Physical mechanisms of offshore propagation of convection in the Maritime Continent
- 1School of Earth and Environment, University of Leeds, Leeds, UK
- 2Centre for Environmental Modelling And Computation, University of Leeds, Leeds, UK
- 3Met Office, Exeter, UK
- 4Department of Meteorology, University of Reading, Reading, UK
- 5Bureau of Meteorology, Australia
- 6National Centre for Atmospheric Science, UK
- 7Centre for Ocean and Atmospheric Sciences, University of East Anglia, Norwich, UK
The Maritime Continent, located within the Indo-Pacific warm pool, experiences some of the most intense convective rainfall on Earth, with a pronounced diurnal cycle. The spatio-temporal variability of convection, its organisation and its offshore propagation away from the islands overnight all depend on many factors including the topography of island coastlines and mountains, and large-scale weather phenomena such as the Madden-Julian Oscillation, El Niño–Southern Oscillation and equatorial waves. However, numerical weather prediction and climate models typically suffer from considerable biases in simulating the diurnal convection and its propagation, hence there is a need to improve our understanding of the underlying physical mechanisms of these phenomena.
While the nocturnal offshore propagation of convection is often thought to be forced by gravity waves triggered by land-based diurnal convection, alternative hypothesized mechanisms exist in the literature, related to the propagation of the offshore land breeze and cold pools. Using convection-permitting simulations of selected case studies of convection propagating offshore from Sumatra, we find a squall line propagating overnight due to low-level convergence caused by the land breeze and environmental winds. This is reinforced by cold pools, which we diagnose using model tracers. However, gravity waves also play a role, triggering localized (non-organized) convection which does not itself propagate, but can appear as propagation along wave trajectories when compositing the diurnal cycle over many days.
The investigation is extended to other coastlines in the Maritime Continent, using convection-permitting simulations for 900 days during boreal winters, to demonstrate broader evidence for these physical mechanisms; to understand why the offshore propagation occurs on some days but not others; and to show how the strength, timing and causes of offshore propagation vary for different Maritime Continent islands, due to variations in the large-scale winds, orography and the topography of coastlines.
How to cite: Peatman, S., Birch, C., Schwendike, J., Marsham, J., Dearden, C., Webster, S., Howard, E., Woolnough, S., Neely, R., and Matthews, A.: Physical mechanisms of offshore propagation of convection in the Maritime Continent, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-15063, https://doi.org/10.5194/egusphere-egu23-15063, 2023.
