Formation of Nocturnal Offshore Rainfall near the West Coast of Sumatra: Land Breeze or Gravity Wave?
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AbstractAfternoon deep convection over the Maritime Continent islands propagates offshore in the evening to early morning hours, leading to a nocturnal rainfall maximum over the nearby ocean. This work investigates the formation of the seaward precipitation migration off western Sumatra and its intraseasonal and seasonal characteristics using BMKG C-band radar observations from Padang and ERA5 reanalysis. A total of 117 nocturnal offshore rainfall events were identified in 2018, with an average propagation speed of 4.5 m s1 within 180 km of Sumatra. Most offshore propagation events occur when the MaddenJulian oscillation (MJO) is either weak (real-time multivariate MJO index < 1) or active over the Indian Ocean (phases 13), whereas very few occur when the MJO is active over the Maritime Continent and western Pacific Ocean (phases 46). The occurrence of offshore rainfall events also varies on the basis of the seasonal evolution of the large-scale circulation associated with the AsianAustralian monsoons, with fewer events during the monsoon seasons of DecemberFebruary and JuneAugust and more during the transition seasons of MarchMay and SeptemberNovember. Low-level convergence, resulting from the interaction of the land breeze and background low-level westerlies, is found to be the primary driver for producing offshore convective rain propagation from the west coast of Sumatra. Stratiform rain propagation speeds are further increased by upper-level easterlies, which explains the faster migration speed of high reflective clouds observed by satellite. However, temperature anomalies associated with daytime convective latent heating over Sumatra indicate that gravity waves may also modulate the offshore environment to be conducive to seaward convection migration.
