Tropical island convection in the absence of significant topography. Part I: Life cycle of diurnally forced convection

Diurnally forced convection was observed over the Tiwi Islands, north of th e Australian continent, as part of the Maritime Continent Thunderstorm Expe riment. Immature peninsula-scale (5-15 km) sea breezes were observed to ini tiate moist convection early each day, principally through convergence that results from the confluence or collision of peninsula breeze fronts. Conve ction initiated by peninsula-scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event. Mature island-sc ale (similar to 100 km) breezes develop by late morning and subsequently pl ay a pivotal role in the forcing and evolution of organized convection. The initiation of mesoscale convective systems (MCSs) is observed to be a d irect consequence of breeze front collisions for only similar to 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 o therwise suppressed. Type A forcing is nature's backup mechanism and it is less likely to produce large or strong mesoscale convective systems when co mpared 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 typ e B forcing emerges 1-3 h before penetration of the sea-breeze fronts to th e interior island. Type B evolution has at least four stages: 1) leeward- o r other preferred-coast sea-breeze showers that develop small cold pools, 2 ) showers that travel inland when their cold pools become denser than the m arine 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 (similar to 100 km). A collision of gust fronts, emanating from separate convective are as over Bathurst and Melville Islands, can excite a fifth stage of developm ent associated with many of the strongest systems. A principal finding of this study is that all MCSs over the Tiwi Islands ca n be traced backward in time to the initiation of convection by island-scal e 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 aspe ct of the theoretical "optimal condition" associated with island convection in a low-level jet regime; however, the data presented here suggest that t he effects of surface layer stagnation may be of greater practical importan ce. Since the aforestated conclusions are based on time series of rather limite d duration, the reader is cautioned as to uncertainty associated with the c limatological frequency of events as described herein. Furthermore, the aut hors have not examined external forcings, which may be associated with larg e-scale circulations.