Downslope Wind Forecasts in a Mountainous Region: Assessing Uncertainty in High-Resolution Modeling over the Las Vegas Forecast Zone

This study focuses on the evaluating of parameterizations for the atmospheric boundary layer (ABL) and its inclusion in operational forecast models such as the Weather Research and Forecasting (WRF) model. The two main issues are the overestimated mountaintop/valley winds as well as surface temperatures. The performance of WRF in complex topography and measurements of winds in the evolving ABL will be examined. The goal of this study is to quantify uncertainty from the model forecasts as well as to evaluate different turbulence closure schemes in the mountain-valley region.

The Terrain-induced Rotor Experiment (T-REX) was conducted over the Owens Valley in California during spring 2006. One of the goals in this study was to learn how the stable boundary layer structure changed in terms of the onset of drainage flow and the occurrence of a nocturnal low-level jet in Owens Valley during quiescent Enhanced Observing Periods (EOPs) during T-REX. The other goal of T-REX was to study the development of topographically generated lee waves and rotor clouds. These phenomena also occur seasonally in the Las Vegas Valley. Both valleys are within the forecast zone of the NWS Forecast Office in Las Vegas and present a challenge to forecasters. Downslope wind events on 15 April 2008 and on 4 October 2009 will be discussed. These events also created problems for the heavy air traffic into the Las Vegas area. The WRF model has been adopted for this research. Vertical resolution was produced with 61 levels in which 23 levels are below 2 km, and the lowest level at 10 m AGL. The working hypothesis is that the model vertical resolution is significant in the error of averaged winds near the surface.

The preliminary results demonstrated remarkable detail of the turbulent mechanisms responsible for surface gustiness. The meso-gamma scale (e.g., 1 km grid spacing) greatly improved its ability to resolve severe downslope wind conditions, and should improve the ability of forecasters to predict similar events. Further attempts to recreate the boundary layer environment will be addressed in order to better configure an operational high-resolution model for the region.