Experimental Daily Forecasts of Northern Atlantic Weather Regimes and Heavy Precipitating Events (HPEs) over Southern France with the Meteo France Global Ensemble System PEARP
Joly, Bruno ; Girardot, Nicole ; Roulet, Bernard ; Labadie, Carole
Northern Atlantic atmospheric low frequency variability can be described as mainly evolving through a fixed number of recurrent states. (Vautard, 1990) identified four weather regimes which have a mean persistency about 10 days. Further studies have shown that these can aid comprehension of large scale dynamics, especially as when also considering interactions with the Atlantic jet (Riviere and Orlanski, 2007). In particular, periods of likely regime transitions may have high impact on the sequence and density of mid-latitudes cyclones (Michel and Riviere, 2011). More recently, as concern on climate change impacts increases, weather regimes are central to new developments in statistical downscaling techniques. Weather regimes may be characterised by applying automatic classification tools over long periods reanalyses. It is then possible to identify local climate properties that occur specifically in association with particular regimes. Global climate models often suffer from a lack of resolution sufficient to describe regional to local atmospheric properties, but they may better reproduce weather regime distributions. An example of such approach is described in Nuissier et al. (2011) over Southern France, which shows that the conditionnal occurrence probability of high precipitation events can be deduced from given regime membership. In the context of weather forecasting, weather regimes analysis in model forecasts could inform forecasters about the next days’ large scale evolution and likely elements of local weather predictability in a more synthetic way. The assumption is that when the regime prediction skill of a model is poor, the predictability of local weather is weak. We present experimental results concerning the forecasting of the 4 Atlantic weather regimes with the operationnal ensemble forecasting system at Meteo France, PEARP. We also show the results of occurrence probability calculations of Heavy Precipitating Events (HPEs) over Southeastern France in one intense case of 2008 autumn season.
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