Eta vs sigma: Placement of storms, Gallus-Klemp test, and 250 hPa wind skill compared to ECMWF in ensemble experiments

Fedor Mesinger
University of Maryland, Earth System Science Interdisciplinary Center
Noon January 27 in Room 2155

Over the years as many as five times documented tests were done comparing the Eta model against the same code but switched to use sigma and in all of them the eta version did better. Among these results, better precipitation scores, and more accurate placement of storms, stand out. A possibility that these results came because the Eta precipitation schemes were “tuned” to work best with the eta would seem to have been eliminated by the results of the parallel test comparing the Eta/EDAS system against the NMM-WRF/GSI system during the 5+ months of 2006. In this parallel the operational Eta although “frozen” for considerable time achieved better precipitation scores than the NMM-WRF that used more advanced data assimilation system, the more so the further one moved away from the data assimilation time. A weakness that received extraordinary notoriety of flow separation in the lee of the Witch of Agnesi topography, is shown to have been removed with the latest refinement of the sloping steps eta discretization. Results that were presented by Veljovic et al. (Meteor. Z., 2010) included those of an experiment in which 26 Eta ensemble members driven by an ECMWF 32-day ensemble mostly had better scores in placing strong 250 hPa winds than their driver members. Trying to identify the primary cause of this perhaps surprising result 10 of the Eta members were driven by switching the vertical coordinate to sigma. While no obvious impact on 250 hPa wind scores stood out, a tendency was seen for more accurate tilt of a 250 hPa trough of the eta compared to sigma members. To test the sensitivity to resolution and also to check on the robustness of this Eta vs ECMWF result to the choice of the period a 10-member Eta experiment was rerun for a more recent ECMWF ensemble, one initialized 4 October 2012, when its resolution was higher than of that used previously. The advantage of the Eta members more frequently than not is seen again, even though this time the resolution of the Eta during the first 10 days of the experiment was about the same as that of the driver ECMWF members. Rerunning the Eta ensemble with the code switched to sigma this time however an advantage of the Eta/eta over the Eta/sigma is seen, quite noticeable during the early 2-6 day period of the experiment when a deep upper-air trough was moving across the Rockies. Candidate reasons for the advantage of the Eta and also Eta/sigma over ECMWF are commented upon.