LAUSR

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.