The dynamics of drainage flows developed on a low-angle slope in a large valley

A two-dimensional version of the Regional Atmospheric Modeling System (RAMS) has been employed to simulate the strong drainage flows observed by simultaneous operations of four tethered balloon sounding systems on an approximately 6-km long, near uniform, low angle (~ 1.6 degree) slope in the southwestern part of the Salt Lake Valley. The simulated drainage flow characteristics agreed well with the tethersonde observations. Detailed analyses of flow dynamics were performed to understand the forcing mechanisms behind the drainage flows that appeared to be stronger and deeper than what was expected for a low-angle slope like this. The analyses show that despite the small slope angle, the buoyancy force dominated the momentum balance. The pressure gradient force or the thermal wind term, which was previously considered negligible for uniform slopes, was found to be important in the downslope flow layer above the maximum drainage wind speed. This pressure gradient force was produced, despite the near uniform slope terrain surface, by the gradual deepening of the drainage flow depths in the downslope direction. When a valley was introduced at the foot of the slope in the simulations with a nocturnal valley inversion building up from the surface, the simulated drainage flow was retarded considerably, consistent with the observations. The analysis of the flow dynamics indicated that this retardation was caused by a reduction of buoyancy force and an increase of pressure gradient force as a result of the pool of cold air in the valley.