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.