Tropical Island Convection in the Absence of Significant Topography. Part I: Life Cycle of Diurnally Forced Convection

Abstract

Diurnally forced convection was observed over the Tiwi Islands, north of the Australian continent, as part of the Maritime Continent Thunderstorm Experiment. Immature peninsula-scale (5?15 km) sea breezes were observed to initiate moist convection early each day, principally through convergence that results from the confluence or collision of peninsula breeze fronts. Convection initiated by peninsula-scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event. Mature island-scale (?100 km) breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection. The initiation of mesoscale convective systems (MCSs) is observed to be a direct consequence of breeze front collisions for only ?20% of the days on which organized convection develops. This is referred to as ?type A? forcing and it occurs when normal convective development is delayed or otherwise suppressed. Type A forcing is nature?s backup mechanism and it is less likely to produce large or strong mesoscale convective systems when compared to the general population of events. On approximately 80% of days during which organized convection develops, a multiple-stage forcing process evolves through complex interactions between preferred sea breezes and convectively generated cold pools. So-called type B forcing emerges 1?3 h before penetration of the sea-breeze fronts to the interior island. Type B evolution has at least four stages: 1) leeward- or other preferred-coast sea-breeze showers that develop small cold pools, 2) showers that travel inland when their cold pools become denser than the marine boundary layer, 3) westward propagation of squalls that result from a merge or maturation of small cold pools, and 4) interaction between a gust front and a zonally oriented sea-breeze front of island scale (?100 km). A collision of gust fronts, emanating from separate convective areas over Bathurst and Melville Islands, can excite a fifth stage of development associated with many of the strongest systems. A principal finding of this study is that all MCSs over the Tiwi Islands can be traced backward in time to the initiation of convection by island-scale sea breezes, usually of type B near leeward coasts. Subsequent convective evolution is characteristic of traveling free convection elsewhere in that it organizes according to cold pool, shear balance, and mean flow factors. The presence of a critical level in the lower troposphere is a unique aspect of the theoretical ?optimal condition? associated with island convection in a low-level jet regime; however, the data presented here suggest that the effects of surface layer stagnation may be of greater practical importance. Since the aforestated conclusions are based on time series of rather limited duration, the reader is cautioned as to uncertainty associated with the climatological frequency of events as described herein. Furthermore, the authors have not examined external forcings, which may be associated with large-scale circulations.