Description and validation of a streamflow assimilation system for a distributed

hydrometeorological model over France. Impacts on the ensemble streamflow

forecasts

G. Thirel, E. Martin, J.-F. Mahfouf (CNRM/Météo-France),

S. Massart, S. Ricci (CERFACS),

F. Regimbeau (DCLIMHYDRO/Météo-France),

F. Habets (UMR Sisyphe Mines ParisTech)

Introduction

2 ensemble streamflow prediction systems (ESPS) at a short- and mid-term range at Météo-France– Based on the distributed hydrometeorological model SIM

– ECMWF EPS-based ESPS (10-day range, 1.5°, 51 members)

– PEARP-based ESPS (60-h range, 0.25°, 11 members)

Need to improve the initial states by an assimilation system– Use of observed discharges

– Description

– Validation over 18 months

Impacts of the assimilation system on the ECMWF EPS-based ESPS

ISBA

Physiographic data for soil and vegetation

+

MODCOU

QrQi

E

H

G

Aquifer

DailyStreamflow

Surface scheme

Snow

SAFRANObservations + NWP modelsPPrecipitation, 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 Ensemble forecasts51/11 members, 11/2-day forecasts

ENSEMBLE FORECASTS

T+ Precip Spatial

DESAGGREGATION

ISBA MODCOU

ENSEMBLE FORECAST

SOIL WAT. TABLES

RIVERS FINAL STATES

ISBA MODCOU

SOIL WAT. TABLES

RIVERS STATE

Initial states of ESPS : need for improvement

Adjusted by BLUE

Strategy

186 stations assimilated over France– Low human influence

– Good quality of observations

– Not too bad results given by SIM

Aim : to use observed discharges in

order to improve streamflow simulation,

by adjusting the ISBA soil moisture

The BLUE equations

Analysed state

Background state

Innovation vector

Jacobian H :

H determines the sensitivity of streamflows to soil moisture variations

Hypothesis : linearity of the model

-> H is computed with SIM runs initialized by perturbed soil moisture states (perturbation around 0.1%)

Observed streamflows

streamflows

x : control variable

Jacobian matrix filling

3 gauging stations Q1, Q2 et Q3.

w1, w2 et w3 moderated sums of soil moistures on the basins

Jacobian matrix :

0

0 0

0

basins

stations

186 stations

Principle of the assimilation system

Experiments (10 March 2005 / 30 September 2006, 186 stations)

6 experiments : 3 variable states * 2 physics of the model

Daily assimilation, daily observations

IS2 (and IS1) will be retained

IS2 combines the best Nash and rmse scores, and the lowest increments

The Doubs at Besançon

Scores for a selection of 148 stations

An exemple of the impacts on the ESPS

IS2

IS1

No assimilation

Some statistical scores

Scores for a selection of 148 assimilated stations, for the 10-day SIM-ECMWF

Spread of the ensemble

RMSE

Scores computed against observed streamflows

Brier Skill Score day 1 and day 10

Day 1

Day 10

Conclusions and perspectives

A streamflow assimilation system has been implemented and validated for the SIM suite

– Better simulation of flows and initial states for the ESPSs (Thirel et al., submitted to the Journal of Hydrology)

Significative improvement of ensemble streamflow forecasts when initialized by the assimilated SIM suite

– Lower RMSE, better BSS and RPSS (due to the assimilation for the first days, then due to the exponential profile)

– Few differences between SIM-PEARP and SIM-ECMWF, difficult to conclude– It is the first time that the ensemblist SIM is compared to observations, not a

reference run

Perspectives : – Optimizing computing costs and the quality of the assimilation system (R and B

matrices to better estimate)– Using another operator (EnKF?)– Implementing the assimilation system into the SIM-ECMWF operational suite

(2012?)

Thank you for your attention!