Examining Multi-Scale Interactions Associated with Offshore Rainfall Near the West Coast of Sumatra with Satellite Multi-Sensor Data, Reanalysis, and WRF Numerical Solutions

This study examines the mesoscale interactions of multi-scale environmental conditions that lead to the formation of offshore rainfall near the West Coast of Sumatra over the Maritime Continent. We use data from NASA GPM satellite-derived IMERG rainfall, NASA MERRA-2 reanalysis, and the ECMWF ERA5 atmospheric reanalysis to characterize the atmospheric and ocean surface conditions associated with the diurnal variations in wind, convection and precipitation near the West Coast of Sumatra. The NASA MERRA-2 and ECMWF ERA5 atmospheric reanalysis data were also used to analyze the large-scale environmental conditions. We focus on two offshore rainfall cases (1 and 4 February 2018) during Madden Julian Oscillation (MJO) phases 6-7. In these cases, precipitation forming inland during the evening time (~6pm LST) begins to propagate offshore through the night and early into the next morning. The Weather Research and Forecasting model (WRF-ARW) was used to downscale ERA5 reanalysis data to 3km resolution to resolve small-scale land-sea interactions and precipitation that ERA5 and MERRA-2 reanalysis are too coarse to describe. BMKG Radar data from Padang is used to validate WRF model simulations.

Previous studies have shown the months of DJF having, on average, strong convergence over the West Coast of Sumatra. However, during the two study cases, MJO phase 6-7 suppressed rainfall over the island and showed weakened background flow, leading to less convergence. Both cases (1 and 4 February 2018) show the offshore propagation of precipitation even in the presence of MJO phases 6-7, indicating that multi-scale processes are involved in the formation and movement of precipitation. The WRF model shows how the diurnal land-sea breeze cycle plays a role in the initiation of convection in the evening to early morning. In addition to the small-scale forcing of the land breeze, large-scale background flow shows evidence of supporting the offshore movement of precipitation into the early morning.