Investigating the effects of surface heterogeneity on the vertical structure of the Saharan convective boundary layer using large eddy simulation

Studies on the convective Saharan Boundary Layer (SABL) show that large scale surface heterogeneities can significantly affect its vertical structure. The widely homogeneous desert region, characterized by high levels of incoming solar radiation, is intercepted by land patches of different soil and vegetation characteristics that alter the surface energy balance. As a result, spatial variations in radiative properties such as albedo, modify the surface fluxes and consequently control the turbulence structure of the overlying convective boundary layer (CBL). In order to investigate the land – atmosphere interactions over this region the National Center for Atmospheric Research's large-eddy simulation code (LES) (Moeng 1984, Moeng and Wyngaard 1988, Sullivan et al. 1994, 1996, Sullivan and Patton 2008) is coupled, in a two-way interaction mode, to the Noah land surface model (LSM) (Ek et al., 2003; Mitchell et al., 2004). Initial conditions for the LES-LSM system are provided by real case simulations carried out with the mesoscale Weather Research and Forecasting model (WRF) (Skamarock et al. 2008) during selected daytime summer periods over the Sahara. Thus, for the LES-LSM system, representative surface forcing and ground boundary conditions are used to drive the LSM while vertical atmospheric profiles of the SABL initialize the LES model. A strip-like surface heterogeneity is imposed on the x - direction (long side) of the rectangular LES-LSM domain and extends along the entire y – direction, perpendicularly to the mean wind flow. The strip is located in the center of the domain, it intercepts a large homogeneous sandy desert area and has different soil, vegetation and radiative properties. Control runs and test cases configured with different strip length and varying surface flux densities are compared and analyzed. Results show that a strip with a halved surface albedo produces almost doubled fluxes in density and stronger convergence near the ground than over the surrounding sandy surfaces. Low order statistics and visualization of the LES flow field reveal that the absolute value of the averaged minimum buoyancy flux near the inversion layer increases above and downwind of the strip. Variances of potential temperature and water vapor mixing ratio, along the base of the inversion, indicate that an entrainment of warm and dry air dominates on the upwind side of the strip. On the other hand, strong thermals penetrate into the inversion layer, above and downwind of the strip, producing a vigorous exchange of air masses between the non turbulent and turbulent regions. The turbulent structure of the simulated SABL plays an important role in processes such as dust uplift and long range transport, influencing weather in nearby and remote climates.