Sea breezes and early convection were observed over the Tiwi Islands, north of the Australian continent, as part of the Maritime Continent Thunderstorm Experiment (MCTEX). Peninsula scale (5-15 km) breezes were observed to initiate the earliest moist convection principally through convergence that results from the confluence or collision of opposing-coast breeze fronts. Such breezes develop in the morning of each day and have a virtual temperature discontinuity of approximately 1 K. Convection initiated by peninsula breezes usually fails to organize beyond the single-cell scale and dissipates as a local event.
A mature convective island boundary layer develops by early afternoon. Owing to residence time over the land, this boundary layer is deepest and warmest near leeward coasts where a well-developed island scale (100 km) breeze maintains a 2-3 K virtual temperature discontinuity. Except for the most inhibited days, island scale sea breezes from opposing coasts (separated by 50 km or more) do not collide, in part because these interact with convectively produced cold pools long before the inland progression of breezes permits an island scale collision. Complexity associated with multiple sea breeze - cold pool interactions is at the heart of convective evolution in the Tiwi Islands and is the daily norm.
On days when convective inhibition is significantly larger than average , sea breezes penetrate inland from the north and south coasts, eventually forcing convection near center island late in the diurnal heating cycle. Owing to sensible heat flux from the islands' surface, the sea breeze front and associated rotor circulation are thermally modified and the virtual temperature discontinuity decreases to < 1 K. Late day initiation of convection near center-island appears somewhat less likely to produce the strongest convective systems, the non-occurrence of which is highly sensitive to small deficits in low level water vapor.
A principal finding of this study is that most mesoscale convective systems (Hectors) can be traced backward in time to the primary moist convection that is initiated by island scale breezes along leeward coasts. Evaporatively produced cold pools, which are cooler than the nearby sea breeze, release this convection from the breeze maintenance mechanism, allowing it to travel and to feed on the heated island boundary layer. Subsequent convective evolution is characteristic of traveling free convection elsewhere in that it organizes mainly according to cold pool and shear balance factors and within constraints normally associated with convective available potential energy (CAPE).
Given that an easterly shear condition prevails in the Tiwi Islands, an optimal condition for strong convective development is often achieved when island boundary layer winds are westerly (or light and variable), CAPE is substantial (500-2500 J/kg), and convective inhibition is <100 J/kg. Under these circumstances, island scale convection originates near the eastern sea breeze front and organizes into a small meridional squall when the cold pool develops. Aloft, easterly "steering" winds favor subsequent westward propagation and mining of the island boundary layer across the full zonal extent of the islands.