A Numerical Study Of Impact Of Topography On Intensity And Pattern Of Sea Breeze Precipitation Over The Hainan Island

The sea breeze precipitation over the Hainan Island during May 31, 2013 is simulated by high-resolution numerical model WRF, the impact of topography on intensity and pattern of local sea breeze precipitation is studied by designing different topography experiments, and the performance of two surface layer parameterization schemes (MM5 scheme and Eta scheme) are discussed for the further study.The results show that, WRF model can simulate the surface wind and sea breeze precipitation intensity reasonably, and the time evolution of simulation and actual precipitation generally approaches well. With the continuous development of sea-breeze circulation, the timing and placement of convective precipitation as well as the sea breeze front almost move inland in phase, and the precipitation area mainly distributed in the front of Li Mu Mountain, which is located in the southwest of studied area. The rainfall structural characteristics are closely associated with the topography feature which is high and upright in the middle area while relatively lower all around in the Hainan Island, the dynamic and thermodynamic influence of terrain has alternate evolution in the whole process of sea breeze precipitation.During 11: 00?16: 00 BST, precipitation is mainly caused by single sea breeze front.The primary mechanism is thermal enhancement duing to the lower height of topography at this stage, and the sea-land thermal flux difference which essentially drive the development of sea breeze is more notable with higher terrain height. While during 17: 00?21: 00 BST, precipitation is mainly caused by the collision of eastern and western sea breeze front, with the inland propagation of sea breeze front, terrain blocking effect takes dominant role as elevation increases gradually, but if the terrain height increases to a certain degree, the blocking effect can rapidly weaken sea breeze circulation and thus reduce rainfall intensity. Nevertheless, all the effects mentioned above depends on the inhomogeneous character of underlying surface, the combination of topography and vegetation can produce larger difference of the land surface energy distribution, and consequently lead to greater influence on local precipitation.The sea breeze circulation and precipitation characteristics are sensitive to the choice of surface layer scheme used in WRF model, the most significant difference between two experiments is mainly reflected on the intensity of the sea breeze and precipitation. Compared with MM5 experiment, Eta experiment produced much stronger sea breeze and low-level convergence, and consequently lead to higher accumulative precipitation, which is reflected by the significantly higher total rain amount, rain cover percentage and maximum rain amount. By analyzing the flux and variable fields in the surface layer, differences in the sensible heat flux (SH) and latent heat flux (LH) over land surface were primarily responsible for the different precipitation amounts and intensity during the daytime in two experiments. The simulated environment at 09: 00 BST in Eta experiment is more conductive to the initiation of convection, and as the thermal turbulent intensity increased gradually in the afternoon, SH and LH increased by 3.57% and 5.65% respectively while the momentum flux decreased by 10.79% when MM5 scheme is replaced with Eta scheme. The higher SH and LH in Eta experiment could lead to comparatively more distinct land-sea temperature difference, and thus result in prosper development of sea breeze and increased low-level instability. Besides, the accumulated instability energy in the warm and wet layer can be more easily triggered with the convergence and strongly ascending motion ahead of the sea breeze front, so the simulated precipitation in Eta scheme is stronger than MM5 scheme.