Stable mixing and transport in complex terrain: A challenge for measurement and modeling studies

Studies of transport, dispersion and mixing processes under stable conditions in complex terrain have been difficult, because these processes are accomplished by small-scale atmospheric features, which do not mix out under the stable conditions. The difficulty arises because such studies require fine resolution in space and time. Constraints of traditional measurement techniques and numerical weather prediction (NWP) modeling capabilities lead to inadequate density of data nodes (measured or simulated) to resolve the important features. Inability of measurements to resolve these structures means that the finescale structure generated by models goes unverified. Because of the importance of these small-scale features under stable conditions, however, it is crucial that they and their effects be accurately represented.

Within recent decades remote sensing instrumentation has become available that has the capacity to measure atmospheric properties at high spatial and /or temporal resolution. Analysis of data from these sensors can be used to verify the small-scale structure produced by NWP output. One such instrument, which will be discussed here, is the Doppler lidar. In this paper results of three field projects showing the effects of small-scale flow features on stable transport and dispersion under nocturnal stable conditions will be presented and reviewed to emphasize their implications to understanding and modeling such flows.

The first study took place just east of the Colorado Front Range at Rocky Flats. Nocturnal drainage flows and an exit jet from one of the major canyons produced thinly layered flow and transport of tracer material in two different directions. In the second study, the Doppler lidar detected outflow from a valley to the east of Vancouver, B.C., Canada. Concurrent air chemistry measurements revealed that this outflow had been cleansed of certain pollutants, including ozone, as a result of vertical and horizontal transport by thermally forced nocturnal flows. In the third study, which took place in the Great Salt Lake basin in Utah as part of the VTMX campaign, the Doppler lidar found a nocturnal down-basin flow, which interacted with the more local canyon outflows and drainage flows. The strength of the down-basin flow significantly affected the ability of the local flows to transport tracer material away from the Salt Lake City urban area.

Small-scale features control the location and concentration of atmospheric contaminants in stable complex-terrain flows. It is thus critical for applications such as air quality, emergency response (including homeland security), fire weather, etc., to understand and be able to model these flows.