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Scaling and kinematics of daytime slope flow systems Reuten, Christian

Abstract

Flows up heated slopes are an elementary component of thermally-driven flows in complex terrain and play a fundamental role in the transport of air pollutants. Our understanding of upslope flows is still incomplete because of the difficulty of carrying out field measurements in complex terrain, the sensitivity of upslope flows to external disturbances, and the difficulty of resolving topographic details in numerical models. In this dissertation I study upslope flows by combining field observations and water-tank experiments. Field observations at a 19° slope showed strong upslope flows of 4 m s⁻¹ in the lower half of the backscatter boundary layer (BBL), determined from lidar scans of aerosol backscatter. A return flow in the upper half of the BBL nearly compensated the upslope volume transport which suggests a trapping of air pollutants in a closed slope flow circulation. I built a bottom-heated water tank with a 19° slope between a plain and a plateau and, using time-dependent scaling, I develop mathematical idealisations of the water tank and the field site. Field and tank observations of non-dimensional thermal boundary layer (TBL) depth agree within 20%. An analysis of the data with probability theory demonstrates that non-dimensional upslope flow velocities in atmosphere and water tank have significantly different functional dependencies on the governing parameters. I demonstrate that the tank flows are fluid-dynamically smooth and explain the similarity violation by a fluid-dynamic feedback: in the water tank, roughness length strongly decreases with increasing upslope flow velocity; by contrast the atmospheric flows were fluid-dynamically rough and roughness length was approximately constant. Flows in the water tank show a persistent eddy with near-surface flows in downslope direction over the plain adjacent to the slope. I argue that the eddy is a result of a TBL depression in the lower part of the slope caused by upslope-flow advection of dense fluid. Water tank experiments suggest that the eddy can cause strongly rising motion over a valley centre for a ratio of about three between valley width and ridge height. In experiments with a plateau length exceeding roughly half the ridge height, independent plain-plateau and upslope flow circulations developed. The upslope flow layer in the water tank agreed with the TBL; the return flow returned dye originally injected over the plain in an elevated layer above the TBL and underneath the plain-plateau flow. When the dye concentrations in TBL and elevated layer became sufficiently similar both layers appeared as one deep BBL. As heating continued two regime changes occurred. First, the TBL merged with the elevated layer, and the upslope flow formed one large circulation with the plain-plateau flow. In a subsequent regime change, the TBL merged with a new elevated layer formed by the large circulation. Upslope flows in the atmosphere are likely to exhibit regime changes at multiple scales. Conditions conducive to re-entrainment of air pollutants are: symmetric topography; weak stratification and larger-scale flows; strong sensible surface heat flux; low ridge height; short plateau; sensible surface heat flux decrements over the slope; and abrupt slope-angle decrements.

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