Search

This paper examines the impacts of increasing horizontal resolution on the performance of mesoscale numerical weather prediction models. A review of previous studies suggests that decreasing grid spacing to approximately 10 km or less generally produces more realistic mesoscale structures, with particular benefits for orographically and diurnally driven flows. There have been only a few long-term objective verification studies of high-resolution forecasts, and these studies suggest, perhaps deceptively, that there are diminishing returns as horizontal grid spacing decreases below approximately 10 km. A multiyear objective verification of the University of Washington MM5 real-time forecasting system compares the realism of predicted surface parameters at 36-, 12-, and 4-km grid spacing over western Washington state for periods up to 48 h. Traditional verification statistics (such as mean absolute, bias, and root-mean-square error) are calculated by interpolating model forecasts to the observation sites. Fo...

The timing and strength of surface troughs forecast by the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5) and the National Centers for Environmental Prediction's (NCEP's) Eta Model are evaluated over the eastern Pacific during the 1997–2000 cool seasons (Sep–Mar). The troughs are verified by comparing observations from eight offshore buoys with 3–48-h model forecasts. An automated approach is used to identify trough events by setting thresholds for 3-hourly cyclonic wind shifts and pressure rises. The mean timing errors (biases) for surface troughs in the 36-km resolution MM5 domain are 1–2 h early (negative) to the north of 42°N and near the coast, and 0.5–1.0 h late (positive) over the offshore waters to the south of 42°N. The mean absolute timing errors range from 2–3 h for the southern offshore sites to 4–5 h over northern offshore waters as well as near steep coastal terrain. For stronger trough events, the MM5 trough mean errors...