Physical mechanisms of offshore propagation of convection in the Maritime Continent

<p>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 Ni&#241;o&#8211;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.</p> <p>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.</p> <p>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.</p>