Terrain-induced focusing of wind fields in the mesoscale

The focusing of wind fields in the atmospheric boundary layer is obtained as a result of the axial symmetry breaking of the wind flowing through terrain depressions and subsequent formation of an organized convective cell. At the cavity center, the airflow coalesces and moves collectively upward along the z-axis while it rotates around the filament of a three-dimensional vortex. Only at a certain height above the surface, the focal height, does a cross-section of the three-dimensional vortex show a perfectly symmetrical pattern for the vertical component of the wind velocity. Focusing is shown to take place at noon when a perfectly symmetric neutrally stratified layer develops at the cavity while outside the atmospheric layer is unstably stratified. The focusing dependence on the shape of terrain depressions and wind profiles, which act as lenses in nonlinear optics, is presented. Results were obtained by using a three-dimensional hydrostatic mesoscale model.