对流冷池对黑风暴沙尘抬升和传输影响的大涡模拟研究

Large Eddy Simulation Study of Effects of Convective Cold Pools on Dust-Uplift and Transportion of Black Storm

利用"西北干旱区陆-气相互作用野外观测实验"加密观测期间敦煌站的温度探空廓线作为大涡模式的初始场,通过在模式中设置冷源的方法,模拟研究对流冷池中的湍流结构特征,以及冷池头部和尾部的湍流对沙尘抬升和垂直传输的影响;进一步通过改变冷源冷却率和冷源半径大小的敏感性试验,探究冷源强度和尺度变化对对流冷池内湍流结构、冷池移动速度和沙尘抬升效率等特征的影响。研究表明:(1)模式模拟的对流冷池结构特征与观测结果较为一致,冷池的头部为较大的湍流涡旋,尾部是受头部下沉气流的阻挡和风切变共同作用形成的湍流涡旋,强度和尺度较小,且距离头部越远,尾部湍流强度也越弱。(2)冷池头部涡旋引起的沙尘抬升潜势较尾部的大,冷池密度流抬升的沙尘大部分在冷池内混合,仅少部分扩散到冷池外,头部的沙尘绝对浓度约是尾部的两倍。(3)增大模式冷源冷却率和冷源尺度,冷池密度流强度增强,头部涡旋移动速度增大,产生的沙尘抬升潜势也增大,沙尘可被冷池内增强的湍流涡旋传输到较高的高度,但由于冷池与环境大气的夹卷作用也增强,冷池消散也较快。

Using temperature profiles observed from Dunhuang meteorological station during the intensive period of a land-atmosphere interaction field experiment over the arid region of North-west China as initial field for the Large Eddy Model （LEM）, characteristics of turbulent structure of convective cold pool and effects of turbulence of the cold pool head and tail on dust uplifting and vertical transportation were investigated by setting cold source in the model. Furthermore, sensitivity tests with varying cooling rate and the size of cold source in the model center were performed to study the effects of intensity and scale of the cold source on the turbulent struc- ture of convective cold pool, the propagation velocity of leading edge of cold pool and the dust uplifted rate, etc. The results show that ： （ 1 ） The simulated structural characteristics of the convective cold pool are consistent with observations. The head of the convective cold pool is a large turbulent vortex. There is a weaker and smaller tur- bulence in the tail due to effects of blocking of downdraft from head＇s vortex and the wind shear. Turbulence in the tail becomes weaker if it is far from the head. （2） Dust uplift potential （DUP） caused by the head vortex is much larger than that of the tail. Dust is largely trapped in the well-mixed cold-pool density current except that only a small fraction of dust diffuses outside the cold pool. The absolute concentration of dust in the head is about twice as much as that in the tail. （3） Simulated results by changing cooling rate and size of cold source in the LEM presented that intensity and moving speed of leading edge of cold-pool density current increases with in- creasing cooling rate and size. It leads to that more dust is uplifted from the surface and more dust is transported to the higher altitude due to the enhanced turbulence of the cold pool, but cold pool becomes less long-lived be- cause of the stronger entrainment of the ambient air with increasing cooling rate and size.