Forecasts of Valley Circulations Using the Terrain-Following and Step-Mountain Vertical Coordinates in the Meso-Eta Model

Abstract

The nonhydrostatic version of the NCEP Meso-Eta Model is used to perform simulations that differ by only the vertical coordinate to determine the differences in forecasted valley circulations associated with the step-mountain and terrain-following vertical coordinates and whether one coordinate produces consistently superior forecasts at meso-? and micro-α scales. A horizontal grid spacing of 850 m is used. The model forecasts are evaluated using data from the October 2000 Vertical Transport and Mixing (VTMX) field campaign in the Salt Lake valley. The forecasts of the diurnal evolution of the dominant circulations in the Salt Lake valley, including valley, slope, and canyon flows, and their modification by synoptic forcing during five intensive observation periods, were qualitatively similar to the measurements. Forecasts produced by the step-mountain and terrain-following vertical coordinates each have their own advantages and disadvantages and neither vertical coordinate outperformed the other overall. In general, the terrain-following coordinate simulations reproduced the observed surface wind directions over the valley sidewalls better, while the step-mountain coordinate simulations of nighttime near-surface temperatures and wind speeds were closer to the observations. Significant differences in wind speed and direction between the simulations were also produced in the middle valley atmosphere at night, with the terrain-following coordinate simulations somewhat better than the step-mountain coordinate simulations. Similar forecast errors produced by both simulations probably resulted from the physical parameterizations, rather than the choice of vertical coordinate.