AbstractAfrican easterly waves (AEWs) are the primary synoptic-scale weather feature found in sub-Saharan Africa during boreal summer, yet there have been few studies documenting the performance of operational ensemble prediction systems (EPSs) for these phenomena. Here, AEW forecasts in the 51-member ECMWF EPS are validated against an average of four operational analyses during two periods of enhanced AEW activity (July-August-September 2007–2009 and 2011–2013). During 2007–2009, AEW position forecasts were mainly under-dispersive and characterized by a slow bias, while intensity forecasts were characterized by an over-intensification bias, yet the ensemble-mean errors generally matched the forecast uncertainty. Although 2011–2013 position forecasts were still under-dispersive with a slow bias, the ensemble-mean error is smaller than 2007–2009. In addition, the 2011–2013 intensity forecasts were over-dispersive, and had a negligible intensity bias. Forecasts from 2007–2009 were characterized by higher precipitation in the AEW trough center and high correlations between divergence errors and intensity errors, suggesting the intensity bias is associated with errors in convection. By contrast, forecasts from 2011–2013 have smaller precipitation biases than 2007–2009 and exhibit a weaker correlation between divergence errors and intensity errors, suggesting a weaker connection between AEW forecast errors and convective errors.
Weather and Forecasting – American Meteorological Society
Published: Feb 6, 2018
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