Simulation of entrainment near a density stratified layer: Laboratory experiment and LIDAR observations

In this paper a simple qualitative model of growth of a mixed layer adjacent to a uniform layer with a stably stratified layer is presented. The entrainment of a stably stratified layer into a turbulent mixed layer in a confined region was studied in laboratory for different Richardson numbers. The results for entrainment near a density stratified layer show that the entrainment rate is a weaker function of Ri than the case with a step density profile. It is also shown that the internal waves in the stratified region, with typical buoyancy period of about 10 s may interact with turbulence near the interface and create a non-uniform entrainment rate as an oscillatory behavior with a typical time scale of 150 s. The process is qualitatively consistent with processes associated with internal waves interaction with turbulence and create a momentarily buoyancy flux with different signs. Results of lidar observations of atmospheric boundary layer also show that the oscillation of aerosol layer is probably due to non-uniform entertainment in the interface between the mixed layer and free atmosphere. Although there seems to be a qualitative similarity between entrainment behavior at the top of the atmospheric mixed layer and laboratory experiments «mixing box», but the two are quite different as lab experiments are in an enclosure without mean flow, while the top of the mixed layer is a free solid boundary flow which may be associated with mean flow shear.