[en] ASCOT (Atmospheric Studies in Complex Terrain) is a multi-laboratory U.S. Department of Energy research program studying the properties of atmospheric boundary layers over non-uniform terrain and the interactions among various scales of motion that influence those properties. Within this context, one of the principal goals of the ASCOT program is to provide information necessary for an accurate description of transport and diffusion processes for atmosphere pollutants that may be released in regions of complex terrain. Three examples from past ASCOT research relevant to this goal are presented. Current and proposed research in the Front Range region of Colorado in the vicinity of the Rocky Flats Plant is also described

[en] A field test program, which assesses the accuracy of the gradient profile method of determining deposition velocities, is described

[en] Field tests of a system to determine deposition resistances of specific pollutants and the eddy diffusivity for pollutants in the surface layer have been conducted, and the data have been analyzed. The results add considerably to the data base available for study of pollutant dry removal processes

[en] The dry removal processes of pollutants associated with emissions from fossil-fuel-fired power plants were studied in field tests. Results of ozone dry deposition tests using both an eddy flux method and a profile method are given. These data are needed as input for modeling long-range transport of atmospheric pollutants

[en] Data from a number of field experiments have been analyzed to study the behavior of the quantity S = sigma/sub y//xsigma/sub theta/. A previously proposed relation is shown to be valid only under rather restricted circumstances, and significant variations with sampling period, averaging time, and release height are shown

[en] A study has been made of atmospheric diffusion over level, homogeneous terrain of contaminants released from non-buoyant point sources up to 100 m in height. Current theories of diffusion are compared to empirical diffusion data, and specific dispersion estimation techniques are recommended which can be implemented with the on-site meteorological instrumentation required by the Nuclear Regulatory Commission. A comparison of both the recommended diffusion model and the NRC diffusion model with the empirical data demonstrates that the predictions of the recommended model have both smaller scatter and less bias, particularly for groundlevel sources

[en] The role of boundary layer mixing is increasingly recognized as an important factor in determining the concentrations of ozone and other trace gases near the surface. While the concentrations at the surface can vary widely due to horizontal transport of chemical plumes, the boundary layer is also characterized by turbulence that follows a diurnal cycle in height and intensity. Surface oxidant concentrations can therefore undergo significant changes even in the absence of photochemistry. A central goal of the Phoenix 2001 Field Campaign was to study vertical mixing with the onset of convection and to quantify the effect of this mixing on chemistry within an urban boundary layer. As part of this study, a series of low altitude aircraft sampling flights were made over the Greater Phoenix area between June 16-30, 2001. The resulting observations, in conjunction with a series of surface measurements and meteorological observations, are being used to study the vertical transport and reactivity of ozone and ozone-precursors shortly after sunrise. Additional details of this campaign are given in Doran, et al. (2002). It was anticipated that turbulence over Phoenix at night would be suppressed as a result of cooling of the boundary layer over the city. By sampling shortly after sunrise, we hoped to collect measurements above the residual nocturnal stable layer and to continue sampling through the developmental period of a convectively active boundary layer. We report here on the first analysis of these observations, made from a Gulstream-1 (G-1) aircraft operated by the U.S. Department of Energy

[en] A network of sodars was operated in the late summer and fall of 1993 to monitor the occurrence of nocturnal winds from a canyon in Colorado's Front Range near the Rocky Flats Plant and to determine the influence of those winds on the flow fields over the plant. The canyon flows could be broadly classified into two categories: well developed and irregular. The well-developed flows were generally stronger, deeper, and more continuous than the irregular ones, and the canyon's influence on the wind fields near the plant site was confined primarily to periods with these flows. These periods, in turn, usually followed days during which a deep mixed layer formed over the plains to the east of the mountains. Following days with shallower mixed layers, the canyon winds tended to be weaker and shallower. Numerical simulations with a nested mesoscale numerical model were used to examine the mechanisms responsible for this behavior. The nighttime simulated temperature gradients between the air near the mountain slopes and the free air over the plains were found to be larger after days with deep mixed layers, resulting in stronger down-canyon flows at night. Marker particles released into the simulated flow fields were used to follow the motion of air parcels from the mountains out over the plains. They revealed a tendency for air parcels to remain elevated when they exit the valley on nights with lighter canyon winds and shallower afternoon mixed layers, thereby reducing the canyon's potential effect on the near-surface winds over the Rocky Flats Plant. Particle trajectories were also used to examine the concept of a well-defined airshed feeding a draining valley; the concept was found to be of limited validity for the topography in this area. 18 refs., 15 figs., 1 tab

[en] The problem of measuring or calculating appropriate surface heat and moisture fluxes for use in general circulation models (GCMs) and single-column models (SCMs) is an important one. For this discussion, three issues are considered: 1. From measurements of surface fluxes at a finite number of points, how can one interpolate/extrapolate to get the average flux over an area covered by an SCM or a GCM grid cell? 2. How are fluxes parameterized in models? 3. How much does it matter if fluxes are not correctly parameterized or if incorrect flux values are used? To address these questions, results from both observations and model simulations are described. These results show differences in boundary layer properties over adjacent areas with differing surface characteristics. Such differences can have important consequences for the determination of area-averaged flux values from point measurements

[en] Measurements of simple nocturnal slope winds were taken on Rattlesnake Mountain, a nearly ideal two-dimensional ridge. Tower and tethered balloon instrumentation allowed the determination of the wind and temperature characteristics of the katabatic layer as well as the ambient conditions. Two cases were chosen for study; these were marked by well-defined surface-based temperature inversions and a low-level maximum in the downslope wind component. The downslope development of the slope flow could be determined from the tower measurements, and showed a progressive strenghtening of the katabatic layer. Hydraulic models developed by Manins and Sawford (1979a) and Briggs (1981) gave useful estimates of drainage layer depths, but were not otherwise applicable. A simple numerical model that relates the eddy diffusivity to the local turbulent kinetic energy was found to give good agreement with the observed wind and temperature profiles of the slope flows