On the impact of surface heterogeneity on a realistic convective boundary layer

Turbulent interactions of momentum and scalars within the convective boundary layer (CBL) over a heterogeneous surface can have a strong influence on both local flow properties and regional weather and climate. Using realistic heterogeneous surface flux data as well as mean vertical soundings obtained from the Soil-Moisture Atmosphere Coupling Experiment 2002 (SMACEX) as boundary and initial conditions in a large-eddy simulation (LES) model, this research attempts to further our understanding of the impact of surface heterogeneity on a growing CBL in a realistic environment. On the basis of a newly proposed method, a set of realizations precisely replicating the basic statistics (mean and variance) and cross correlation of surface flux boundary conditions (relative to the actual fields at the SMACEX) are used for a series of LES experiments. The impacts of heterogeneity on two aspects of the CBL are discussed in this study: (1) the characteristic vertical profiles of CBL properties and (2) the vertical mixing scales of the CBL. Within the set of conditions examined in this study, the potential temperature is more sensitive to the surface heterogeneity scale than to specific humidity, and the variation of velocity (both horizontal and vertical) clearly reflects the impact of surface organization. The vertical profile of the coefficient of variation calculated from the potential temperature field is introduced to identify the thermal blending height, and a good agreement is obtained compared to the predicted values using formulations from the literature. A commonly used formulation for the momentum blending height was found to inadequately represent the vertical mixing. Another stability-dependent formulation combining both shear and convective characteristics is shown to provide more reasonable results.