Study Of Large Eddy Simulation Of Boundary Layer Convection And Vertical Tracer Transport In Arid Area

Boundary layer convection plays an important role in the transportation of heat, water vapor, momentum and aerosols. Sand-dust storms occur frequently in the arid region in Northwest China. It is very important to study the structure and the evolution of the boundary layer and boundary layer convection in arid area due to the energy and the material of a sand-dust storm being transported through the atmospheric boundary layer. Using observations measured in Dunhuang meteorological station during the intensive period of Land - atmosphere interaction field experiment over arid region of North-west China, together with a large eddy model (LEM), the structure and the evolution of the deep boundary layer in arid area in summer are investigated. The simulated convective boundary layer overall agrees with the observations. A series of numerical sensitivity experiments are performed to analyze the effects of the surface heat flux, wind shear, the initial inversion strength and model resolution on the formation and strength of convection and the distribution of the physical magnitude, and boundary layer convection on tracer uplift and transport. Theoretical models of quantitative relationship between surface heat flux and tracer uplift and transport, wind shear and tracer uplift and transport, respectively. The following conclusions are drawn.(1) Using observed potential temperature, specific humidity and wind profiles at07:00on June3and time varying surface heat flux to initialize and force the large eddy model LEM, a series numerical sensitivity experiments with different surface heat flux and wind shear are applied to diagnose the effects of surface heat flux and wind shear on boundary layer convection, tracer uplift and transport. The results show thatsurface heat flux increases with constant wind shear will give rise to a thicker and warmer CBL, stronger convections due to intense surface turbulence transporting more energy to the upper layer. Wind shear increases with constant surface heat flux leads 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.The least square analysis reveals that the tracer uplift rate increases linearly with the surface heat flux when wind shear is less than10.5x10－3s－1owing 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 than462.5W/m2because 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 depend on the magnitude of surface heat fluxes.(2) Using observed potential temperature, specific humidity and wind profiles at12:00on June3to initialize the LEM model which is forced by time varying surface heat flux, numerical experiments with different inversion strength are carried out to investigate the effects of inversion strength on boundary layer convection and tracer transport. It shows that the thicker boundary layer with stronger convections and tracer transported to the higher altitude is simulated in the experiment with weaker inversion layer before it is penetrated by the boundary layer convection. Once the boundary layer convection runs through the inversion layer, deeper boundary layer is formed due to the mixing of the CBL and the overlying neutral layer. The intense convection which transports tracer to a higher level is found in the weaker inversion test.Before the inversion layer is penetrated by the boundary layer convection, the stronger entrainments with a thicker entrainment layer are presented in the experiment with weaker inversion layer. Potential temperature variances become large because of the increased entrainments while the vertical and horizontal wind variances increase due to the stronger updrafts. When the inversion layer is damaged by the convection, boundary layer convection which used to be constrained in a shallower CBL with an intense inversion layer tends to be more vigorous with larger variances of potential temperature and the wind speed. The distributions of PDFs of vertical velocity and potential temperature show that there are more and stronger downdrafts at the top of the boundary layer before the inversion layer is penetrated in the case with weaker inversion layer. The distributions of potential temperature and tracer are flatter at the top of boundary layer due to the enhanced entrainments. Whereas the more and the stronger up and downdrafts are simulated at the top of the boundary layer after the inversion layer is penetrated in the case with intense inversion layer. The distribution of potential temperature becomes flatter with the larger mean magnitude at the upper boundary layer.(3) Using sounding profiles at12:00on June3to initialize the LEM model, sensitivity experiments with different model resolutions are performed to study the impacts of model horizontal resolution on the structure and evolution of the boundary layer, both model horizontal and vertical resolutions on the entrainment layer and on vertical tracer transportation. The results show that there is a great impact of model horizontal resolution on the simulation of mean structure of the boundary layer. The smaller potential temperature with the gradient less than zero is simulated in the case with the larger grid distance due to the weakness of the entrainment with decreasing the model resolution. The potential temperature variances in the entrainment layer increase with the higher model horizontal resolution because of the more warm air entrained downward from the inversion layer. Meanwhile, the vertical velocity variances which is contributed by the smaller scale eddies increase significantly after the motion of them are calculated accurately with increasing model horizontal resolution. The results from a quadrant analysis of the heat flux in the inversion layer indicate that the heat flux in the entrainment zone increase in all quadrants with the largest contribution coming from the upward-moving cooler air with increasing model horizontal resolution. The wider range of the vertical velocity and the potential temperature at different levels are shown with increasing model resolutions by interpreting the distributions of PDFs of the vertical velocity and the potential temperature. Tracer can be transported to the higher level in the case with the smaller model grid distance. The characters of the distributions of physical magnitudes in the boundary layer can not be simulated with the horizontal grid spacing of1000m.There are great changes in heat fluxes in the entrainment layer with varying model vertical resolutions. The simulated profiles of the heat flux in the entrainment layer resemble each other from the cases with30m, 20m and10m vertical grid spacing, respectively. The same thing happens in the cases with50m and100m vertical grid spacing. The largest contribution to heat flux in the entrainment layer is from the upward-moving cooler air with increasing model vertical resolution, In this study, the numerical experiment with200m horizontal resolution and30m,20m or10m vertical resolution can simulate not only the thermal structure but also the finite entrainments at the top of the boundary layer.