青岛近岸一次海陆风过程中大气波导成因的数值研究

Numerical study on the formation of atmospheric duct during a sea-land breeze process in the nearshore of Qingdao

采用新一代区域大气数值模式WRF对2006年8月21日青岛地区一次典型的海陆风过程中的大气波导现象进行了详细的数值模拟研究,以探究海陆风过程中大气波导的形成机理。WRF数值结果较好地再现了海陆风及大气波导的发生发展过程。基于高时空分辨率的WRF数值模拟结果,对海陆风过程中大气波导的形成机制进行了详尽的分析,结果表明:湿度突变层的存在是导致波导发生的主要因素,此次波导过程无逆温层的存在;波导随海风的出现而形成。午后,海陆温差增大,海风在海陆热力性质差异的驱动下形成,并将大量水汽带到近岸,使近岸底层大气相对湿度增大,陆上暖干空气在海风环流作用下向海洋一侧平流,最终在海洋上空下沉,海洋大气边界层发展成为具有稳定层结的热内边界层;同时,下沉的干空气与底层湿空气形成较大的水汽垂直梯度,有利于大气折射指数随高度减小,形成大气波导。海风环流减弱至陆风环流形成发展后,水汽垂直混合均匀,大气波导随之减弱、消失。地形因素对波导的形成作用显著,地形的阻挡使海风携带的大量水汽在向海一侧积聚,在海风环流作用下波导形成于陡峭地形向海一侧。

The atmospheric duct in a typical sea-land breeze process occurred in Qingdao on 21 st August,2006 is investigated using the state-of-the-art WRF.The WRF numerical results reasonably reproduces the occurrence and development of the sea-land breeze and atmospheric duct.Based on the WRF numerical simulation with high temporal and special resolution,a detailed analysis of the formation mechanism of atmospheric duct in the process of sea-land breeze is conducted in this paper.The results show that the existence of the layer with abrupt humidity change is the main factor leading to the occurrence of atmospheric duct,and there is no temperature inversion layer in this duct process.Moreover,the atmospheric duct occurs with the appearance of sea breeze.The land-sea temperature difference increases in the afternoon,and the sea breeze is formed driven by the difference in thermal properties between the sea and the land,which brings a large amount of water vapor to the shore and increases the relative humidity in the bottom atmosphere.Meanwhile,the warm and dry air on land is advected to the ocean side under the influence of sea breeze and sinks over the ocean in the end.The oceanatmosphere boundary layer develops into a thermal inner boundary layer with stable stratification.The sinking dry air and the bottom moist air causes a larger vertical gradient of water vapor,which is favorable to the decrease of atmospheric refractive index along with height and the formation of atmospheric duct.When the sea breeze circulation weakens and the land breeze forms and develops,the water vapor is mixed evenly in the vertical direction,and the atmospheric duct weakens and disappears.Topography plays a significant role on the formation of duct.A large amount of water vapor carried by the sea breeze accumulates on the seaward side due to the terrain obstruction,and the duct is formed on the seaward side of the steep terrain under the influence of the sea breeze circulation.