Understanding Hector: The dynamics of island thunderstorms

Linear and nonlinear models are used to examine the development of island t hunderstorms, in particular the Hector convective system that forms over th e 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 tha t the low-level convergence is maximized when the flow is weak and along th e major axis of the heat source. A dry version of the nonlinear model verif ies the trends predicted by the linear model except at very low flow speeds where the convergence is bounded in the nonlinear model but increases inde finitely 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 nonli near model. The sensitivity of the convective strength (defined by the accu mulated rainfall and total condensate) to wind speed and direction, surface fluxes, and low-level moisture is then examined. It is shown that the stre ngth 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 in creased low-level convergence from the linear and nonlinear dry models. Sen sitivity experiments indicate that the convective strength increases as bot h 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 soundi ng increases, the accumulated rainfall over the islands increases monotonic ally; however, the total condensate reaches a maximum at a CAPE of around 1 500 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. F inally, simulations with a single coastline are performed to show that one of the reasons the Hector convective system is so strong is that it develop s over an island where the land-sea circulation from all coastlines can con tribute.