Diurnal Precipitation Cycle over a Mesoscale Tropical Island

Hainan island, which has the nickname of “Chinese Hawaii”, is one of the most favorite tourist attractions in China. The oval shaped island with complex terrains and peak altitude of 1500 m has a long axis of ~300km and a short axis around ~200km with central latitude of ~15N located in the South China Sea and Mainland China. The diurnal precipitation is a very important phenomenon which has profound impacts on the local and regional weather, climate and economy but has rarely been studied in published literature.

Through analyzing the 10-year high-resolution precipitation dataset from NOAA's Climate Prediction Center Morphing Technique (CMORPH), we found that the diurnal cycle of precipitation on the island is very regular. The local precipitation maximum first develops on the leeside of the island mountain chains right around noontime (12Z, Local Standard Time or LST) which gradually enhances with peaking at around 17Z. The precipitation decreases thereafter and ends at 21Z. The diurnal cycle is also further confirmed through rain gauge observations. The four stations located on the windward side of the island, however, do not display the same diurnal characteristics in precipitation.

To further understand the diurnal cycle of the precipitation on the Hainan island, a series of convection-permitting numerical experiments using the WRF model are performed to examine the warm-season diurnal variation of the precipitation. Despite some differences in the rainfall intensity, the experiments can successfully simulate the diurnal precipitation cycle over this island which is dominated by the island sea breeze. The diurnal variation of rainfall coincides with the convergence or divergence of the land-sea breeze circulation. In the sensitivity simulation that replaces the island with all ocean surface, not surprisingly there was no diurnal cycle of precipitation and land-sea breeze. The impacts of the different initial time and fixed boundary conditions also been examined in the sensitivity experiments, results show that the large-scale diurnal boundary condition have some influence on the precipitation near the northeast and northwest areas but does not have much influence on the precipitation on the island. With the different idealized oval island orientations and different island location, it was found that the island direction and position have much influence on the precipitation distribution on the island while do not the primary responsible for the diurnal cycle of precipitation. More detailed analyses of these simulations will be presented at the conference.