Governing factors of the unprecedented extreme rainfall over Rameswaram Island

Abstract

On 20-21 November 2024, extreme precipitation, the first of its kind to occur in the past 125 years, resulted in severe flooding across the southern coastal region of Tamil Nadu in particular over Rameswaram. The dynamic and thermodynamic conditions in the atmosphere that precede this extreme precipitation event are examined using reanalysis datasets of ERA5, brightness temperature, and Sea Surface Temperature. The analysis reveals deep convective clouds, a large amount of precipitable water, elevated specific humidity, and wind convergence in the lower troposphere at Rameswaram. The positive sea surface temperature (SST) anomaly of 2°C in the Bay of Bengal could lead to significant evaporation. Intense moisture is transported from the Bay of Bengal by easterly winds, and the westward-propagating tropical easterly waves facilitate the advection of positive PV anomalies towards Rameswaram. The presence of a cyclonic circulation progressing towards the southwest coast of the Indian Peninsula is evident in the moisture flux divergence. Convection was supported and sustained by enhanced upward vertical velocity between 850 and 200 hPa. Moreover, the positive potential vorticity (PV) tower extending from 850 hPa to 200 hPa, points to the development of dynamic instability. Vertical and horizontal PV dipoles, signatures of deep convection and instability, are also identified over Rameswaram during heavy rain. The time-height evolution of the divergence, vertical velocity (ω), and PV anomaly, along with the mid-level vortex, suggests that the 'top-down' dynamics can be responsible for the torrential downpour over Rameswaram.