Water tank studies of atmospheric boundary layer structure and air pollution transport in upslope flow systems

{[}1] Heated mountain slopes sometimes vent air pollutants out of the boundary layer over the slope top and at other times trap pollutants in closed circulations. Field, numerical, and water tank studies of fair weather atmospheric conditions over complex terrain have shown more complicated vertical distributions of temperature, moisture, and aerosols than over horizontal terrain. To study these phenomena, we analyze flow fields, densities, and dye distributions in a bottom-heated salt-stratified water tank over a 19 degrees slope with adjacent plain and plateau and compare with field and numerical model data. Vertical layering of dye results from upslope and plain-plateau circulations. The thermal boundary layer (TBL, the bottom layer up to neutral buoyancy height), coincides with the lower branches of these circulations. The return flow branches form elevated layers (EL) with properties intermediate between the TBL and environmental background. As heating continues, the TBL rapidly entrains the ELs, leading to deeper circulations with new ELs at greater heights. Field data suggest that successive formation and entrainment of ELs occurs at multiple scales in the atmosphere. If the aerosol loading of an EL is too high to distinguish it from the underlying TBL on lidar backscatter scans, then both layers and the associated closed circulation appear embedded in one deep backscatter boundary layer. The findings suggest defining the atmospheric boundary layer in complex terrain on the diurnal heating timescale rather than the commonly used 1-hour timescale, which is more appropriate for flat terrain. We discuss conditions leading to venting versus trapping of air pollutants.