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Regional climate modeling: Should one attempt improving on the large scales? Lateral boundary condition scheme: Any impact?

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A considerable number of authors presented experiments in which degradation of large scale circulation occurred in regional climate integrations when large-scale nudging was not used (e.g., von Storch et al., 2000; Biner et al., 2000; Rockel et al., 2008; Sanchez-Gomez et al., 2008; Alexandru et al., 2009; among others). We here show an earlier 9-member ensemble result of the June-August precipitation difference over the contiguous United States between the "flood year" of 1993 and the "drought year" of 1988, in which the Eta model nested in the COLA AGCM gave a rather accurate depiction of the analyzed difference, even though the driver AGCM failed in doing so to the extent of having a minimum in the area where the maximum ought to be. It is suggested that this could hardly have been possible without an RCM's improvement in the large scales of the driver AGCM. We further revisit the issue by comparing the large scale skill of the Eta RCM against that of a global ECMWF 32-day ensemble forecast used as its driver. Another issue we are looking into is that of the lateral boundary condition (LBC) scheme. The question we ask is whether the almost universally used but somewhat costly relaxation scheme is necessary for a desirable RCM performance? We address this by running the Eta in two versions differing in the lateral boundary scheme used. One of these is the traditional relaxation scheme and the other is the Eta model scheme in which information is used at the outermost boundary only and not all variables are prescribed at the outflow boundary. The skills of these two sets of RCM forecasts are compared against each other and also against that of their driver. A novelty in our experiments is the verification used. In order to test the large scale skill we are looking at the forecast position accuracy of the strongest winds at the jet stream level, which we have taken as 250 hPa. We do this by calculating bias adjusted equitable threat scores (Mesinger 2008) and frequency bias scores for wind speeds greater than a chosen wind speed threshold, with the ECMWF analyses used as truth. We also calculate a traditional RMS difference between the forecast and analyzed winds at this same level. Our results show the Eta RCM skill in forecasting large scales with no interior nudging to be just about the same as and usually even slightly higher than that of the driver model. As to the LBC impact, no disadvantage compared to relaxation was seen from using the Eta scheme, in spite of its requiring information from the outermost RCM boundary only.

Eine größere Reihe von Autoren hat numerische Untersuchungen unternommen, die Fehler in den großskaligen Feldern aufwiesen, wenn "large-scale nudging" nicht verwendet wurde (unter ihnen z. B., von Storch et al., 2000; Biner et al., 2000; Rockel et al., 2008; Sanchez-Gomez et al., 2008; Alexandru et al., 2009). Wir zeigen hier Ergebnisse einer älteren Simulation der Juni-August-Niederschlagsdifferenz über den zusammenhängenden Staaten der USA zwischen dem "Überschwemmungsjahr" 1993 und dem "Dürrejahr" 1988 mit einem neun Mitglieder umfassenden Ensemble, bei dem das Eta-Modell in das COLA AGCM hineingenestet war. Diese Ergebnisse stimmen mit den analysierten Messwerten recht gut überein, obwohl das antreibende AGCM insofern versagte, als es ein Minimum dort auswies wo ein Maximum sein sollte. Es wird hier die Vermutung geäußert, dass dies nur durch Verbesserungen durch das RCM auf der großen Skala möglich war. Hierzu wird die großskalige Vorhersagegüte des Eta RCM mit der des es antreibenden globalen EZMW 32 Tage-Ensemble-Vorhersage verglichen. Weiterhin wird für die Randbedingungen untersucht, ob das allgemein gebräuchliche aber aufwändige Relaxationsschema für das gewünschte Verhalten des RCM notwendig ist. Hierzu wird das Eta-Modell mit zwei unterschiedlichen Randbedingungen gerechnet; zum Einen mit dem gebräuchlichen Schema, zum Anderen mit dem Eta-Modell-Schema, das Informationen nur am äußersten Rand vorgibt und nicht alle Variablen am Ausströmrand vorgibt. Die Vorhersagegüte dieser beiden RCM-Vorhersagen wird gegeneinander und gegen die des Antriebs verglichen. Die übliche rms-Differenz zwischen Vorhersage und analysierten Winden wird ebenfalls berechnet. Unsere Ergebnisse zeigen, dass die Vorhersagegüte des Eta-Modells ohne inneres ,,nudging" auf den großen Skalen meistens nur wenig über der des treibenden Modells liegt. Der Einfluss der Randbedingungen brachte keine Nachteile, obwohl nur Informationen vom äußersten Rand notwendig waren.
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Document Type: Research Article

Publication date: 2010-06-01

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  • Meteorologische Zeitschrift (originally founded in 1866) is the joint periodical of the meteorological societies of Austria, Germany and Switzerland. It accepts high-quality peer-reviewed manuscripts on all aspects of observational, theoretical and computational research out of the entire field of meteorology, including climatology. Meteorologische Zeitschrift represents a natural forum for the meteorological community of Central Europe and worldwide.
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