The Contrasting Role of Soil Moisture on the Diurnal Cycle of Precipitation During Dry and Wet Seasons Over the Maritime Continent

Soil moisture strongly regulates land–atmosphere interactions, yet its influence on rainfall over the Maritime Continent remains uncertain. Using a coupled land–atmosphere regional model, we compare control simulations (CTL) initialized with realistic soil moisture against sensitivity experiments (SENS) initialized with dry soil conditions for two dry (May 2013, 2023) and two wet (December 2014, 2023) seasons. The results reveal a pronounced seasonal asymmetry in the role of soil moisture on precipitation over the islands of the Maritime Continent. In dry seasons, reduced soil moisture in the SENS experiment suppresses rainfall by reducing transpiration and direct soil evaporation relative to CTL. This leads to a sharp reduction in surface latent heat flux and weaker afternoon convection. In contrast, wet-season rainfall increases in SENS due to increased moisture advection from surrounding ocean despite reduced terrestrial evaporation. Analysis of the diurnal cycle shows a pronounced suppression of the afternoon precipitation peak in the dry seasons in SENS, whereas in the wet seasons, the diurnal peak is weakened and flattened, but nocturnal rainfall is enhanced. Evaporation partitioning helps explain this seasonal contrast: in the dry seasons, transpiration is reduced under SENS, while in the wet seasons, canopy evaporation persists due to frequent rainfall that sustains canopy wetness. These findings highlight the important role of land–atmosphere coupling in the dry seasons and greater oceanic control in the wet seasons, and call for better observation of these processes to improve rainfall predictions over tropical islands.