Abstract
To determine the effect of switching between the eta and the sigma coordinate in numerical weather prediction involving topography, five sets of tests were performed. The eta version did better in all of them particularly with precipitation scores and more accurate placement of storms. However, a problem of flow separation in the lee of the bell-shaped topography discovered by Gallus and Klemp seemed to many to suggest the eta coordinate to be ill suited for high-resolution models. Flow separation is shown not to occur following a refinement of the eta discretization. Trying to identify a primary cause of the improvement in 250 hPa winds previously demonstrated in Eta ensemble members over their ECMWF driver members, ten of the Eta members were run switched to sigma. At a critical time, the Eta members in eta mode showed a tendency for more accurate tilt of a 250 hPa trough than the members run in sigma mode. The experiment was rerun for a more recent and higher resolution ECMWF ensemble, and for an increased number of members. The advantage of the Eta over ECMWF is seen again, even though this time, the Eta resolution during the first 10 days of the experiment was about the same as that of driver members. Rerunning the Eta ensemble switched to sigma showed an advantage in the Eta/eta 250 hPa wind scores used, again associated with an upper-air trough’s movement across the Rockies. Better positioning of lee lows ahead of these troughs using Eta/eta is suggested to be making significant contributions to its better precipitation scores. Implications of experiments done for regional climate modeling are discussed as well.
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Acknowledgements
It was under the directorship of Eugenia Kalnay of the Development Division of the then National Meteorological Center in the early nineties that critical steps took place enabling the Eta model to achieve its prominence, this having been made possible by Eugenia’s respect for results—as opposed to a pre-established plan as to what should happen. Attention to the horizontal diffusion of the Eta was prompted by comments from Sandra Morelli, of the University of Modena and Reggio Emilia, Italy; and experiments that led to the discovery of the horizontal diffusion problems at 1-km grid distance were carried out by Jorge Luis Gomes, and Gustavo Sueiro, of the Center for Weather Prediction and Climate Studies (CPTEC), Cachoeira Paulista-SP, Brazil. Eric Rogers, of NCEP, supplied us with the original of one of the plots of Fig. 4, damaged in its cited version. Fedor Mesinger’s work on the development of the “sloping steps” discretization of the Eta dynamical core has been partially funded by NCEP/EMC via grants to ESSIC, University of Maryland, College Park, MD, awarded by Stephen Lord, then EMC Director. Subsequent work on the Gallus–Klemp problem reported on here was done within a visit of Fedor Mesinger to CPTEC, hosted by Sin Chan Chou, under a grant No. 400792012-5, awarded by the National Council for Scientific and Technological Development (CNPq), Brazil. Our work has partially been supported by the Serbian Academy of Sciences and Arts, via grant F-147, and that of Katarina Veljovic by the Ministry of Science and Technological Development of the Republic of Serbia, under Grant No. 176013. Comments and suggestions of three anonymous reviewers of an earlier version of this manuscript, and three of the present version, are appreciated as they significantly helped improve the presentation. Last but not least, Tom Black, of NCEP, not only kindly assisted with the readability of the text and with removing English imperfections of our original, but, in addition at places, suggested formulations that more appropriately summarized some of the results we are presenting.
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Mesinger, F., Veljovic, K. Eta vs. sigma: review of past results, Gallus–Klemp test, and large-scale wind skill in ensemble experiments. Meteorol Atmos Phys 129, 573–593 (2017). https://doi.org/10.1007/s00703-016-0496-3
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DOI: https://doi.org/10.1007/s00703-016-0496-3