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

ISBA

Physiographic data for soil and vegetation

+

MODCOU

QrQi

E

H

G

Aquifer

DailyStreamflow

Surface scheme

Snow

SAFRANObservations + NWP modelsPrecipitation, temperature, humidity, wind, radiations

Hydrological modelPoor

Weak to moderate

Good

Nash

Habets et al. (2008)

Meteorological analysis

The SIM hydro-meteorological model

The SIM based ESPS

ObservationsMeteor. models

ANALYSIS RUN (daily)

SAFRAN10-year

climatology Wind, Rad.,

Humidity

SOIL WAT. TABLES

RIVERS FINAL STATE

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

ISBA MODCOU

SOIL WAT. TABLES

RIVERS STATE

• Simple method, based on SAFRAN and an altitudinal gradient

10 October 2004

Non-perturbed run, 240h total

Distance Interpolation

1/r2

ECMWF EPS Precipitations (1.5° grid)

Interpolated on the SAFRAN zones

Interpolated on the 8km ISBA meshes

Spatial disaggregation of ECMWF EPS precipitations (Rousset-Regimbeau et al.)

Altitudinal gradientsT : -0.65 K/100mPrecip : • 2 mm/m/year where altitude < 800m • 0.7 mm/m/year where altitude > 800m

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)

SAFRAN/PEARP precipitation ratio over 1 year

PEARP : 0.25° grid

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

Observations (5000 rain gauges)

ECMWF (Day 1)

PEARP (Day 1)

Results over the test period : 10 March 2005 – 30 September 2006

Statistical scores of day 1 and 2 are better for PEARP than for ECMWF rainfall

Distribution by basin size (BSS against reference)

Basins sizes

Q10 Day 1

Q10 Day 2

Q90 Day 2

Q90 Day 1

ECMWF

PEARPBasins sizesBasins 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 statesof the ESPS by assimilation(presentation on Wednesday during the COST WG2 session)

Simple method as a first approach (ROUSSET-REGIMBEAU,2007)

DEMETER Precipitation (2.5° grid)

Interpolation on ISBA mesh - 8km

Interpolation on SYMPOSIUM zones

Vertical GradientT : -0.65 °K/100m

Rain : + 0.7 to 2 mm/year/1m

Interpolation1/r²

Distance

Cumulated rain in mm over March-April-May 1998

SAFRANReference

Seasonal hydrological forecasts (Tanguy et al.)

Soil Water Index

Temporal correlation calculated against the SIM reference on March-April-May months over 31 years

1.0

-1.0

High correlation for mountainous areas : good snow melting simulation (?)

0.0

Riverflow forecasts The Durance at Embrun : 3-month

average (MAM)

SIM 88 m3/sMeans :

Ensemble mean 105 m3/s

785 m3/s

758 m3/s

Means :

The Garonne at Tonneins : 3-month

average (MAM)

Mean riverflow in m3 /s

SAFRAN Ensemble mean Members

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!