An Evaluation of Mesoscale Model Predictions of Down-Valley and Canyon Flows and Their Consequences Using Doppler Lidar Measurements during VTMX 2000

Abstract

A mesoscale model, a Lagrangian particle dispersion model, and extensive Doppler lidar wind measurements during the Vertical Transport and Mixing (VTMX) 2000 field campaign were used to examine converging flows over the Salt Lake valley in Utah and their effect on vertical mixing at night and during the morning transition period. The simulated wind components were transformed into radial velocities to make a direct comparison with about 1.3 million Doppler lidar data points and to evaluate critically the spatial variations in the simulated wind fields aloft. The mesoscale model captured reasonably well the general features of the observed circulations, including the daytime up-valley flow; the nighttime slope, canyon, and down-valley flows; and the convergence of the flows over the valley. When there were errors in the simulated wind fields, they were usually associated with the timing, structure, or strength of specific flows. The simulated flow reversal during the evening transition period produced ascending motions over much of the valley atmosphere in the absence of significant ambient winds. Valley-mean vertical velocities became nearly zero as down-valley flow developed, but vertical velocities between 5 and 15 cm s?1 occurred where downslope, canyon, and down-valley flows converged, and vertical velocities greater than 50 cm s?1 were produced by hydraulic jumps. A fraction of tracer released at the surface was transported up to the height of the surrounding mountains; however, higher concentrations were produced aloft for evenings characterized by well-developed drainage circulations. Simulations with and without vertical motions in the particle model produced large differences in the tracer concentrations at specific locations and times, but the amount of tracer moving out of the valley atmosphere differed by only 5% or less. Despite the stability, turbulence produced by vertical wind shears mixed particles several hundred meters above the surface stable layer for the particle model simulation without vertical motions.