Multi-Scale Interactions Associated with Two Offshore Rainfall Events Near the West Coast of Sumatra

This study investigates the offshore propagation of precipitation off the west coast of Sumatra on February 1 and 4, 2018. While previous studies tend to focus on seasonality, this study focuses on specific events. On February 1, convection organizes north of Padang and propagates about 100 km offshore. On February 4, convection organizes south of Padang, forms a multicellular system, and travels over 150 km offshore. The study uses ERA5, MERRA-2, IMERG, Padang radar data, and downscaled simulations with WRF-ARW V4.1.3 to understand the roles of the land breeze, gravity waves, and convective outflow.

Large-scale environmental conditions reveal stronger westerly wind from the Indian Ocean on February 1 and strong northeasterly wind from the South China Sea on February 4. Both events are associated with high surface humidity and weaker 900 mb winds along the coastline. WRF simulations reveal that on February 1, the sea breeze uniformly transitions to a land breeze, while on February 4, it forms chaotically. Convective outflow enhances offshore precipitation, especially on February 4, by interacting with the land breeze to create additional convergence zones, intensifying convection. Gravity waves played a minor role, contributing to atmospheric instability.

The study concludes that offshore moving convection is mainly driven by the land breeze, with gravity waves playing a minimal role. Convective outflow enhances and sustains offshore propagation by creating additional convergence zones and intensifying convection. Moisture advection occurs before nocturnal propagation and increases atmospheric instability, facilitating favorable conditions for nighttime offshore precipitation.