Observations of nocturnal slope flow have been made at two sites with quite different topography and vegetation. In both cases, continuous measurements of wind and temperature profiles were made from towers that extended through the depth of the katabatic flow. At the simpler site, which approximates a tilted plane, three towers were located at different distances down the slope to measure the development of slope flow with downslope distance. Slope flow depth, downslope wind speed, and temperature deficit are found to change with downslope distance at rates that are consistent with the predictions of Manins and Sawford's (1979) layer-averaged model of slope flow, while measured entrainment rates are found to be comparable to those predicted by Ellison and Turner's (1959) laboratory experiments. The depth of slope flow is found to be roughly 0.05 times the vertical drop from the top of the slope, a relationship that also follows from combining Manins and Sawford's model and Ellison and Turner's laboratory data. Analysis of the wind spectra and a simple numerical model suggest that the turbulent kinetic energy profiles in slope flow are dependent on the speed and direction of the ambient wind and can differ substantially from those found over flat terrain. At the more complex of the two measurement sites, the occurrence of slope flow was found to correlate well with a dimensionless number 5 that is a function of the ridge-top wind speed and of the strength and depth of the inversion and that is an estimate of the ratio of the buoyancy deficit to the external horizontal pressure gradient.

Nocturnal drainage flow on simple slopes

Results of a case study of katabatic winds utilizing tethered balloon profiling equipment are presented and discussed. The data are unique in the detail they provide of both vector wind and potential virtual temperature throughout the flow.



It is concluded from the data that the one-dimensional models so far proposed in the literature are inappropriate. Direct computation shows that the divergence of radiation responsible for cooling the air is much smaller than the surface radiation flux used in these models. The inadequacy of these models is not due to the underlying assumption of temporal invariance, but rather the implication is that the flow is significantly three dimensional.



Use of surface stress as the retarding stress in the dynamical balance is also shown to be erroneous, at least so long as ambient wind does not assist the katabatic flow. The stress between the flow and the ambient air is much more important and this stress is inextricably tied in with the downslope development of the flow which is thus inferred to play a major part in the dynamics of katabatic winds.



Katabatic flow is an example of an internal mixing process. The data are shown to suggest a critical gradient Richardson number of about 1/4 for the maintenance of internal mixing above slope flows.

Katabatic winds: A field case study

Many mechanisms and factors are responsible for winds in mountainous areas. A mathematical model was developed on the premise that mountain winds are the result of vector addition of wind components generated and modified by these influences. The functions and techniques used in the model were formulated to define and integrate such components of the resultant wind in mountainous areas as valley-mountain wind, slope wind, sea breeze and land breeze, larger scale wind systems, and sheltering and diverting effect of topography. The model is designed primarily for use in estimating wildland fire danger and computing fire spread in southern California. Data were obtained mainly from the San Bernardino Mountains of southern California. Model winds were computed and compared with available observed winds at four research sites in the San Bernardino Mountains. The model output compared well with observed wind velocities.

A Mathematical Model for Diagnosis and Prediction of Surface Winds in Mountainous Terrain.

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.

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Katabatic Flow Mechanisms on a Low-Angle Slope

Spectra of the time series of wind speeds and temperatures measured in the outflow area of a large drainage region are presented. Peaks are found in these spectra at frequencies corresponding to periods of oscillation of ∼1.5 h. A simple model is given, which accounts qualitatively for some of the observed features.

Velocity and Temperature Oscillations in Drainage Winds

A very simple model of night drainage winds is presented giving the wind velocity of a cooled layer as a function of the following parameters: net radiation loss, friction coefficient, the angle of the slope and the lapse rate in the environment above the treated layer.

A. Hočevar

Papers

Night drainage winds