Katabatic flow over long slopes: Velocity scaling, flow pulsations and effects of slope discontinuities

Theoretical and field studies on mean velocity and temperature fields of a quasi-steady nocturnal atmospheric boundary layer on sloping surfaces will be reported, including hydraulic adjustment at slope discontinuities. Based on the scaling of layer-averaged equations of Manins & Sawford (J. Atmos. Sci., 36, 619, 1979), velocity scaling is proposed for katabatic winds under high (negligible entrainment) and low stability (intense entrainment) conditions, which is evaluated using field observations conducted during the Vertical Transport and Mixing (VTMX) campaign as well as using controlled laboratory experiments. A threshold Richardson number between the two regimes is identified. Other predictions include the depth of the katabatic flow, along-slope variation of buoyancy and period of oscillations of the flow. It is shown that under highly stratified conditions the pulsation of katabatic flow is not necessarily a consequence of the widely accepted adiabatic heating mechanism proposed by Fleagle (J. Meteor., 7, 227, 1950) but a result of along-slope buoyancy oscillations. The observations agree well with the former analysis. Laboratory experiments on slope discontinuities show interesting patterns of hydraulic adjustment and associated mixing patterns, depending on the appropriately defined Froude number.