Interactions Between Downslope Flows and a Developing Cold-Air Pool

Downslope flows and regions of enhanced cooling have important impacts on society and the environment. Parameterisation of these often subgrid-scale phenomena in numerical models requires a sound understanding of the underlying physical processes, which has been the overarching aim of this work. A numerical model has been used to characterise the development of a region of enhanced cooling in an idealised alpine valley with width and depth of order 10 and 1 km, respectively, under stable, decoupled, poorly-drained conditions. A focus of this work has been to remove the uncertainty surrounding the forcing mechanisms behind the development of regions of enhanced cooling. The average valley-atmosphere cooling has been found to be almost equally partitioned between radiative and dynamics effects. Complex interactions between the downslope flows and the region of enhanced cooling have been quantified for the first time. For example, relatively large variations in the downslope flows are generally restricted to the region of enhanced cooling and cannot solely be attributed to the analytical model of [McNider, 1982a]. These flow variations generally coincide with return flows above the downslope flows, where a thin region of unstable air occurs, as well as coinciding with elongated downslope flow structures. The impact of these interactions on the dispersion of passive pollutants has been investigated. For example, pollutants are generally trapped within the region of enhanced cooling. The concentration of pollutants within the region of enhanced cooling, emitted over the lower half of the slopes, increase as the emission source moves away from the ground-based inversion that expands from the bottom of the valley. The concentration of pollutants within the region of enhanced cooling is very similar when varying the location of the emission source over the top half of the valley slopes. This work includes a test of the effects of varying the horizontal numerical grid resolution on average valley-atmosphere temperature changes.