风切变对边界层对流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"加密观测期间在敦煌站的观测资料以及大涡模式,模拟了对流边界层的发展,以及示踪物从混合层向残留层传输的时空变化。模拟的对流边界层的结构及演变特征与实测结果基本一致。进一步通过有风切变和无风切变的敏感性数值试验,研究了风切变对垂直速度、位温和示踪物浓度的水平分布以及示踪物传输高度的影响。研究结果表明,在有风切变的试验中(甚至风切变仅存在于近地层中),对流边界层的增长加强,而且示踪物被传输的高度也较高。与浮力驱动的对流边界层相比,由浮力和风切变共同驱动的边界层中上升气流较弱而下沉气流较强,但前者的上升气流与下沉气流的分布在垂直方向上更为倾斜。由于夹卷作用的增强,浮力和风切变共同驱动的对流边界层较浮力驱动的对流边界层暖。在夹卷层,浮力和风切变共同驱动的边界层对流的上升气流和下沉气流都比浮力驱动的边界层对流中的强,而且垂直速度的概率密度函数分布也较对称,其位温和示踪物浓度的概率密度函数分布也比浮力驱动的边界层中的平直。对湍流动能收支的分析也表明风切变对湍流动能有重要影响,尤其对夹卷层中的湍流动能切变产生项影响较大。示踪物浓度的概率密度函数垂直分布显示,浮力驱动的边界层中示踪物浓度随高度变化较小,而浮力和风切变共同驱动的边界层中示踪物浓度随高度递减,但是示踪物传输的高度比较高。

Study of the impacts of wind shear on boundary layer convection based on the large eddy simulation

Using the observations measured in the Dunhuang meteorological station during the intensive period of Land-atmosphere Interaction Field Experiment over the Arid Region of Northwestern China, together with a large eddy model (LEM), the effects of wind shear on the growth of convective boundary layer (CBL) are investigated, and spatial and time variations of the tracer transport from the CBL into the residual layer are analyzed. The simulated convective boundary layer agrees overall with observations. A series of sensitivity experiments with and without wind shear are performed to understand the effect of wind shear on the tracer transport. In wind shear cases (even wind shear exists near the surface layer), the growth of the boundary layer is enhanced and tracer can be transported to a higher level. Compared with buoyancy-driven CBL, weaker updrafts and stronger downdrafts existed in the shear-buoyancy-driven CBL. However, the simulated buoyancy-driven convection exhibits a more skewed distribution of updrafts and downdrafts, with weaker downdrafts than those in the wind shear cases. The shear-buoyancy-driven CBL is warmer than the buoyancy-driven CBL due to enhanced entrainments. In the entrainment layer, the shear-buoyancy-driven convection shows more symmetrical distributions of updrafts and downdrafts with stronger updrafts and downdrafts than those in the simulated buoyancy-driven convection. The distributions of potential temperature and tracer are flatter in wind shear cases than that in cases without wind shear. The analysis of the turbulent kinetic energy (TKE) budget shows that wind shear modifies the vertical profiles of different terms in the TKE budget, especially the shear production term in the entrainment layer. The vertical distribution of probability density functions (PDFs) of tracer concentration shows that tracer concentration keeps constant with the increasing height in buoyancy-driven case, while it declines with tracer being transported to a higher level in wind shear cases.