10.1 Water tank modelling of air pollution transport over heated slopes

Wednesday, 23 June 2004: 8:30 AM
Christian Reuten, Univ. of British Columbia, Vancouver, BC, Canada; and D. G. Steyn and S. E. Allen

The transport of air pollution by upslope flows during fair-weather conditions in the daytime is of critical importance to the concentration of pollutants in cities surrounded by mountains. Previous investigations indicated that there are two modes whereby air pollution is transported over mountain slopes: venting of air pollutants into the free atmosphere and trapping within the convective boundary layer. This raises a number of questions regarding the degree to which either mode dominates for given topography and environmental conditions and how transitions between the two modes occur.

In the past, upslope flows were investigated by means of field studies, numerical models, and a few theoretical models, and only very few water tank models. With regard to the open questions, water tank experiments have great advantages over other means of investigation. In comparison with field studies, water tank experiments are less expensive and organizationally involved, can be repeated under predetermined conditions, allow manipulation of more parameters over a wider range, and permit more and better measurements. Water tanks also allow more freedom in the choice of topography, ranging from idealized situations to scaled copies of real topographies. Numerical models share these advantages of water tank models. However, in the case of steep topography, strong advection and an active boundary layer, their approximations and assumptions make them less reliable for predicting atmospheric behavior. In comparison, if a complete scaling of all relevant parameters is possible, water tank experiments permit quantitative atmospheric predictions. Finally, theoretical models are limited to idealized and strongly simplified situations. The major disadvantages of water tank modeling are the technical challenges, the limited domain leading to boundary effects, and the restriction to dry atmospheric processes. Moreover, simultaneous scaling of all relevant parameters is often very difficult.

We built a tank consisting of three sections: a plain, a 19-degree slope, and a plateau. This geometry is an idealization of the topography in the vicinity of Minnekhada Park in the Lower Fraser Valley, British Columbia, Canada, where slope flow measurements were taken as part of the air pollution field study Pacific 2001. The tank has a length of 1.35 m, a width of 0.4 m, and a depth of 0.6 m. Decreasing salt concentration with height stratifies the water. The tank bottom is a bent stainless steel plate. Thirty-four strip heaters are attached underneath the tank bottom, each with a maximum output power of 500 W. Surface heat flux was controlled via power input to the strip heaters. By varying surface heat flux and stratification we could model dry atmospheric conditions covering the range of naturally occurring conditions. We calculated vertical profiles of specific volume, corresponding to potential temperature in the atmosphere, by converting measurements of conductivity and temperature (C&T) with ultra fast and sensitive C&T probes over the foot, centre, and top of the slope. A two-dimensional velocity field was measured by particle image velocimetry. Neutrally buoyant particles were illuminated in a vertical plane along the length of the slope and their movement captured with a digital video camera. By determining particle positions in consecutive video frames the velocity field was calculated. Studies of the dispersion of dyes injected in the tank provided qualitative insight into air pollution dispersion over heated slopes.

Applying scaling and simplifying assumptions, good quantitative agreement was found between atmospheric and water tank observations. Dispersion experiments agreed with qualitative atmospheric observations. For example, dyes injected at the foot of the slope quickly formed a dispersion boundary layer, which was thickest over the foot of the slope and became thinner towards the top of the slope. We also confirmed lidar field observations suggesting that dispersion boundary layer and convective boundary layer, defined via the neutral buoyancy height of rising thermals, do not coincide over complex terrain. In agreement with theoretical predictions and qualitative observations, inhomogeneous surface heat flux lead to a split of the upslope flow into a component returning horizontally toward the plain and a second component continuing upslope. Some theoretical models predict closed slope flow circulations, similar to sea breezes. However, it is still open where the circulation is initiated. In our experiments a return flow started at the top of the slope at about the same time as the upslope flow started at the bottom of the slope. In a water tank model with different topography, Chen et al. (1996) found a critical value of the ratio of ridge height and neutral buoyancy height over the plain, at which a mode switch seemed to occur between venting and trapping of air pollution. Our water tank experiments indicate that the critical value may be dependent on the topography.

The water tank experiments we performed add new insights into the process of air pollution venting over mountains. Surface heat flux inhomogeneities over the slope lead to partial trapping. The ratio of trapping to venting seemed to increase with increasing heat flux inhomogeneities. In the case of homogeneous heating over the slope, however, discrete changes between venting and trapping occurred when the heat flux was increased. The question, how topography impacts any critical parameters, will remain open until a wider variety of topographies will have been studied.

References: Chen, R.-R., N. S. Berman, D. L. Boyer, and H. J. S. Fernando, 1996: “Physical model of diurnal heating in the vicinity of a two-dimensional ridge”, J. Atmos. Sci., 53(1), 62-85.

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