Characteristics of occasional poor medium-range forecasts for Europe
Rodwell, Mark J. ; Magnusson, Linus ; Bauer, Peter ; Bechtold, Peter ; Cardinali, Carla ; Diamantakis, Michail ; Källén, Erland ; Klocke, Daniel ; Lopez, Philippe ; McNally, Tony ; Persson, Anders ; Prates, Fernando ; Wedi, Nils
A feature of medium-range weather prediction is the occasional strong dip in forecast skill. Such events are often referred to as drop outs' or busts'. Although frequencies have decreased, even a single bust is inconvenient for users of ECMWF products, and it can have a significant impact on seasonal-mean scores. The ECMWF Working Group on Diagnostics has carried out a study aimed at understanding the nature of forecast busts over Europe, and exploring possibilities of further reducing their frequency or severity. This article outlines what has been found out about the general characteristics of European busts. It is established that a large proportion of these busts are associated with increased forecast uncertainty, particularly associated with blocking onset. Much of this uncertainty, particularly in spring, appears to arise from sensitivities to initial conditions over the United States and, in agreement with Grazzini & Isaksen (2002), mesoscale convective systems (MCSs) over the USA play a key role. A companion article in this edition of the ECMWF Newsletter shows how the bust of 10 April 2011 corresponds to the general characterisation found here. It then examines this case in more detail with a view to identifying key factors that could help reduce the frequency or severity of forecast busts.
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