Katabatic Flow Mechanisms on a Low-Angle Slope

Abstract

Momentum and heat budget equations for katabatic flows on sloping surfaces are revisited. Terms in these equations are evaluated using wind and potential temperature data from four tethered-balloon data collection systems on a 3-km line running down a 1.6° slope at the foot of the Oquirrh Mountains in Utah?s Great Salt Lake valley. The analyses focus on the development with downslope distance of the katabatic flow and the associated negatively buoyant layer under synoptically undisturbed conditions. With strong ambient stratification, the katabatic flow shows little variation between sites, suggesting a state close to local equilibrium. When the ambient stratification is weaker, the acceleration of the katabatic flow between two tethersonde sites is systematically larger than what would be predicted based on observed buoyancy. Comparison of observed flow direction with the local topographic gradient indicates that slope curvature, associated with small deviations from the basically planar slope, may be responsible for the anomalous increase. It is concluded that the cross-slope homogeneity of the flow, which is assumed in simplified katabatic flow models, may be significantly disturbed even on slopes that appear to be planar to the observer.