Thunderstorms form over the islands within the Maritime Continent almost every afternoon during the build up to the monsoon. These thunderstorms are strongly diurnally forced. Therefore, the diurnal evolution of the boundary layer over the islands is a crucial factor in understanding the thunderstorm life cycle. The use of wind profilers, regular radiosonde launches, an extensive AWS network and other instruments, has for the first time given the opportunity to study the development and circulations within a tropical island boundary layer during the build up to the monsoon.
Four distinct phases in the boundary layer characteristics over the Tiwi islands were observed during MCTEX:
1) A break down in the surface inversion and a deepening of the mixed layer through the morning and early afternoon. Sea breeze circulations developed on all the coastlines, replacing the nocturnal land breeze and drainage flows.
2) Thunderstorms modified the island boundary layer, stabilising the surface layer, and suppressing the sea breeze circulations.
3) A partial recovery of the island boundary layer occurred. The formation of a surface inversion followed this at sunset, with a residual mixed-layer above.
4) A strong surface inversion developed, with a residual mixed-layer of oceanic origin over much of the islands. Below about 100m land breeze and drainage flows dominated. Above this level, synoptic scale circulations led to the advection of oceanic air over the islands.
The timing of thunderstorm activity and the resulting cloud cover, and external synoptic scale disturbances are important factors limiting the growth of the island boundary layer and inland penetration of the sea breeze. The boundary layer development will be shown for cases of an early thunderstorm onset, a late thunderstorm onset, and a period where external synoptic scale disturbances affected the circulation across the whole island.