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)
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Description and validation of a streamflow assimilation system for a distributed hydrometeorological model over France. Impacts on the ensemble streamflow.
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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
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