海南岛海风雷暴结构的数值模拟

本文利用高分辨率WRF模式对2012年7月20日发生在海南地区的一次海风雷暴过程进行模拟,探讨了海南岛复杂地形下海风雷暴的结构、发展演变过程及其触发机制.结果表明,海南岛北部向内陆传播的海风与南部受地形阻挡的海风相遇后会形成海风辐合带,辐合带能影响当地的散度和涡旋特征,为雷暴的发生发展提供有利的动力和热力学条件.海南岛受热带海洋的影响较大,当地的水汽条件和对流潜势长期保持着有利于对流发展的状态,自由对流高度始终处于较低的位置,一旦海风辐合带来的抬升运动克服对流抑制到达自由对流高度后,对流就能自主地发展起来,所以单纯的海风辐合也常常能触发当地的强雷暴.雷暴发生发展过程中对流参数存在明显的变化,其演变曲线的突变位置对雷暴的发生有一定的指示作用.海南岛的海风雷暴过程与当地的复杂地形密切相关,地形的动力阻挡作用影响着低层海风的辐合以及对流的发展.

A numerical simulation of sea breeze thunderstorm structure over the Hainan Island

The land-sea breeze system is a wind pattern that is observed in coastal regions. It is generated by the differences in the heat budgets of the land and sea surfaces. The role of the sea breeze circulation, including the sea breeze front, in triggering thunderstorm has long been recognized. The afternoon thunderstorms triggered by sea breeze can affect the basic characteristics of the sea breeze and its change. In this paper, the thunderstorm caused by sea breeze is called sea breeze thunderstorm. Surrounded by the sea, Hainan Island has strong sea breeze and rich water vapor under the influence of tropical ocean. It has a high incidence of sea breeze thunderstorm, which is triggered by interaction of local circulations such as land-sea breeze and mountain-valley breeze.The WRF-ARW model(Version 3.6) coupled with the Noah land surface is used to simulate the sea breeze thunderstorm over the Hainan Island during July 20, 2012. The characteristics of thunderstorm over complex terrain are analyzed with radar, satellite, sounding and surface observations data. The structure and evolution of thunderstorm as well as its trigger mechanism are also discussed. This paper is intended to represent the mechanism of sea breeze thunderstorm and improve the forecasting performance. As a typical sea breeze thunderstorm day, there was a significant wind shift around the island. The low-level sea breeze and land cover caused a unique water vapor distribution, which provided the conditions for local thunderstorm to produce precipitation. The convective instability layer emerged in the southern part of the island, which was conducive to the formation and development of the convective activity. While the cold air in the north of island broke the unstable layer, the convection occurrence became more difficult in this region. After the sea breeze formed along the coast, it penetrated inland and developed gradually. Because of the topography forcing, northern and southern sea breezes met in the vicinity of Baoting station. As a result, a significant sea breeze convergence zone has formed, affecting local divergence and characteristics of vortex. Under the favorable dynamical condition, the local thunderstorm weather occurred. When sea breeze thunderstorm over Hainan is discussed, we should not only concern with the development of the sea breeze front, but also need to analyze the local distribution of convective inhibition. The evolution of local energy and convective parameters can indicate the arise of sea breeze thunderstorm in temporal and spatial scales. The large convective available potential energy(CAPE) and small convective inhibition(CIN) have provided favorable conditions for the development of thunderstorm before it occurred. As the occurrence of thunderstorm, the instability energy was released. The CAPE decayed rapidly and the CIN began to rise. It was a symbol that thunderstorm system has entered into the decline stage.This study indicated that the characteristics of the sea breeze and thunderstorm can be reasonably simulated by WRF model. Under the influence of the tropical ocean, the water vapor, convective potential energy and the level of free convection keep long-lasting development state which is favorable for the convection. The convection would develop autonomously when the uplift produced by sea breeze convergence overcome the convective inhibition and reach free convection level, so the sea breeze convergence can often trigger thunderstorm in Hainan. The sea breeze thunderstorm is closely related to the local terrain, which affects the spatial and temporal distributions of low-level wind and convection convergence zone. This study is conducive to understand sea breeze thunderstorm over Hainan Island and the key factor to forecast it, but we still need further studies of more cases to support the relevant conclusion.