Diagnostic measures for assessing numerical forecasts of African Easterly Waves

Abstract

The utility of a number of diagnostic measures for assessing forecasts of the synoptic-scale flow over West Africa and the eastern Atlantic is investigated. The forecasts were carried out using the COSMO Model provided by the Deutscher Wetterdienst (DWD) for a three week period in 2004. During this period a number of African Easterly Waves (AEWs) were observed, three of which subsequently developed into the Hurricanes Danielle, Frances and Ivan. A sequence of 72 h forecasts were initialised twice daily from the DWD global analysis, using analyses and 12 h forecasts for the boundary conditions. A variety of diagnostics were used to assess the forecasts including objective analyses of jet and trough axes and Hovmoeller plots. The zonal wind was averaged along the objectively analysed jet axes over West Africa and the Atlantic for the forecasts and analyses. This provides a robust measure of the jet strength that takes into account the spatial variability of the jet location and is not tied to either the maximum wind speed or a particular geographic location. Application of this measure to assess the forecasts showed that overall the jet strength was well represented. The largest errors were associated with local jet variations due to misrepresentation of the African Easterly Waves in the forecasts. The objectively analysed trough axes are used to give a visual indication of the forecast quality. Hovmoeller plots proved useful for assessing the evolution of the AEWs, although the interpretation was difficult when convection in the model produced small-scale but strong vorticity anomalies. The results of this study will be applied to future case studies based on the African Monsoon: Multidisciplinary Analysis (AMMA) special observing periods. Zusammenfassung Um Vorhersagen der synoptischskaligen Strömung über Westafrika und dem Ostatlantik zu bewerten, wurde die Anwendbarkeit verschiedener diagnostischer Methoden untersucht. Für einen dreiwöchigen Zeitraum des Jahres 2004 wurden mit Hilfe des vom Deutschen Wetterdienst (DWD) zur Verfügung gestellten COSMO Modells Simulationen durchgeführt. Während dieser Periode wurden zahlreiche African Easterly Waves (AEWs) beobachtet, von denen sich anschließend drei zu den Hurrikanen Danielle, Frances und Ivan entwickelten. Es wurde eine Abfolge von 72-Stunden-Vorhersagen, deren Randwerte alle 12 Stunden mit Analysen und Vorhersagen des globalen Modells des DWD initialisiert wurden, erstellt. Verschiedene diagnostische Methoden, wie die objektive Analyse der Jet-und Trogachsen sowie Hovmöller-Plots wurden angewandt, um die Vorhersagen zu bewerten. Entlang der so bestimmten Jetachsen über Westafrika und dem Atlantik wurde der Zonalwind sowohl für die Vorhersagen als auch die Analysen gemittelt. Dies liefert ein robustes Maß für die Stärke des Jets, welche die räumliche Variabilität der Lage des Jets miteinbezieht aber weder an die maximale Windgeschwindigkeit, noch an eine bestimmte geographische Region gebunden ist. Bei der Beurteilung der Vorhersagen zeigte die Verwendung dieser...

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