Convection Initiation Associated With Ambient Winds and Local Circulations Over a Tropical Island in South China

Convection is highly active on tropical islands. Based on a 3‐year radar climatology over Hainan Island, a mountainous tropical island in South China, we identified a distinct spatial dependence of convection initiation (CI) on the speed of the large‐scale low‐level flow. Convection initiates most prominently over the lee‐side plains in high‐wind conditions while over mountains in low‐wind conditions. Quasi‐idealized numerical simulations suggest that mountain ridges stimulate the formation and growth of horizontal convective rolls (HCRs) over the lee‐side plains in high‐wind conditions. These HCRs subsequently intersect with sea‐breeze fronts, which enhances the low‐level convergence favoring CI on the lee‐side plains of the island. During low‐wind conditions, thermally driven circulations control and favor CI over the mountains during the day. The findings highlight that the response of local circulations to the lower‐troposphere ambient flow and diurnal thermal heating is an important aspect of convection initiation on the island. Plain Language Summary Skillfully predicting the location and timing of the formation of thunderstorms in the tropics is a challenging problem. Using radar observations, this study investigates how the key characteristics of low‐level winds influence the formation of thunderstorms on a mountainous tropical island (Hainan Island in South China). Modeling results demonstrate that the spatial occurrence of the thunderstorm formation is sensitive to the speed of low‐level winds. The thunderstorm formation occurs most prominently on the downwind plains and over the upwind mountain regions under the high‐wind and low‐wind conditions, respectively. During the high‐wind condition, the perturbed ambient flows downwind of ridges help to enhance the convergence in the boundary layer, and thus contribute to the thunderstorm formation in the plain region in the afternoon. Under low‐wind conditions, the local daytime heating is responsible for the frequent formation of thunderstorms over mountains. Considering that large‐scale circulations can be predicted reasonably well by numerical models, the findings of this study may assist operational forecasts of the formation of thunderstorms on tropical islands. Key Points The location of convection initiation on the mountainous Hainan Island depends on the speed of the large‐scale low‐level flow Convection initiates most prominently over the lee‐side plains in high‐wind condition and over the mou