Seasonal changes in diurnal rainfall over Sri Lanka and possible mechanisms

Abstract

This study examines the diurnal rainfall characteristics (including phase and amplitude) and their seasonality over the second-largest island in the tropical Indian Ocean, Sri Lanka (hereafter LKA), and discusses the possible causes. Analyses of satellite precipitation estimates (2001–2019) show that over LKA, (1) diurnal rainfall maximum appears at 12 UTC (17:30 local time) in all seasons, (2) diurnal rainfall amplitude is larger in spring and autumn than in winter and summer, and (3) the dominant propagation direction of diurnal rainfall is eastward in summer and westward in spring, autumn, and winter. Examinations on the atmospheric modulations show that, in all seasons, land surface warming maximum appears around 8–9 UTC, followed by an upward motion and low-level wind convergence maxima around 10–11 UTC. These time-lagged changes in atmospheric conditions further help transport more moisture from the nearby seas to LKA and lead to a diurnal rainfall peak at 12 UTC in all seasons. Additionally, all these maintenance mechanisms show larger diurnal amplitude in spring and autumn than in winter and summer, resulting in observed seasonal differences in diurnal rainfall amplitude. Furthermore, the causes of differences in diurnal rainfall propagation over LKA are attributed to the combined effects of the interaction between the diurnally varying land-sea breeze and orography, the interaction between the land breezes from LKA and nearby South India, and the modulations from the seasonal difference in prevailing low-to-mid-level winds. These findings reveal how multiple-scale atmospheric changes can affect diurnal rainfall over a tropical island in the Indian summer monsoon lead region.