Numerical simulations of turbulent flow within and in the wake of a small basin



[1] Small terrain features, such as small valleys, basins, sinkholes, low hills, and outcrops, while generally associated with mountainous regions, can also be found over plains. In this study, we present a numerical investigation of the effect of a small terrain feature (a 30 m high rim) on the mean and turbulent flows inside and downstream of an enclosed basin it surrounds. Results from high-resolution numerical simulations (10 m isotropic spatial resolution) indicate that small terrain features in the proximity of larger ones can induce relatively large modifications to the mean and turbulent flows. The 30 m high rim is found to have an effect on the mean wind speeds at least 600 m upstream from the basin. The main effect is a 10% reduction in wind speed up to 120 m above ground level due to the upstream blocking effect of the rim. The presence of the rim can also double the turbulent kinetic energy (TKE) both inside and downstream of the basin compared to an otherwise identical basin without a rim. The slopes of the basin play an important role in first creating and then defining the wake, and in intermittent wind regimes most of the scalar transport from near the slope of the basin happens through slope roll vortices that define the edge of the downstream wake region of the basin. Inside the basin, the rim acts to limit momentum transfer in the lower half of the basin, which suggests a mechanical forcing effect induced by the rim on lower basin environments that could interact with thermal buoyancy effects in heated or cooled basins. Some of the wake features resemble wind-eroded surfaces in the wakes of Martian craters. Results also reveal a critical height level (43 m below the rim height) that acts as the most favored location for TKE production and destruction, which could be important for the top-to-bottom turbulence erosion of basin boundary layers. These results stress the importance of resolving small-scale terrain features, as their effects can be nonlocal.