多云天气下海南岛海风环流结构的数值模拟

A numerical study of sea breeze circulation under cloudy conditions over Hainan Island

利用WRF-Noah耦合中尺度模式对海南岛2012年7月5日的多云海风个例进行三维高分辨率数值模拟,重点分析多云天气条件下复杂地形区域的海风环流结构及其演变特征。通过观测资料与模拟结果的对比发现,WRF模式能够合理地模拟出岛屿四周的海风演变特征。与少云海风日相似,多云海风日中全岛海风于12时开始形成,15时海风发展最为强盛,影响范围最广,18时全岛海风的辐合程度最强,海风辐合区是主要的潜在降水区域。对比山区与平坦地区的海风环流发现,山区海风环流强盛期为13—18时,而平坦地区海风环流强盛期为15—18时。复杂的山地对海风环流结构有直接和间接的影响:一方面在山地地形动力阻挡和抬升作用下,海风环流变得更加清晰完整,间接延长了海风环流的维持时间;另一方面局地地形热力作用形成的谷风环流与海风几乎同时产生和消亡,两者汇合后,谷风的瞬间加强会引起海风锋锋消,瞬间减弱会引起海风锋锋生;两者同相叠加会使得海风环流结构更加完整。相比之下,平坦地区的海风受到的地形动力和热力作用小,海风水平分布比较规则,海风环流垂直结构的变化主要取决于不同方向海风之间的相互作用。

In this study,to explore the strength and structure of sea breeze under cloudy conditions over Hainan Island,the3D high resolution numerical simulation of sea breeze on July5,2012was conducted by means of non hydrostatic mesoscale model WRF,coupled with the Noah land surface model.Hainan Island is a tropical island with oval topography.The frequency of occurrence of sea breeze is high throughout the island,due to its geographical location and unique topography.In addition,cloudy skies under weak stable weather conditions are conducive to the triggering of sea breeze,according to previous researches regarding the statistical analysis of sea breeze over Hainan Island.Therefore,it is significant to study the structure of sea breeze circulation on July5,2012,when the sea breeze observed was conspicuous,and there was a large number of clouds over the island during the evolution of the sea breeze.Compared with the observational data,the results show that the evolution and characteristics of sea breeze can be reasonably simulated by WRF.The evolution of sea breeze under cloudy conditions resembles sea breeze under clear conditions.Formed at12:00BST throughout the whole island,sea breeze circulation was strong during the time range of15:00-18:00BST in flat areas,and strong during13:00-18:00BST in mountainous areas.A significant convergence line was finally observed on the island at18:00BST.Strong convective weather could be easily triggered under the convergence of sea breezes for the deep convective instability energy and abundant water vapor,i.e.the convergence zone of sea breezes is the main potential precipitation area.The complex terrain has direct and indirect effects on the structure of sea breeze circulation.It determines the distribution of the low level horizontal wind field,then affects the characteristics of sea breeze in all directions indirectly.The sea breeze circulation becomes clear,the sea breeze front is almost perpendicular to the ground and its strength enhances,then the sea breeze convection can last for a longer time,indirectly due to the blocking and lifting effect of the mountain.At the same time,the valley breeze circulation forms and disappears almost simultaneously along with the sea breeze.Therefore,the valley breeze circulation interacts with the sea breeze circulation on the same side of the mountain.On one hand,this can cause frontolysis of the sea breeze front when the valley breeze strengthens suddenly,otherwise it causes frontogenesis of the sea breeze when the valley breeze weakens.On the other hand,the circulation becomes more pronounced and strengthened when the valley breeze overlaps with the sea breeze.However,in flat areas,the dynamic and thermal effects of the terrain are weak.The sea breeze pattern in flat areas is more regular than in mountain areas.Sea breezes from different directions gather together,and some convective clouds merge in the convergence of sea breezes.Furthermore,the variation of sea breeze circulation mainly depends on the interaction with sea breezes from different directions.