PNNL

Abstract

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 coordinate and whether one coordinate produces consistently superior forecasts at meso-gamma and micro-alpha scales. 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 has its own advantages and disadvantages and neither vertical coordinate out-performed 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.