3 - Centre National de Recherches Météorologiques - Météo France

1 Data coverage from AMSU-A sounders used in operations in the ARPEGEmodel assimilation, over a 6 hour assimilation period (MetOp is in blue).23 Geometry of the GPSradio occultation technique:the atmosphere refracts theradio signal sent bya GPS transmitter;the refraction angleobserved by a GPS receiverenables to characterizethe atmospheric refractiveindex at the tangentpoint (T).4 The 08/11/2007 strong North Sea storm which amplified all day long.The related cold front, well marked, undergoing further internal fine scaleperturbations that caused severe damages in the North of France (compositeradar reflectivity composite, top left panel). The Arpege operational version(bottom left panel) does not represent such fine structures, but it did forecaststhe storm. On the other hand, the new version of Arpege (right panels) furtherforecasts the front organization and it suggests the internal perturbation, both inits dynamical components (bottom) and in its cloud structure (top). Top rightpanel: 3 layer composite cloud cover, high level clouds (H), mid-level clouds (M)and low-level ones (B). Bottom: wind (barbs, kt) and relative vorticity (shading)at about altitude 1500 m. A maximum vorticity band is a good indicator of afrontal zone, while a spotty maximum outlines a vortex. Contour interval: warmcolours 5 10 -5 s -1 from light red, 2,5 10 -5 s -1 below, cold colours, negative values.PEARP 16/01/2007 18TUvalid. 18/01/2007 18TU (+48h)1. 3. 5. 10. 15. 20. 25. 30. 35. 40.mgp Example of a 48 h uncertainty forecast by the upgraded global PEARP fromthe 16/01/2007 18 UTC, for the 18/01/2007 18 UTC. On that day, a strongstorm hit the North Sea and nearby south-eastern countries. Contours are500 mbar geopotential height (a pressure-like field, altitude about 5.6 km,contour interval 10 damgp) forecast by the ensemble mean. Shading showsthe uncertainty as the standard deviation of the differences between ensemblemembers (interval: 5 mgp). Far from being uniform, uncertainty grows for specificareas shown in red and where it turns out that active bad weather systemsare often located.A first upgradeof the Arpege ensembleprediction system(PEARP)PEARP, for Arpege ensemble forecasting inFrench, is Météo-France ensemble predictionsystem, initially developed by theForecast Department. It is an ensemble targetedtowards short range probabilistic predictions,that is for the following next 3days.On top of an unperturbed control forecast,PEARP consists in 10 more forecasts whoseinitial conditions are subtly non-randomlymodified. The differences between the evolutionsin time of the 11 forecasts provide ameasure of the uncertainty and enable tooutline the probability distributions of thepredicted parameters. In the operationalversion, the perturbations that are used toprepare the various initial states are onlycomputed within a domain restricted toNorth-Atlantic and Europe. CNRM initialaction on PEARP has been to change theseperturbations. Firstly, they are, from now on,computed over the whole globe. Secondly,part of the forecast uncertainty comes fromthe past, through the evolution of perturbationsfrom previous runs during the periodpreceding the initial forecast time.Finally, a small part of the uncertainty aboutthe knowledge of the initial state is derived5 . Research and development: annual report 2007from the latest variationnal analysis and isused to adjust the amplitude of the final perturbationsthat are then added to that analysis.This is the first step of an evolution thataims to develop a state-of-the-art probabilisticforecast system, a step mostly directedtowards improving Météo-France contributionto the WMO supported TIGGE project ofcollecting global ensemble forecast. Thisnew version of PEARP became operational infebruary 2008. 4