Physical Mechanisms Controlling the Offshore Propagation of Convection in the Tropics: 2. Influence of Topography
Résumé
A set of idealized convection-permitting simulations is performed to investigate the influence of topography on the physical mechanisms responsible for the nocturnal offshore propagation of convection around tropical islands. All simulations have an idealized island in the middle of a long channel oceanic domain, with constant sea surface temperature and without rotation. To diagnose the impact of topography, we compare a flat island simulation with two simulations with mountain ranges of different shapes. The topography over the island has a strong impact on the diurnal cycle of convection as clouds tend to remain all day over the highest topography. This weakens the diurnal cycle and the land breeze front and triggers a comparatively less frequent long-distance offshore propagation of convection. As in the flat simulation, the distance of offshore propagation is particularly sensitive to humidity and temperature at the top of the boundary layer. A shallow circulation that is asymmetric with respect to the island influences the boundary layer top humidity and can favor propagation on one side of the island or the other. These results mimic cloud and precipitation patterns observed prior to the Madden-Julian Oscillation propagation over the Maritime Continent. The shape of the topography does not seem to influence the offshore propagation of convection significantly except for mountain-valley breezes that reinforce the land breeze and the establishment of the asymmetric shallow circulation. Plain Language Summary In Part I of this paper, we looked at the mechanisms controlling the offshore propagation of convection around an idealized flat tropical island. But tropical islands generally have high topography reaching thousands of meters over Papua New Guinea, Borneo, and Sumatra. The impact of topography is investigated using the same framework as before but with a mountain range on the island. We test two different shapes to investigate valley breeze effects and the dependence on the mountain shape: one is a simple ridge and the other has a peak and a pass. The presence of a mountain range strongly affects the offshore propagation of convection as clouds tend to stay above the highest topography at night, which weakens the land breeze. Gravity waves triggering convection far from the coast are present but only for days when clouds move away from the coast at night. The valley is found to reinforce the land breeze and, more surprisingly, allows the development of asymmetric offshore convection relative to the island. This asymmetry, also found with the flat island simulation, is responsible for the advection of a dry anomaly that forces convection to stay close to the coast more frequently.
Domaines
Océan, Atmosphère
Origine : Publication financée par une institution