Medium-range Ensemble Streamflow forecast over France

1. Ensemble Prediction System SAFRANAnalysis Of atmospheric forcing: Rain, snow, humidity, radiations, temperature, wind… Ensemble forecasts from ECMWF 51 members of meteorological parameters, 10-day forecasts OR Surface scheme ISBA E + Physiographic data for soil and vegetation H 51 runs ISBA MODCOU G Snow Qr Qi Daily streamflow 51 streamflow forecasts Aquifer MODCOU Hydrological model 1-day forecast 10-day forecast Talagrand histograms for the Seine at the Paris gauging station, computed from 4 September 2004 to July 2005, with the operational SIM streamflow as reference Initial ECMWF mean ensemble for 17 October 2004 on its 1.5° grid Result of the spatial disaggregation on the ISBA 8km grid SAFRAN observation analysis over France for the calibration period (from 4 September 2004 to 31 July 2005) Result of the downscaling cumulative rainfall of the ensemble mean of the ECMWF EPS for the calibration period Medium-range Ensemble Streamflow forecast over France F. Rousset-Regimbeau (1), J. Noilhan (2), G. Thirel (2), E. Martin (2) and F. Habets (3) 1 : Direction de la climatologie, Météo-France, France 2 : CNRM-GAME, Météo-France, CNRS, GMME/MC2, France 3 : UMR SISYPHE, UPMC, ENSMP, CNRS, Paris, France (guillaume.thirel@meteo.fr, +33 (0) 5 61 07 97 30) 3. Statistical analysis of streamflow forecasts (September 2004 – July 2005) Example of outputs : Ensemble streamflow forecast for the Seine at the Paris gauge station, 15 January 2006 Probability of the streamflow exceeding the 700m^3/s level, for the Seine at Paris, forecasts from 1 to 20 March 2006 A medium-range Ensemble Streamflow Prediction System was set up at Météo-France and is running in real time since September 2004. It is based on the hydrometeorological chain SAFRAN-ISBA-MODCOU (SIM). SIM is composed of a meteorological analysis system (SAFRAN), a land surface scheme (ISBA) and a distributed hydrological model (MODCOU). SIM is forced by the 10-day ensemble forecasts from the ECMWF. The study period is : 4 September 2004 – 31 July 2005. Daily streamflow of the Seine at the Paris gauging station, from 4 September 2004 to 31 July 2005 Rank histograms : Rank histograms are quite similar for all the stations. They show an over-population of extreme ranks (U-shape) especially for the first days. This may show a lack of variability in the ensemble, particularly in the first days of forecast. 1. The SAFRAN-ISBA-MODCOU hydrometeorological model over France The SIM system is composed of a meteorological system (SAFRAN), a land surface model (ISBA) and a hydrogeological model (MODCOU). SIM simulates the evolution of water and energy budget at the surface, the soil wetness and the discharge at 900 stations over France Brier Skill Scores : Brier Skill Scores computed for 1, 5 and 10-day forecasts for the Q90 (floods), following the basin area. The SIM climatology (1981/2004) is used as reference. Good statistical scores for the majority of river gauges, as well for low flows as for floods. BSS decrease with the lead time, and increase with the basins sizes. Two particular flood events : 2. Downscaling method for rainfall ensemble forecasts The Seine basin is relatively large (44000km²), with a slow hydrological response, and a strong influence of the aquifer. This long duration flood is well analysed by SIM, the date of the peak is accurately forecasted several days in advance, as well as the intensity of the event. Daily streamflows for the Hérault at Gignac, for the end-September 2006 flood. Streamflow members are superimposed for 10-day forecasts. Forecasts from 15 September 2006 (up) and 21September 2006 (down) are shown. • 2 steps : • Data from ECMWF are interpolated horizontally onto the SAFRAN zones using distance-dependent weights. • Altitude effects are taken into account with a vertical gradient relative to the difference between ECMWF and ISBA grid altitudes. Daily streamflow of the Seine at Paris for the March 2001 ten-year flood. Ensemble members (10-day long) are superimposed for each day of prediction. The Hérault basin is quite small (1400km²), in a hilly area, with fast response to precipitation. Flash floods occur currently because of convective events. Even if the accuracy is not perfect, the 15 September is already giving an early warning 10 days before the event. The 21 September forecast is more accurate for the date of the peak, but under-estimates the flood. Good overall distribution of the mean ensemble rainfall forecasts from the ECMWF, when compared with the reference SAFRAN analysis. The mean amount of rainfall is well reproduced. Some differences are observed in North-Western zones (over-estimation) and in South-Eastern zones (under-estimation). 4. Conclusions The analysis of the downscaled ensemble precipitations show good results overall. Despite of the lack of dispersion, the classical scores (BSS, RPSS) show satisfying values. Results are good for both high and low flows. Moreover, the system has a potential for being a reliable tool to hydrological forecasters, as well for pre-alert as for alert. The study of short-range ensemble streamflow forecasts has been done with the PEARP ensemble rainfall data, and a streamflow assimilation system is being developed. Improvements on the ensemble system could be done in the future by taking into account model errors.