Unraveling Atmosphere and Surface Boundary Interactions Behind Extreme Tropical Rainfall: A Case Study in Indonesia Using Fully Coupled Atmosphere-Hydrology Simulations

This study investigates the interactions of ocean and land surface boundary conditions with the atmosphere during extreme rainfall associated with the flooding over South Kalimantan on January 10–18, 2021. The interactions are analyzed through a set of sensitivity tests that involve perturbations in ocean and soil surface conditions. Our results indicate that wet soil conditions alleviate inland rainfall propagation from the ocean by promoting deep convection through the modulation of background winds, while dryer soil weakens the convective rainfall and limits the convective initiation to localized regions. Additionally, flooded surfaces enhance wind flow, allowing moisture to spread over a broad area. Ocean surface feedback to rainfall only exhibits positive feedback over the ocean, while inland rainfall is more complex, driven primarily by wind dynamics. Under extreme synoptic conditions, background winds, forced by gravity waves and Madden-Julian Oscillation, play a more significant role in modulating rainfall propagation than land-sea breezes or diurnal temperature cycle. Ultimately, in an energy-controlled regime, atmospheric dynamics are found to be the dominant factors in the ocean-land-atmosphere interactions, with surface fluxes partitioning playing a secondary role.