Abstract
The National Weather Centre of Meteo-France has developed a tool to correct the state of the atmosphere, within the ARPEGE operational global model, by adjusting the potential vorticity in cases of disagreement between the initial condition and some available observations. Among them, data from geostationary satellites are the primary source of information. Tropopause coherent structures in the model can be assessed using ozone and water-vapour signatures that can be deduced from Meteosat data. The inversion of the modified potential vorticity provides a new spatial distribution of wind and temperature that can be used as a new initial state from which a new forecast run is possible. The weakness of this approach lies in its qualitative nature. We present a case of forecasting a storm (Klaus, January 24, 2009) where modif ications of an initial state by different experts show significant differences but nevertheless share some features. The structure called consensus – the lowest common denominator – is highlighted by principal component analysis. It represents about half of the variability carried by the various corrections. Besides the observed improvement of the forecast, the good agreement between the different modifications to the initial conditions demonstrates the robustness of the approach.
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