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

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Abstract
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/no rain” category. Thus, reasons other than resolution are responsible for most of the advantage of the Eta Model over the NGM. An increase in vertical resolution from 17 to 38 layers resulted in an additional modest improvement of skill at the medium and at most of the higher intensity categories. An increase in horizontal resolution from 80 to 40 km brought greater improvements, this time at the lightest and at the medium intensity categories. Another feature we have investigated is the use of the eta coordinate. As a sequel to our earlier tests, we have recently done 16 forecasts running the Eta Model as an eta and also as a sigma system model. In contrast to the earlier experiments, in the sigma mode no “eta-like” discretization of mountains was performed. At all categories the “Eta” has achieved higher scores than the “Eta/sigma”, its advantage being particularly large for more intense precipitation. A fitting question on this occasion is, “What are our views on possible further progress in weather prediction through developments in numerical methods?” From the vantage point of the Eta Model, which we take as a state-of-the-art standard, we identify five areas which are offering or may offer promise of additional increase in skill. These are the two areas referred to above: resolution and the choice of the vertical coordinate, and those of the choice of the vertical grid, of the numerics of the propagation of gravity-inertia waves, and of the box-average vs. point-sample treatment of predicted variables. We comment on prospects for benefits to be achieved via efforts in the direction of each of the five areas listed.