Boundary-layer plumes over mountainous terrain in idealized large-eddy simulations

Coherent plume structures in the convective boundary layer over non-flat terrain are investigated using large-eddy simulation. A conditional sampling method based on the concentration of a decaying passive tracer is implemented in order to identify the boundary-layer plumes objectively. Conditional sampling allows quantification of the contribution of plume structures to the vertical transport of heat and moisture. A first set of simulations analyzes the flow over an idealized valley, where the terrain elevation only varies along one horizontal coordinate axis. In this case, vertical transport by coherent structures is the dominant contribution to the turbulent components of both heat and moisture flux. It is comparable in magnitude to the advective transport by the mean slope-wind circulation, although it is more important for heat than for moisture transport. A second set of simulations considers flow over terrain with a complex texture, drawn from an actual digital elevation model. In this case, conditional sampling is carried out by using a simple domain-decomposition approach. We demonstrate that thermal updrafts are generally more frequent on hill tops than over the surroundings, but they are less persistent on the windward sides when large-scale winds are present in the free atmosphere. Large-scale upper level winds tend to reduce the vertical moisture transport by the slope winds. Conditional sampling allows quantification of the contribution of boundary-layer plumes to the vertical transport of heat and moisture in the convective boundary layer. Over mountain slopes, coherent structures can be seen as a part of the slope-wind layer. Mostly, they dominate the turbulent transport over mountainous terrain. For moisture, turbulent transport can be surpassed by advective transport by the slope-wind circulation. Accordingly, moisture exchange is very sensitive to upper level winds. Generally, large-scale winds reduce the plume frequency on windward slopes.image