PNNL

Abstract

Momentum and heat budget equations are written for katabatic flows on sloping surfaces. 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 Salt Lake Valley. The analyses focus on the development with downslope distance of the katabatic flow and the associated negatively buoyant layer. Under conditions of strong stratification, the katabatic flow shows little variation between sites, suggesting a state close to local equilibrium. With weaker ambient stratification, 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 assumption of cross-slope homogeneity of the flow which is assumed in simplified katabatic flow models is not even found on slopes which appear planar to the visual observer.