Abstract

Short-lived afternoon heavy rainfall events may form over Central O‘ahu during seasonal transition periods (June and October) under favorable large-scale settings. These include a deep moist layer with relatively high precipitable water (> 40 mm), blocking pattern in mid-latitudes with a northeast-southwest moist tongue from low latitudes ahead of an upper-level trough, absence of a trade wind inversion, and weak (< 3 m s-1) low-level winds. Our high-resolution (1.5 km) model results show that immediately before the storm initiation, daytime land surface heating deepens the mixed layer over Central O‘ahu and the top of the mixed layer reaches the lifted condensation level. Meanwhile, the development of onshore/sea-breeze flows, driven by land-sea thermal contrast, brings in moist maritime air over the island interior. Finally, convergence of onshore flows over Central O‘ahu provides the localized lifting required for the release of instability. Based on synoptic and observational analyses, nowcasting with a lead time of 2–3 hours ahead of this type of event is possible. In the absence of orographic effects after removing model topography, processes that lead to heavy rainfall are largely unchanged, and subsequent development of heavy showers over Central O‘ahu are still simulated. However, when surface heat and moisture fluxes are turned off, convective cells are not simulated in the area. These results indicate that daytime heating is crucial for the development of this type of heavy rainfall event under favorable large-scale settings.

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