The Influence of Sea- and Land-Breeze Circulations on the Diurnal Variability of Precipitation over Hainan Island

Diurnal cycle is one of the most fundamental mode of precipitation in the world, while the mechenisms of how the land-sea breeze circulations trigger and impact on the precipitation still remain murky. This study examines the diurnal variation of precipitation over a tropical island in China using hourly gauge-based rainfall observations from 1950 to 2010 and satellite estimated precipitation CMORPH from 2006 to 2015，as well as numerical simulations. Precipitation is most significant from April to October, and exhibits a strong diurnal cycle. More than 60% of the total annual precipitation over the island is attributed to the diurnal cycle, though this fraction varies significantly by month. The diurnal signal is the result of land/sea breeze circulations, which are enhanced by the elevated terrain in the island’s interior. The CMORPH and rain gauge datasets generally agree well, except that the CMORPH data underestimates precipitation and has a 1-h delay in the precipitation peaks relative to the rain gauge observations . The diurnal cycle of the rainfall and the related land/sea breeze circulations during the first rainy season (May and June) over the island were well captured by convection-allowing numerical simulations with WRF, which were initiated from 10-year average ERA-interim reanalysis, despite slightly overall overestimation and 1-h delay of the rainfall peak. Results showed that the diurnal precipitation is due to a diurnal cycle of moist convection. The precipitation initiates at 1200 LST under the interaction of low-level wind convergence (land/sea breeze) and the moisture transported from the ocean by the prevailing wind as well as from low levels by updraft. The precipitation intensifies rapidly thereafter and peaks at 1700 LST with its inland propagation and the collisions of sea breezes from different sides of the island. The cold pools of the convective storms contribute to the inland propagation of the sea breeze. The precipitation dissipates quickly in the evening owing to the cooling and stabilization of the lower troposphere and with the decrease of moistur e. It was found that orography may not be a dominant factor to the diurnal variation of the precipitation over the island.