This paper describes the use of the Advanced Regional Prediction System(ARPS, developed at the Center for Analysis and Prediction of Storms, University of Oklahoma) for simulation of air flows like those observed during the first field campaign of the Vertical Transport and Mixing eXperiment (VTMX). The first VTMX field campaign was carried out in the Salt Lake Valley in October, 2000. The ARPS data assimilation system has been used to interpolate observed data and a background field from the ETA model (developed at the National Centers for Environmental Prediction) to a coarse ARPS model grid with horizontal spacing of 4 km. Outputs from model runs on that grid have served as inputs for one-way nested grid runs on finer grids with horizontal resolution of 1 km, 500 m, 250 m and 100 m. This paper describes the results from the high resolution simulations of flow in the Jordan Narrows area which separates the Utah Lake and Salt Lake Valleys.

The availability of VTMX field data makes it possible to compare detailed flow simulations with objective analyses of observed data for the same meteorological conditions. When the coarser model results agree with the observed data, the simulations can provide more details of the flow than can be observed. This helps us to understand the flow physics better, especially as related to vertical mixing in stable, urban atmosphere, and more important, in regions of complex topography.

During the fourth Intense Operation Period (IOP4) of the VTMX field campaign, which took place the night of October 8-9, 2000, the model output captures the evening transition period well. When drainage flows developed during the night, the high resolution model results revealed some interesting flow features that resembled hydraulic jumps, bores, wave initiation and Bernoulli effects. For instance, in the afternoon, when there was a northerly flow through the Jordan Narrows, lee waves appeared to develop in the lee of both the Oquirrh Mountains, which mark the west side of the Salt Lake Valley, and Traverse Mountains, which separate the Salt Lake and Utah Valleys. When the wind is strong, there is flow separation and a large hydraulic jump downstream of the mountains. We expect that these simulation results can be used to guide the interpretation of measured data from the same region, and also aid in the design of future field experiments.

This research was supported by the U. S. Department of Energy under the auspices of the Environmental Meteorology Program of the Office of Biological and Environmental Research.