Observation impact on data assimilation with dynamic background error formulation ALEXANDER BECK alexander.beck@univie.ac.at Department of, Univ. Vienna, Austria Thanks to: Martin Ehrendorfer, Patrick Haas, and Mike Fisher
Motivation! Poor-man s OSSEs within a conceptual data assimilation system.! Focus on fundamental issues.! Extract as much as possible information from given set of observations.! Quantify impact of background-error formulation for different observational configurations. Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 2
Background error covariance modeling! Specification of B is crucial for DA-system! Several approaches (NMC method, ensemble of analyses, )! Operational implementations use static B! Does a flow-dependent background-error formulation improve the analyses and subsequent forecasts? Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 3
Static & dynamic background errors Z500 variance field implied through B and (QG3L model)! Static B: Flat background specification! Errors of the day are not taken into account! available from Kalman filter Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 4
The quasigeostrophic assimilation system! quasigeostrophic T21/L3 model (n=1449; Marshall & Molteni 1993)! Artificial observations sampled from truth-run with specified observation errors! Static background-error covariance matrix B obtained from NMC approach! Tuning of static B matrix for each observational configuration Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 5
Tuning of static B matrix! B matrix needs to be adapted given model dynamics, length of assimilation interval and observational configuration.! Avoid systematic trend in analysis quality from cycle-to-cycle.! Within QG-framework: Estimate B from statistics. Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 6
Data assimilation in cycling environment static dynamic Analysis error covariance matrix obtained from Hessian of costfunction. Forecast error covariance matrix available from Kalman filter theory. Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 7
Role of initial B specification in KF Evolution of trace for dynamic background-error formulation (KF) started with different initial covariance matrices. Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 8
Results: Assimilation window length Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 9
Z200 Increments: 12h vs. 48h window static dynamic 12h 48h Analysis increments more similar for 48h exp s compared to 12h exp s. Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 10
Results: Spatial observation density Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 11
Temporal distribution & observation errors Impact of observation quality Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 12
Summary! Cycling experiments in QG/4D-Var system! Comparison of static and dynamic background-error formulation! Pre-specified artificial observations! Analysis & forecast errors computed with respect to truth-run Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 13
Conclusions! Mean analysis error decreases with increasing length of assimilation window.! Impact of dynamic background-error formulation is reduced for long ass. windows.! Dynamic background formulation is beneficial for non-uniformly distributed observations.! Importance of model error needs to be addressed ( weak-constraint 4D-Var ). Thank you for your attention! Third WMO Workshop on the Impact of Various Observing Systems on NWP; Alpbach, Austria, 9-12 March 2004 14