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

Abstract

[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°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.Citation: Reuten, C., D. G. Steyn, and S. E. Allen (2007), Water tank studies of atmospheric boundary layer structure and air pollution transport in upslope flow systems,

Daytime Heat Transfer Processes Related to Slope Flows and Turbulent Convection in an Idealized Mountain Valley

“…The formation of an elevated weakly turbulent mixed layer and its progressive merging with a CBL growing underneath is similar to processes occurring in the vicinity of isolated slopes, observed by Reuten et al (2007) and modeled by Serafin and Zardi (2010).…”

Air Pollution Transport in an Alpine Valley: Results From Airborne and Ground-Based Observations

“…After sunrise thermally-induced upslope winds carry pollutants that have accumulated during the night at the valley floor, upward along the slope (Bader and Whiteman 1989;Whiteman 1989). Pollutants remain either trapped in a closed slope-flow circulation (Reuten et al 2005) or are vented out of the valley into the free atmosphere (Kossmann et al 1999;De Wekker et al 2004); factors favouring trapping or venting are discussed in Reuten et al (2007). An elevated inversion layer favours trapping due to a reduction of the mass flux in the slope wind layer (Vergeiner and Dreiseitl 1987).…”

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

“…They obtained a similar circulation pattern in their summertime simulation, whereas in the wintertime simulation a more symmetric solar heating of the valley sidewalls led to a more symmetric tracer distribution. Reuten et al (2005Reuten et al ( , 2007 collected observations in the Lower Fraser Valley, British Columbia, Canada, and conducted water-tank experiments in order to investigate air pollution transport in the CBL in mountainous terrain. They studied the conditions leading to either venting of pollutants into the free atmosphere at the mountain top or the occurrence of a return flow that causes a closed slope-flow circulation and traps the pollutants within the CBL.…”