Model Forecast Improvements with Decreased Horizontal Grid Spacing over Finescale Intermountain Orography during the 2002 Olympic Winter Games

Abstract

Abstract Forecasts produced for the 2002 Olympic and Paralympic Winter Games (23 January–25 March 2002) by a multiply nested version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are examined to determine if decreasing horizontal grid spacing to 4 km improves forecast accuracy over the finescale topography of the Intermountain West. The verification is based on high-density observations collected by the MesoWest cooperative networks, including approximately 200 wind and temperature sites and 100 precipitation sites across northern Utah. Wind and precipitation forecasts produced by the 4-km MM5 domain were more accurate (based on traditional measures) than those of its parent 12-km domain. The most significant improvements in wind speed forecasts occurred at night in valleys and lowland locations where the topography of the 4-km domain produced more accurate nocturnal flows. Wind direction forecast improvements were most substantial at mountain sites where the better topographic resolution of the 4-km domain more accurately reflected the exposure of these locations to the free atmosphere. The 4-km domain also produced quantitative precipitation forecasts that were either equally (small events) or more (large events) accurate than the 12-km domain. Precipitation bias errors varied substantially between the two domains since the representation of the region’s narrow, steeply sloped, basin-and-range topography improved dramatically at 4-km grid spacing. Curiously, the overall accuracy of temperature forecasts by the 4-km domain was not significantly better than that of the 12-km domain. This was due to an inability of the MM5 to properly simulate nocturnal and persistent cold pools within mountain valleys and the lowlands upstream of the Wasatch Mountains. Paradoxically, the added resolution of the 4-km domain, coupled with the failure of this version of the MM5 to fully capture the nocturnal and persistent cold pools, resulted in poorer skill scores. At upper elevations, which are typically above the cold pools, the 4-km domain was substantially more accurate. These results illustrate that decreasing horizontal grid spacing to less than 10 km does improve wind and precipitation forecasts over finescale Intermountain West topography. It is hypothesized that model improvements will ultimately enable the advantages of added model resolution to be fully realized for temperature forecasts over the Intermountain West.