Understanding Hector: The Dynamics of Island Thunderstorms

Linear and nonlinear models are used to examine the development of island thunderstorms, in particular the Hector convective system that forms over the Tiwi Islands just north of Australia. The linear model is used to examine the flow response to an isolated, elliptical, heat source. It is found that the low-level convergence is maximized when the flow is weak and along the major axis of the heat source. A dry version of the nonlinear model verifies the trends predicted by the linear model except at very low flow speeds where the convergence is bounded in the nonlinear model but increases indefinitely in the linear model. Deep convection develops over the heat source when a moisture profile with positive convective available potential energy (CAPE) is added to the nonlinear model. The sensitivity of the convective strength (defined by the accumulated rainfall and total condensate) to wind speed and direction, surface fluxes, and low-level moisture is then examined. It is shown that the strength increases as the wind speed decreases and as the wind direction turns toward the major axis of the island. in agreement with the prediction of increased low-level convergence from the linear and nonlinear dry models. Sensitivity experiments indicate that the convective strength increases as both the heat and moisture fluxes increase. The strength is more sensitive to the heat flux since this drives the large-scale convergence and sea breezes that generate convection. As the low-level moisture in the upstream sounding increases, the accumulated rainfall over the islands increases monotonically; however, the total condensate reaches a maximum at a CAPE of around 1500 J kg-1 and then decreases thereafter. It is shown that the low-level moisture is an important predictor of the form of convective development. Finally, simulations with a single coastline are performed to show that one of the reasons the Hector convective system is so strong is that it develops over an island where the land-sea circulation from all coastlines can contribute.