An experiment called the Maritime Continent Thunderstorm Experiment (MCTEX) was conducted in 1995 over the tropical Tiwi Islands north of the Australian content. Part I of this two paper series describes the evolution of the island sea breezes and shows they were responsible for the initiation of small squalls. Squalls on the leeward cost eventually evolve to produce outflows or cold pools of sufficient magnitude to initiate other storms or maintain the parent storm. This paper describes how these cold pools and sea breeze front interact to produce the large thunderstorm complex called Hector and what factors control its evolution.
Based on the analysis of 14 storm days from MCTEX it was determined that: 1) Hector results from the interaction of a gust front with a line of cumulus clouds; typically this line of clouds is associated with the dominant sea breeze front, 2) the gust front that produces Hector forms from smaller storms along the sea breeze front, 3) only under conditions of suppressed early convection do the north and south sea breeze fronts collide; it is only in this situation that Hector can be said to form from the collision of the sea breezes, 4) the reorientation of Hector from east-west to north-south occurs as a result of cells forming and merging along the gust front; this process is favored when the cell motion and gust front motion are nearly equal, and 5) reorientation of Hector was infrequent during MCTEX this was probably because the steering level winds were generally less than the speed of the gust front motion; thus the gust front frequently moved away from the storms.
The initial location of Hector is controlled by the low-level environmental wind. Hector's occurrence on the north or south coast is dependent on the meridianal wind component and its location on the east or west side of the island by the zonal wind component. Thus if the wind is southwest Hector will first form on the northeast side of the islands and if the wind is northeast it will be on the southwest side. The reason being the low level wind controls whether the sea breeze front along the north coast or south coast will be dominant. Whether Hector initially forms on the east or west side is probably related to the length of the wind directory over land. As shown in Part 1 the depth of the mixed layer and the temperature increases with increasing residence time of the boundary layer air over the land.
It was desired to test several findings from other experiments; these were: a) storm initiation usually occurs along boundary layer convergence lines particularly at locations where the convergence line intersects a pre-existing line of cumulus, b) the collision of convergence lines favors the formation of the more intense storms, c) long-lived lines of thunderstorms (squall lines) are more likely when the updrafts along the convergence line are vertical and the convergence line and storms have similar motions. The results from the Tiwi Islands definitely confirm these early findings.
It is interesting to compare storm evolution in the Tiwi Islands to that observed in Florida during the CaPE Project (Wilson and Megenhardt (1997). In the Tiwi Islands the steering level winds are easterly parallel to the orientation of the sea breeze, where in Florida the tendency is for the steering level winds to be aligned perpendicular to the sea breeze fronts. Because of the weak steering level flow in both locations the gust fronts tend to move away from the storms leading to short lived storms. However in the Tiwi Islands the gust front is continually intercepting cumulus clouds and squalls along the sea breeze front which results in a continuous initiation of storms near the intersection point, thus giving the appearance of a long lived storm system. In Florida the only significant interaction between a gust front and the sea breeze front occurs when gust fronts from storms generated on the sea breeze on the windward side of the island move across the Florida peninsula and collide with the leeward sea breeze front. This results in an intense line of convection that is relative short lived. This is because the gust front, generated by these collision produced storms, moves away from the line of storms because of the light steering level winds.