Slope flow mechanisms are crucial for the transport of air pollutants in regions of complex terrain. In previous investigations on slope flows, little attention has been paid to the relationship between return flows and the top of the boundary layer. Based on few observations and modelling results the common understanding is that daytime upslope flows vent air pollutants out of the boundary layer air into the free atmosphere. Return flows are understood to be either very small or occur above the top of the boundary layer. Investigations primarily concentrate on the fate of the pollutants after having been vented into the free atmosphere, like re-entrainment into the boundary layer after having been advected to a different region.

During the air pollution field study Pacific 2001 in the Lower Fraser Valley, British Columbia, Canada, we studied slope flow mechanisms by taking measurements of wind speed, lidar backscatter of particulate matter, temperature, and specific humidity. The results presented here are based on measurements taken on July 25-26, 2001 during weak synoptic winds, clear skies, and strong daytime solar heating.

Measurements of the three air flow components were performed with a Doppler sodar in Minnekhada Park at the foot of a SSE-facing slope with an average angle of 19° and a ridge height of approximately 1000m in a shallow maritime boundary layer with a maximum mean height of approximately 900m. At a nearby site in the adjacent plain, a scanning lidar system and a tethersonde were used to measure the backscatter of particulate matter and the temperature, wind speed and direction and specific humidity, respectively.

Our observations show strong daytime upslope flows paired with often equally strong and deep return flows. Most remarkably, we observed that the return flows occurred below the mean top of the boundary layer. Depths of up to 500 m each for the upslope and the return flow and maximum values of the wind components parallel to the slope of up to 6 m/s were observed. The height of the maximum velocity of the upslope wind field was found to be close to one half of the total depth of the upslope flow.

These observations show that air pollutants can remain trapped under the top of the boundary layer rather than being vented into the free atmosphere, resulting in higher concentrations of air pollutants than previously expected. Furthermore, existing analytical models do not explain the shape and the height of the maximum velocity of the upslope wind fields observed during Pacific 2001.