近地层参数化对海南岛海风降水模拟的影响

利用WRF模式探讨了两种近地层参数化方案(MM5方案和Eta方案)对2013年5月31日海南岛一次海风降水过程模拟的影响.结果表明,改变近地层方案可对当地的海风环流及相应的降水特征产生明显影响,两个试验最重要的差别主要体现在模拟的海风及降水的强度差异上,与MM5试验相比,Eta试验的低层海风及辐合程度更强,相应的降水也更强,表现为岛屿总格点降水量、大于10mm的降水区域、最大格点降水三个量化指标均比较大.通过分析两种方案在不同降水阶段的地表通量及近地层变量场,发现Eta试验模拟的降水前环境场更有利于对流的启动,随着午后热力湍流的不断增强,将MM5方案替换为Eta方案可使近地层感热通量、潜热通量分别增加约3.57%、5.65%,动量通量减小约10.79%,感热、潜热的增加使Eta试验中近地层大气的加热加湿作用更加明显,相应的低层大气不稳定度更高,再配合海风锋前较强的辐合上升运动,局地不稳定能量的释放变的更加容易,因此降水强度更大.

Impact of surface layer parameterizations on simulated sea breeze precipitation over the Hainan Island

Sea breeze is a prototypical mesoscale circulation in coastal areas caused by a thermal difference between sea and land during the daytime, the previous studies have shown that the simulation of sea breeze is closely linked to the surface layer parameterization, which calculate the surface fluxes of heat, moisture and momentum and interact with other model physics including cloud, radiation and precipitation processes. The performance of two surface layer parameterization schemes (MM5 scheme and Eta scheme) are discussed using Weather Research and Forecasting Model to determine how the sea breeze precipitation is affected over the Hainan Island. Results show that 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.