On the importance of meteorological downscaling for short, medium and long-range hydrological ensemble prediction over France G. Thirel (1), F. Regimbeau (2), G. Tanguy (1), E. Martin (1), L. Franchisteguy (2), J.-P. Céron (2), J. Noilhan (1) and F. Habets (1) CNRM-GAME, Météo-France, CNRS, France (2) Direction de la climatologie, Météo-France, France (3) UMR-SISYPHE, ENSMP, CNRS, France (guillaume.thirel@meteo.fr, +33 (0) 5 61 07 97 30)

1 model, 3 ensemblist applications (ESPS) SIM : an hydrometeorological model distributed over France Medium-range ensemble streamflow forecasts (Regimbeau et al.) Forced by the ECMWF EPS (1.5°, 10-day range, 51 members) Pre-alert Short-range ensemble streamflow forecasts (Thirel et al.) Forced by the PEARP EPS (0.25°, 60-h range, 11 members) Alert, localized and severe events Seasonal ensemble streamflow forecasts (Tanguy et al.) Forced by DEMETER (2.5°, 4-month range, 9 members) Water management

The SIM hydro-meteorological model Meteorological analysis SAFRAN Observations + NWP models Precipitation, temperature, humidity, wind, radiations E Surface scheme H ISBA + G Snow Physiographic data for soil and vegetation Qr Qi Nash Daily Streamflow Hydrological model Poor Weak to moderate Good MODCOU Aquifer Habets et al. (2008)

The SIM based ESPS SAFRAN ISBA MODCOU ISBA MODCOU ANALYSIS RUN (daily) ECMWF/PEARP/DEMETER Ensemble forecasts 51/11/9 members ENSEMBLE FORECASTS T+ Precip Spatial DISAGGREGATION ISBA MODCOU ENSEMBLE FORECAST SOIL WAT. TABLES RIVERS FINAL STATES Observations Meteor. models 10-year climatology Wind, Rad., Humidity SAFRAN SOIL WAT. TABLES RIVERS STATE SOIL WAT. TABLES RIVERS FINAL STATE ISBA MODCOU

Spatial disaggregation of ECMWF EPS precipitations (Rousset-Regimbeau et al.) Simple method, based on SAFRAN and an altitudinal gradient ECMWF EPS Precipitations (1.5° grid) Interpolated on the 8km ISBA meshes Distance Interpolation 1/r2 Altitudinal gradients T : -0.65 K/100m Precip : 2 mm/m/year where altitude < 800m 0.7 mm/m/year where altitude > 800m Non-perturbed run, 240h total 10 October 2004 Interpolated on the SAFRAN zones

Visualization of the outputs Run every day, quasi operational Internal website updated everyday Several types of output graphs transmitted to the SCHAPI Flood risk Persistance of the flood risk

Spatial disaggregation of PEARP EPS precipitations (Thirel et al.) Spatial interpolation on the SAFRAN zones Altitudinal gradient : does not work! Differences not linked to relief Point by point bias removal (calibrated over a 1-year period) PEARP : 0.25° grid SAFRAN/PEARP precipitation ratio over 1 year

Precipitation disaggregation ECMWF : altitudinal gradient (climatological) 2 mm/m/year where altitude < 800m 0.7 mm/m/year where altitude > 800m PEARP : bias removal calibrated over one year Results over the test period : 10 March 2005 – 30 September 2006 Observations (5000 rain gauges) ECMWF (Day 1) PEARP (Day 1) Statistical scores of day 1 and 2 are better for PEARP than for ECMWF rainfall

Distribution by basin size (BSS against reference) Q10 Day 1 Q90 Day 1 ECMWF PEARP Basins sizes Basins sizes Q10 Day 2 Q90 Day 2 Basins sizes Basins sizes

Short/Mid-term ESPS conclusions 2 simple downscaling methods A first approach, to improve! Encouraging results of ensemble streamflow forecasts SIM/PEARP improves the scores for small basins and floods High interest for flood alert SIM/ECMWF brings useful pieces of information at a medium-range scale Useful for water management Better PEARP and ECMWF EPS are now available Further improvements are expected Improvement of the initial states of the ESPS by assimilation (presentation on Wednesday during the COST WG2 session)

Seasonal hydrological forecasts (Tanguy et al.) Simple method as a first approach (ROUSSET-REGIMBEAU,2007) Interpolation on SYMPOSIUM zones Interpolation 1/r² Distance Vertical Gradient T : -0.65 °K/100m Rain : + 0.7 to 2 mm/year/1m DEMETER Precipitation (2.5° grid) Interpolation on ISBA mesh - 8km Cumulated rain in mm over March-April-May 1998 SAFRAN Reference

Soil Water Index Temporal correlation calculated against the SIM reference on March-April-May months over 31 years 1.0 High correlation for mountainous areas : good snow melting simulation (?) 0.0 -1.0

Riverflow forecasts The Durance at Embrun : 3-month average (MAM) The Garonne at Tonneins : 3-month average (MAM) Mean riverflow in m3/s SAFRAN Ensemble mean Members Means : Means : SIM 88 m3/s 785 m3/s Ensemble mean 105 m3/s 758 m3/s

Seasonal ESPS conclusions First approach of the topic at Météo-France Better results than using the climatology Good potential to provide reliable pieces of information on soil moisture and streamflows Water management

Conclusions and perspective 3 differently time/spatial-scaled ESPSs implemented at Météo-France Flood alert, pre-alert and water management can be dealt with The three systems need further work on meteorological disaggregation and model uncertainty Current PEARP and ECMWF EPS have better resolution and time-range -> improvement of ESPSs expected! PERSPECTIVES ECMWF-SIM will be in an operational mode (2010-2011) Implementation of the assimilation system in this operational ESPS PhD Thesis to work on the seasonal forecasts (?)

Thank you for your attention!