Eta vs sigma: Precipitation scores, Gallus-Klemp test, and added value at large scales in NWP/RCM experiments

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. 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. A result of some years ago strongly suggesting that in regional climate modeling significant added value is possible in large scales within the domain used was presented in Veljovic et al. (Meteor. Z., 2010), as well as that of an experiment in which 26 Eta ensemble members driven by those of 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, with no obvious deterioration of 250 hPa wind scores. However a tendency was seen for more accurate tilt of the 250 hPa trough of the eta compared to sigma members. To test the impact of resolution and also to check on the robustness of the 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 of the 32 days was about the same as that of the driver ECMWF members.