Impact of Resolution and of the Eta Coordinate on Skill of the Eta Model Precipitation Forecasts

During the last somewhat more than a year of the operational running of the Eta Model at the U.S. National Meteorological Center (NMC) a considerable body of statistics has been accumulated demonstrating the model's significantly increased accuracy in forecasting precipitation compared to that of NMC's Nested Grid Model (NGM). The model has shown a smaller but just as consistent advantage in skill against that of NMC's global spectral model. An obvious question is whether there are design features of the Eta Model which could be identified as responsible for this generally improved performance. One feature on which we have data is spatial resolution. In an experiment we performed, a sample of 148 forecasts was formed from each of four models: three versions of the Eta Model differing in resolution only, and the NGM. The Eta Model of that time had achieved a substantial advantage over the NGM with no increase in resolution at all of our eight precipitation categories except at the lightest “rain/n...