Abstract: Shear-induced bottom boundary layer convection in stratified basins: A modeling study



Author(s): J. Becherer, L. Umlauf


Recent studies have shown evidence for inverse density-stratification inside turbulent bottom boundary layers (BBLs) on the slopes of stratified basins. These data suggest that the near-bottom vertical current shear acting on the cross-slope buoyancy gradient may create unstable stratification and therefore strongly modify turbulence and mixing in the BBL.  In addition to shear production, this mechanism provides an additional source for turbulent kinetic energy with a yet unknown impact on the basin-scale mixing. Here, we present results from a process-oriented modeling study investigating this effect in lakes, where the near-bottom shear and turbulence result from internal seiching motions. The simulations have been performed with a 3d-high-resolution numerical shallow-water model using topography-following coordinates and a state-of-the-art second-moment turbulence model. In agreement with available data our results suggest that mixing occurs in a BBL of a few meters thickness, and dominates the basin-scale mixing for the systems we have investigated (Lake Constance, Lake Alpnach). Turbulence during BBL convection was found to be strong with diffusivities approximately one order of magnitude larger than during stable stratification. However, because of the weak stratification, the contribution of unstable boundary layers to overall mixing was found to be negligible. Conversely, during periods of downwelling, the stabilizing effect of the shear greatly enhances the mixing efficiency thus leading to strong mixing in spite of the comparatively small diffusivities. Several mechanisms for the communication of mixed BBL fluid with the interior will be discussed.