OROGRAPHIC AND STABILITY EFFECTS ON DAYTIME, VALLEY-SIDE SLOPE FLOWS

The effects of orography and initial stability upon the magnitude and configuration of daytime, valley-side slope flows were investigated. A three-dimensional, time-dependent, non-hydrostatic numerical model pr ovided simulations over a range of idealised valley forms for a range of vertical stabilities. The model's short-wave radiation scheme was i mproved and the runs were for a virtually dry atmosphere. Airflow over the valley is influenced by two distinct stability regimes, separated by a sharp threshold value of 0. 37-degrees-C km-1. At lower stabilit ies, flow is strong and predominantly downward. Above the threshold, u plift occurs for all stabilities, decreasing in magnitude with increas ing stability. Cross-valley flow increases in the stability range 0.06 -degrees-C-0.6-degrees-C/100 m and decreases at higher stabilities. Fo r a given stability above the threshold value, vertical velocities are directly related to slope angle. Horizontal velocities increase with slope at low angles but there is a suggestion that they decrease with increasing slope angle at high angles. The effect of valley half-width is much smaller than that of slope; greater valley width leads to a w eaker cross-valley circulation. Conditions for the development of vall ey-slope flow configuration in harmony with the underlying orography a re derived. A quantitative relationship between the magnitude of the a verage flow and the average slope and the initial stability is present ed.