边界层对流对示踪物抬升和传输影响的大涡模拟研究

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实测资料以及大涡模式, 通过一系列改变地表热通量和风切变的敏感性数值试验, 分析了地表热通量和风切变对边界层对流的强度、形式, 以及对对流边界层结构和发展的影响。模拟结果显示风切变一定, 增大地表热通量时, 由于近地层湍流运动增强, 向上输送的热量也较多, 使对流边界层变暖增厚, 而且边界层对流的强度明显增强, 对流泡发展的高度也较高。当地表热通量一定, 增大风切变时, 由于风切变使夹卷作用增强, 将逆温层中的暖空气向下卷入混合层中, 使对流边界层增暖增厚, 但是对流泡容易破碎, 对流的强度也较弱。另外通过在模式近地层释放绝对浓度为100的被动示踪物方法, 用最小二乘法定量地分析了地表热通量和风切变分别与示踪物抬升效率和传输高度的关系。分析结果表明, 风切变小于10.5×10-3 s-1时, 增大地表热通量加强了上层动量的下传, 使示踪物的抬升效率也线性增大;地表热通量小于462.5 W m-2时, 增大风切变减弱了边界层对流的强度, 从而使示踪物的抬升效率减弱。当风切变一定时, 示踪物的平均传输高度随地表热通量增加而增大, 而地表热通量一定, 只有风切变大于临界值时, 示踪物平均传输高度才随风切变的增加而增大, 而临界风速的大小由地表热通量决定。

Study of a Large Eddy Simulation of the Effects of Boundary Layer Convection Tracer Uplift and Transmission

Using a Large Eddy Model (LEM) and observed data from Dunhuang meteorological station during the intensive period of a land-atmosphere interaction field experiment over the arid region of Northwest China, a series of sensitivity experiments were performed to investigate the effects of the surface heat flux and wind shear on the strength and organization of boundary layer convection, as well as the growth of the Convective Boundary Layer (CBL). The results show that increases of surface heat flux with constant wind shear will give rise to a thicker and warmer CBL, stronger convection, and larger thermal eddies due to intense surface turbulence transporting more energy to the upper layer. On the other hand, increases of wind shear with constant surface heat flux lead to a thicker and warmer CBL because of the entrainment of warm air from the inversion layer to the mixed layer, while the boundary layer convection becomes weaker with broken thermal eddies. To investigate the quantitative linkage of surface heat flux and wind shear with the tracer uplift rate and transport height, a passive tracer with a constant value of 100 was added at all model levels below 100 m in all simulations. The least square analysis reveals that the tracer uplift rate increases linearly with the surface heat flux when wind shear is less than 10.5×10^-3 s^-1, owing to the enhancement of the downward transport of higher momentum. However, the tracer uplift rate decreases with increasing wind shear when the surface heat flux is less than 462.5 W m^-2 because of the weakened convection. The passive tracer in the model is also shown to be transported to the higher altitude with increasing surface heat flux and under constant wind shear. However, under a constant surface heat flux, the tracer transport height increases with increasing wind shear only when the shear is above a certain threshold, and this threshold depends on the magnitude of surface heat fluxes.