The role of density currents and gravity waves in the offshore propagation of convection over Sumatra

Peatman, Simon C., Birch, Cathryn E., Schwendike, Juliane, Marsham, John H., Dearden, Chris, Webster, Stuart, Neely, Ryan R. and Matthews, Adrian J. ORCID: (2023) The role of density currents and gravity waves in the offshore propagation of convection over Sumatra. Monthly Weather Review, 151 (7). 1757–1777. ISSN 0027-0644

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The Maritime Continent experiences some of the world’s most severe convective rainfall, with an intense diurnal cycle. A key feature is offshore propagation of convection overnight, having peaked over land during the evening. Existing hypotheses suggest this propagation is due to the nocturnal land breeze and environmental wind causing low-level convergence; and/or gravity waves triggering convection as they propagate. We use a convection-permitting configuration of the Met Office Unified Model over Sumatra to test these hypotheses, verifying against observations from the Japanese Years of the Maritime Continent field campaign. In selected case studies there is an organized squall line propagating with the land-breeze density current, possibly reinforced by convective cold pools, at;3 m s 21 to around 150–300 km offshore. Propagation at these speeds is also seen in a composite mean diurnal cycle. The density current is verified by observations, with offshore low-level wind and virtual potential temperature showing a rapid decrease consistent with a density current front, accompanied by rainfall. Gravity waves are identified in the model with a typical phase speed of 16 m s 21. They trigger isolated cells of convection, usually farther offshore and with much weaker precipitation than the squall line. Occasionally, the isolated convection may deepen and the rainfall intensify, if the gravity wave interacts with a substantial preexisting perturbation such as shallow cloud. The localized convection triggered by gravity waves does not generally propagate at the wave’s own speed, but this phenomenon may appear as propagation along a wave trajectory in a composite that averages over many days of the diurnal cycle.

Item Type: Article
Additional Information: Funding information: This research was funded by the Met Office Weather and Climate Science for Service Partnership (WCSSP) Southeast Asia, as part of the Newton Fund; and the TerraMaris project, funded by National Environment Research Council (NERC) Grant NE/R016739/1. Data availability statement. Himawari data are provided by the Japan Meteorological Agency; we thank the ICARE Data and Services Center ( for providing access to the Himawari data used in this study. IMERG data are provided by the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center’s Earth Science Division and Precipitation Processing System, which develop and compute the IMERG as a contribution to GPM; and archived at the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC; GLOBE data are provided by the National Oceanic and Atmospheric Administration ( Radar and AWS data from the R/V Mirai are provided by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC;
Uncontrolled Keywords: cold pools,diurnal effects,gravity waves,maritime continent,sea breezes,squall lines,atmospheric science ,/dk/atira/pure/subjectarea/asjc/1900/1902
Faculty \ School: Faculty of Science > School of Natural Sciences
Faculty of Science > School of Environmental Sciences
UEA Research Groups: Faculty of Science > Research Groups > Centre for Ocean and Atmospheric Sciences
Related URLs:
Depositing User: LivePure Connector
Date Deposited: 02 Mar 2023 13:31
Last Modified: 20 Jul 2023 09:30
DOI: 10.1175/MWR-D-22-0322.1


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