Topographic Generation of Mesoscale Vortices in Mixed-Layer Models

Abstract

Abstract The fundamental assumptions underlying mesoscale mixed-layer models of the atmospheric boundary layer are that 1) the flow is hydrostatic, and 2) the Reynolds-averaged potential temperature and horizontal velocity are vertically well mixed. These assumptions determine completely the Reynolds-stress profile, which to a good approximation is a quadratic function of height, and has curvature proportional to the horizontal buoyancy gradient. The only significant source of the vertical component of vorticity in mesoscale mixed-layer models is the curl of the divergence of the Reynolds stress, which can generate quasi-stationary vortices downstream of three-dimensional topography in flow containing buoyancy gradients. We provide quantitative guidance about conditions sufficient for these vortices to form under the mixed-layer modeling assumptions. We caution that observations do not appear to support strongly the assumption that velocity is vertically well mixed in baroclinic, convective boundary layer...