Formation of Nocturnal Offshore Rainfall near the West Coast of Sumatra: Land Breeze or Gravity Wave?
Loading...
Date
2021-03
Journal Title
Journal ISSN
Volume Title
Publisher
Monthly Weather Review
Abstract
fternoon 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 s21 within 180 km of Sumatra. Most offshore propagation events occur
when the Madden–Julian oscillation (MJO) is either weak (real-time multivariate MJO index , 1) or active over the Indian
Ocean (phases 1–3), whereas very few occur when the MJO is active over the Maritime Continent and western Pacific Ocean
(phases 4–6). The occurrence of offshore rainfall events also varies on the basis of the seasonal evolution of the large-scale
circulation associated with the Asian–Australian monsoons, with fewer events during the monsoon seasons of December–
February and June–August and more during the transition seasons of March–May and September–November. 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 prop-
agation 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.
Description
Related research data sets can be found on the Texas Data Repository website (see DOI's here for link).
Keywords
Gravity waves; Diurnal effects; Rainfall; Sea breezes; Radars/radar observations
Citation
Bai, H., and Coauthors, 2021: Formation of Nocturnal Offshore Rainfall near the West Coast of Sumatra: Land Breeze or Gravity Wave?. Mon. Wea. Rev., 149, 715–731, https://doi.org/10.1175/MWR-D-20-0179.1.