ABSTRACT

Some of the scientific questions we intend to investigate include:

• What are the meteorological mechanisms contributing to near-surface convergence and divergence over the basin floor during stable conditions?
• What are the spatial and temporal variations of the convergence and divergence zones over the basin floor? Are there preferred regions of the basin where rising or sinking motions occur or where pollutants tend to accumulate? Do local terrain variations significantly affect the location of the convergence and divergence zones?
• What is the magnitude of upward and downward motions caused by surface convergence and divergence zones over the basin flow during stable conditions? Where are the mean vertical motions large enough to increase the effective vertical mixing?
• How does the interaction of synoptic, mesoscale, and boundary layer processes affect the vertical mixing of pollutants within the basin atmosphere during stable conditions?
• To what degree does the effects of the urban canopy perturb basin-scale circulations and convergence and divergence patterns that affect the vertical mixing of pollutants?
• How much data does a mesoscale model need to assimilate to adequately represent the wind and temperature fields in a basin?

By addressing these specific scientific questions, we can contribute to answering the more general questions listed in the VTMX science plan that include:

• What are the fundamental processes that control vertical transport for stable and transition boundary layers?
• How do pollutants move through residual layers above a stable or convective surface layer and to what extent can pollutants penetrate stable and residual layers aloft ?
• What is the nature of the interaction of terrain-induced flows with cold air pools in basins, and how do such flows affect the formation and erosion of those pools and the dispersion of pollutants in them?
• How do traveling weather systems remove stable stagnant air out of a basin, and under what conditions do these removal mechanisms fail?

If our hypothesis is correct, then an understanding of the mean vertical motions associated with the local thermally-driven circulations and multi-scale interactions with larger-scale flows will be essential to address each of these questions.

The VTMX science plan states that Salt Lake City or Phoenix are likely candidates for the field campaign. We will assume for the purposes of our proposal that a field campaign will take place in Salt Lake City over a four-week period. There are many advantages for choosing Salt Lake City as a field campaign site. For example, an extensive network of surface stations already exists in the vicinity of Salt Lake City and there will be opportunities to collaborate with atmospheric scientists at the University of Utah, U.S. Army Dugway Proving Grounds, and the Utah Department of Environmental Quality (UDEQ). The steep slopes of the Wasatch Mountains will have a strong influence on the vertical exchange mechanisms associated with convergence and divergence and multi-scale interactions of local, regional, and synoptic flows.

Our proposal will support the operation of a radar wind profiler and five surface meteorological stations; we anticipate that several other meteorological measurements will be supported by other EMP projects. In addition to meteorological measurements, we will collaborate with NOAA's Environmental Technology Laboratory (ETL) in deploying the Depolarization and Backscatter Unattended Lidar (DABUL) as part of the field campaign to measure characteristics of the aerosol distribution. Information on the particulate distribution will be obtained from the backscatter lidar to determine the vertical extent of mixing within the nocturnal boundary layer and the presence of layers above the boundary layer that may be due to recirculating flows. In collaboration with DOE's Brookhaven National Laboratory, a series of perfluorocarbon tracer experiments will also be perfomed. By measuring tracer concentrations at approximately 55 surface sites throughout the basin, we will quantitatively measure how the convergence and divergence patterns affect transport and mixing during stable conditions. The tracer data will also provide a time history of vertical and horizontal transport that can be used to infer the winds between the observation locations. We expect to conduct tracer experiments on six nights of the field campaign that is expected to last four weeks. Five different perfluorocarbons will be released, each at a different location.

A series of analyses of the field campaign measurements will identify processes that can influence the mean vertical motions within the basin atmosphere. Our approach is not to rely on analyzing data from one particular instrument, but to combine several types of measurements to aid in the description of atmospheric processes that affect vertical mixing. Interpretation of these analyses will be closely coupled to the mesoscale model simulations.

A series of modeling studies, with and without 4DDA, will be performed using a coupled mesoscale meteorological (RAMS) and Lagrangian particle dispersion model to elucidate the interactions of synoptic, mesoscale, microscale, and boundary layer processes responsible for the vertical advection and diffusion of pollutants. During the first project year, the mesoscale model will be employed to 1) examine basin circulations and vertical exchange processes associated with fall and winter pollution episodes for Salt Lake City and 2) identify recommended instrumentation sites for the VTMX field campaign.

After the VTMX field campaign, a series of simulations that employ continuous 4DDA will be performed to produce high spatial and temporal resolution analyses within the basin and in the regions surrounding the basin. The u and v component of the wind, potential temperature, and humidity measurements from the radar wind profilers, mini-sodars, airsondes, rawinsondes, and the mesonet will be incorporated by the mesoscale model. The 4DDA simulations will reconstruct the mesoscale features of the basin atmosphere with a high degree of confidence so that the predicted vertical motions associated with regions of convergence and divergence can be used to interpret how they affect the vertical transport of pollutants within the basin atmosphere. The Lagrangian particle dispersion model will then be used with two release scenarios. For the first scenario, particles will be released from point sources to mimic the emission of perfluorocarbon tracers from the surface and the tops of tall buildings. The second scenario is similar to the first, except that particles will be released from a surface area encompassing the urban and suburban areas so that particles are likely to be mixed horizontally and vertically throughout the basin. The predicted particle concentration fields will be compared to the aerosol measurements made by DABUL. We will look for evidence of layering in the basin atmosphere and use the particle trajectory characteristics to determine the role of mean vertical motions in producing those layers.

An additional series of mesoscale model and Lagrangian particle model simulations will be performed, except that 4DDA will not be used. The purpose of these simulations is to determine 1) mesoscale model forecast errors and their effect on simulated vertical transport, 2) whether the urban canopy influences the basin-scale circulations and convergence and divergence patterns, and 3) how synoptic, regional, and local flows interact to either enhance or suppress the mixing of pollutants in the basin. Forecast and 4DDA simulations for individual periods during the field campaign will be made at the same time. If the forecast simulations indicate ways of improving the model performance, then the 4DDA simulations described in the previous section will be repeated. We intend to minimize the impact of 4DDA so that the model's dynamic and thermodynamic relationships are the primary drivers that determine the circulations that affect vertical transport and diffusion processes in the basin atmosphere.

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