Investigation of an extreme rainfall event over the Western Ghats of India using radar data

Extreme rainfall events (ERE) over complex orographic regions are high-impact and often lead to catastrophic cascading disaster due to the underlying terrain effect. We investigated an extreme rainfall event that occurred over a high-altitude site (HACPL) in Mahabaleshwar over the Western Ghats of India on July 16, 2018. Disdrometer installed at HACPL recorded a rainfall amount of 256 mm. Data collected with an X-band radar was used to examine the spatio-temporal features of precipitating clouds during ERE. The ERE was associated with concurrent occurrence of three prominent systems, a low-pressure system over Bay of Bengal, a mid-tropospheric cyclone and an off-shore monsoon trough. Most of the ERE rainfall (60%) was associated with prolonged moderate rain (>10 mm/h) through nocturnal and morning hours. Rainfall microphysics indicated ERE dominated by fewer and bigger raindrops, with unimodal distributions of Dm, Z and log10(Nw) and modes around 1.1 mm, 36 dBZ and 4, respectively. Radar observations revealed that cumulus congestus clouds played a key role, occurred only 27% of the time but contributed 42% of the total ERE rainfall. The spatial extent of the ERE was found to be confined over the elevated slopes of the Ghats, reflecting strong orographic lifting. Time series of reflectivity profiles showed a transition from shallow/warm clouds to cumulus congestus on July 16, followed by a shift to shallow clouds. The vertical development of clouds on ERE day was found to be associated with enhanced moisture transport to the lower and mid-tropospheric levels by the intensified monsoon low-level jet. These results highlight the coupled role of orography, dynamics and microphysics in producing extreme rainfall event over complex terrain.