moisture transport is the key factor for KRA and MUM events. Besides, unique convective signatures such as sudden and intense vertical velocity, flaring of relative vorticity, convergence, and moisture accumulation are seen for UTK, and thereafter, these variables rapidly weaken, indicating a short duration cloud burst type HRE. This is in clear contrast to other HRE events where convective conditioning and weakening of key variables occur in a gradual manner before and after D0, respectively. The ice hydrometeors are the major source of heavy rainfall in HREs except for UTK, where snow is the major contributor of rainfall in terms of rainwater and cloud liquid water. Further, it is revealed that an elevated moist static energy both at the lower and upper troposphere is found for UTK and MUM events compared to CHE and KRA. The intense lower troposphere moisture, low-level convergence, upper-level divergence, and intense relative vorticity, along with vertical motions, are key precursors for these HREs. Finally, a consolidated schematic flow of mechanisms responsible for HRE is proposed. The findings of this study are highly useful for improving early warning and disaster preparedness.