Idealised Simulations of Daytime Pollution Transport in a Steep Valley and its Sensitivity to Thermal Stratification and Surface Albedo

Numerical simulations of tracer transport in an idealised, east-west aligned valley are performed with the Regional Atmospheric Modeling System (RAMS), both two-dimensional and three-dimensional. The results are qualitatively consistent with wintertime observations in the Austrian Inn Valley. The simulations show an asymmetry in wind circulation and tracer distribution between the valley sidewalls according to the orientation of the slope with respect to the sun. Two-dimensional sensitivity experiments are run to investigate the influence of vertical inhomogeneities in thermal stratification and vegetation coverage on the slope-wind circulation and therewith the tracer transport. It is shown that a transition to a layer of higher stability or to a region with higher surface albedo causes a reduction of the mass flux in the upslope-wind layer and due to mass continuity a quasi-horizontal transport out of the slope-wind layer.