Jeff$Whitaker*,$TomHamill$ John …xs1.somas.stonybrook.edu/~na-thorpex/tigge_workshop_2012... ·...
Transcript of Jeff$Whitaker*,$TomHamill$ John …xs1.somas.stonybrook.edu/~na-thorpex/tigge_workshop_2012... ·...
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The new NOAA Hybrid Ensemble Kalman Filter / Varia:onal Data Assimila:on System
Jeff Whitaker*, Tom Hamill
John Derber, Daryl Kleist, Dave Parrish
Xuguang Wang
*me
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Basic Idea • Incorporate ensemble informa2on directly into the background term of the varia2onal cost func2on. ➊ To obtain flow-‐dependent informa:on on background errors (B).
Current genera2on 4DVar provides some flow-‐dependence in B by evolving with TLM over assimila2on window, but cannot propagate B from one assimila2on cycle to the next.
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Basic Idea • Incorporate ensemble informa2on directly into the background term of the varia2onal cost func2on. ➋ To obtain a beHer representa:on of cross-‐variable covariances.
ps ob
First-Guess SLP contours
PWAT increment
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Basic Idea • Incorporate ensemble informa2on directly into the background term of the varia2onal cost func2on. ➌ To make beHer use of massively parallel computers. Ø 4DVar involves running of a reduced resoluIon linear perturbaIon model and its adjoint sequen&ally, improving flow-‐dependence of B involves lengthening the assimilaIon window. Only scales as well as perturbaIon model.
Ø Ensembles are “embarassingly parallel” – may provide the same improvement in B as long-‐window 4DVar.
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Advantages of the hybrid approach Features from EnKF Features from VAR
Extra flow-‐dependence in B Localiza2on done correctly (in model space)
More flexible treatment of model error (can be treated in ensemble)
Reduc2on in sampling error in 2me-‐lagged covariances (full rank evolu2on of B in assimila2on window in 4DVar).
Automa2c ini2aliza2on of ensemble forecasts, propaga2on of covariance info from one cycle to the next.
Ease of adding extra constraints to cost func2on
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Two ways of using ensemble to specify B
• Use ensemble to set values in parameterized covariance model (ECMWF “EDA” system, based on wavelet transforms).
• Use ensemble directly by adding extra control variables to represent ensemble informa2on (extended control variable approach – UKMO, Env Canada, NOAA).
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Terminology • Prolifera2on of names (EDA, Ensemble Var, hybrid Var/EnKF…). Can be confusing.
• Andrew Lorenc has proposed this terminology: 1) Ensemble 3(4)DVar: uses a ensemble (generated
externally, typically by an EnKF) to define B in the Var cost func2on at the beginning of the window.
2) 4D Ensemble Var: uses an external ensemble to specify B throughout the window (replacing TLM and adjoint).
3) Ensemble 4D Ensemble Var: as above, but ensemble comes from running ensembles of Var, with perturbed obs, boundary condi2ons, as in ECWMF EDA. No external ensemble, no TLM/adjoint.
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EnKF member update
member 2 analysis
high res forecast
GSI Hybrid Ens/Var
high res analysis
member 1 analysis
member 2 forecast
member 1 forecast
recenter analysis ensemble
NOAA Dual-‐Res Coupled Ensemble 3DVar
member 3 forecast
member 3 analysis
Previous Cycle Current Update Cycle 8
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Important Details • EnKF uses same forward operator, same obs as GSI Var,
runs at ½ the resolu2on. Covariance localiza2on and infla2on used to represent under sampled sources of error, crucial for maintaining ensemble spread.
• Localiza2on in GSI included in extended control variable formula2on – but occurs in model space (not ob space as in EnKF).
• Blending of sta2c and ensemble covariance currently controlled by a single parameter (¾ ensemble currently)
• UKMO implementa2on is “ensemble 4DVar”, ours is “3D ensemble Var” (confused yet?). NCEP ensemble component can func2on as stand-‐alone DA system (Var is op2onal).
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Single Observa:on
Single 850mb Tv observa&on (1K O-‐F, 1K error)
3DVAR (all sta:c)
½ Sta:c, ½ ensemble All ensemble
First guess T
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Single Observa:on (Hurricane Ike)
Single 850mb zonal wind observaIon (3 m/s O-‐F, 1m/s error) in Hurricane Ike circulaIon
All ensemble
3DVAR (all sta:c)
½ Sta:c, ½ ensemble
First guess SLP, wind
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NCEP parallel test
Became opera&onal May 22
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Stats for 1st month since implementa:on
Stats for one year ago
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Extension to 4D
• 4D ensemble Var: Use ensemble to propagate covariance within the assimila2on window (instead of TLM).
• GSI now includes this capability • Roughly twice as expensive as 3D ensemble Var.
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Observa2on 2me at beginning (T-‐3h) of 6 hour window Evolu2on of increment (every 3h) for 4D algorithms (Top to Bojom : -‐3h / 0h / +3h)
4DVAR (TL/AD)
HYBRID 4DVAR (TL/AD)
4D-‐ENS-‐VAR (NO TL/AD)
HYBRID 4D-‐ENS-‐VAR (NO TL/AD)
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Summary • A Hybrid 3D ensemble Var DA system has been implemented in NOAA opera2ons. – Result of a produc2ve collabora2on between research and opera2ons.
– Best of both worlds (allows ensemble to propagates B, leverages advantages of Var solver).
– 4D extension now being tested. • For the first 2me, improvements to ensemble system can lead directly to improvements in analysis. – Synergis2c interac2on between ensemble + DA development.
– Need to represent all sources of uncertainty in ensemble!
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