A modeling study of katabatic flows over slopes with changing slope angle

A mesoscale model is used to study katabatic flows over terrain that includes a change in slope angle. Mesoscale model initialization packages often include terrain smoothing algorithms that can alter valley and mountain profiles in complex terrain airsheds that are inadequately resolved. One common effect of terrain smoothing routines on concave mountain profiles is that of terrain-raising in the vicinity of a slope angle change. The characteristics of katabatic flows over slopes that include a slope angle change are explored, and compared to an idealized simple slope representing smoothed over terrain. In the case of a steeper slope angle followed by a more gradual slope, as the flow progressed over the slope angle change, a decrease in downslope velocity, and increase in potential temperature deficit of the katabatic flow was noted. Examination of the momentum budget revealed that the ratio of vertically integrated mixing and drag to buoyancy increases sharply as the flow adjusts to the new slope angle. Next, a comparison between characteristics of katabatic flows over a simple slope and a combined slope was made, with the simple slope having the same horizontal distance and slope height as that of the combined slope, which includes a slope angle change. It is shown that the katabatic flow over the simple slope has larger downslope velocities and warmer potential temperatures than that of the combined slope near the foot of the slope, while the combined slope is warmer and faster than the simple slope further upslope. Insight into the difference between these two flows is gained through examination of the process of energy conversion in katabatic flows, where kinetic energy is derived from buoyant potential energy, which is proportional to the temperature deficit and height of a parcel. Since the simple slope is everywhere higher than the combined slope, the simple slope has a larger reservoir of buoyant potential energy to convert into kinetic energy, resulting in stronger, deeper katabatic jet near the foot of the slope than that of the combined slope.