evaluation of an ensemble kalman filter for wrf against a ...evaluation of an ensemble kalman filter...
TRANSCRIPT
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Evaluation of an EnsembleKalman Filter for WRF against a
3D-Var Assimilation System
Alain CayaJeff Anderson, Chris Snyder, Dale Barker
National Center for Atmospheric Research
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Outline
• Motivations• Brief introduction to data assimilation• Description of the EnKF and 3D-Var
methods• Model, observations, experimental design• Observation space diagnostics• Imbalance introduced by the assimilation• Conclusions
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Motivations
• Develop an Ensemble Kalman Filter forthe research community;
• Potential for a wide range of applicationsand scales;
• Ensemble gives an error estimate;• Base for ensemble forecasting.
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Data Assimilation Research Testbedhttp://www.image.ucar.edu/DAI/DART
• Ensemble-based data assimilation schemes• Compliant models:
1. Many low-order models (Lorenz63, L84, L96, ...)2. Global 2-level PE model (from NOAA/CDC)3. CGD’s CAM 2.0 & 3.0 (global spectral model)4. GFDL FMS B-grid GCM (global grid point model)5. MIT GCM (from Jim Hansen)6. Weather Research and Forecast model7. NCEP GFS (assisted by NOAA/CDC)8. GFDL MOM3/4 ocean model9. ACD’s ROSE model (upper atmosphere withchemistry)
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Data Assimilation
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True forecast-error statistics
• Are– Inhomogeneous, anisotropic, non-stationary
• Depend on– Model: error statistics, resolution, forecast
range, …– Observations: error statistics, distribution,
frequency, …
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(Ensemble) Kalman filter
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Ensemble Kalman filter
• x′a are obtained using the square-root filterapproach.
• 56 ensemble members.• No inflation.• Observations assimilated serially, one at a
time.• Horizontal localization half-width: ~ 2500 km.
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Temperature Increments(T-obs at 850 hPa, initial time)
3D-VarEnKF With Localization
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Zonal-wind Increments(T-obs at 850 hPa, initial time)
3D-VarEnKF
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WRF 3D-VARhttp://box.mmm.ucar.edu/wrf/WG4/
• Background covariance model given byrecursive filters (Wu et al., 2002, MWR)
• isotropic - inhomogeneous• Control variables:
• streamfunction (ψ),• unbalanced velocity potential (χ),• unbalanced temperature (T),• unbalanced surface pressure (ps),• pseudo-relative humidity.
• Linear balance equation
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WRF 3D-VAR Background ErrorCovariance
• “NMC” method.• 48 – 24-hour GFS forecasts with T170
resolution valid at the same time for 357cases distributed over one year.
• Amplitudes and scales have been tunedfor WRF.
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http://wrf-model.org
• Regional, atmospheric model• Non-hydrostatic, fully compressible• Multiple nested domains• Movable grids
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Observation Network
Real observations:• NCEP radiosonde data set• U, V wind components• Temperature• No background check
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Observational ErrorsNo observational-error covariances
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Experimental Setup
• North America, 45x45 grid, 200 km resolution,• Real observations available every 12 h,• 3D-Var uses NCEP/FNL global analyses as ICs
(1 Jan 03) and LBCs,• EnKF uses same for initial ensemble mean and
ensemble mean LBCs,• EnKF ICs and LBCs perturbations are drawn
from 3D-Var background-error statistics.
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EnKF
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EnKF
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EnKF vs 3D-Var
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EnKF vs 3D-Var
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EnKF vs 3D-Var(F – O)
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EnKF vs 3D-Var(F – O)
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Imbalance
3D-Var: ~0.057EnKF: ~0.048
Free: ~0.026
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Conclusion 1• Results indicate good performance of
the EnKF at synoptic scale with WRF.• The 12-hour forecasts from EnKF are
closer to the observations than 3D-Var12-hour forecasts:– Consistent with earlier OSSE experiments.– Non-stationary forecast-error statistics in
the EnKF are beneficial.– Both schemes may improve with further
tuning.
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Conclusion 2• The EnKF introduces somewhat less
imbalance than 3D-Var:– OSSE experiments reported earlier
indicated that more imbalance is introducedby the EnKF
– Initialization? (digital filter, build balanceconstraints in the EnKF scheme?)
– Need larger ensemble?– More sophisticated localization? (group filter)
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Ongoing work with WRF EnKF
• Humidity observations assimilation• Satellite data assimilation (retrievals)• Surface observations• Radar data assimilation:
- Radial velocities- Reflectivities