Global ocean reanalysis at CERFACS

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Global ocean reanalysis at CERFACS

• A. T. Weaver and N. Daget
• CERFACS, Toulouse

CLIVAR/GODAE Ocean Reanalysis Workshop, ECMWF, 31
Aug. 1 Sept.
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CERFACS reanalysis system specifications

• Ocean model
• OPA version 8.2 (Madec et al. 1998)
• Global, free surface configuration (ORCA grid)
• 2 deg. zonal resolution, variable meridional
  resolution with equatorial refinement (1/2 deg.
  at equator)
• 31 vertical levels with 16 in the upper 200m
• Surface forcing fields
• ERA-40 with bias-corrected precipitation
  (Troccoli and Kallberg 2004)
• Delayed adjustment of P-ER based on global mean
  sea level drift (adjustment timescale 1 day)
• Relaxation fields
• Strong relaxation (-200Wm-2K-1) to gridded
  Reynolds SST
• Weak relaxation (3-year timescale) to subsurface
  temperature and salinity from gridded Levitus 98
  
• Part of the common experimental design in the
  ENACT project
• (see www.ecmwf.int/research/EU_projects/ENACT
  and Davey et al., CLIVAR Exchanges, Vol. 11, No.
  3, 2006)

CLIVAR/GODAE Ocean reanalysis workshop, ECMWF, 31
Aug. 1 Sept.
3
CERFACS reanalysis system specifications

• Assimilation algorithm
• Based on the OPAVAR source code as described in
  various references Weaver et al. (2003), MWR
  Vialard et al. (2003), MWR Ricci et al. (2005),
  MWR Weaver et al. (2006), QJRMS
• Assimilated observations and observation error
  specification
• T and S data profiles from the quality-controlled
  ENACT/ENSEMBLES data-set, version EN2v1a (Ingleby
  and Huddleston 2006, JMR)
• Profiles vertically thinned (max. 5 measurements
  per model layer)
• Cut-off depth at 1000m cut-off latitude at
  65oN/S
• No data assimilated in closed or in some
  semi-enclosed seas
• Observation errors assumed uncorrelated
• Observation error variance profiles specified
  from the globally-averaged estimates of Ingleby
  and Huddleston (2006) increased near coastlines.

CLIVAR/GODAE Ocean reanalysis workshop, ECMWF, 31
Aug. 1 Sept.
4
CERFACS reanalysis system specifications

• Version of OPAVAR used for the reanalysis
  experiments
• Multivariate 3D-Var
• Incremental formulation
• Innovation computed with the full model on the
  outer loop
• Increment computed by quadratic CG minimization
  in the inner loop
• 2 outer iterations are used, with 10 30 inner
  iterations for each outer iter.
• Control variables are temperature and the
  unbalanced part of salinity
• Multivariate constraints based on background T-S
  conservation, hydrostatic balance, dynamic
  height, geostrophy are used to compute the
  balanced parts of S, u, v and SSH
• Background error statistics for T and
  unbalanced S
• These variables are assumed to be mutually
  uncorrelated
• Horizontal and vertical (univariate) correlations
  are assumed to be approximately Gaussian and are
  modelled using a diffusion equation
• Zonal/meridional length scales are
  stretched/shrunk near the equator
• Vertical correlation length scales are made
  resolution-dependent
• T and unbalanced S variances are parameterized in
  terms of the background vertical T gradient and
  background vertical mixing coefficient,
  respectively
• Cycled using a 10-day assimilation window and
  Incremental Analysis Updates (Bloom et al. 1996,
  MWR)

CLIVAR/GODAE Ocean reanalysis workshop, ECMWF, 31
Aug. 1 Sept.
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Summary of CERFACS ocean reanalysis experiments

• Exp. CFCS2 CERFACS Control (ENACT) Stream 2
• OPA-ORCA2, ERA-40, no data assimilation,
  1962-2001
• Exp. CFAS2 CERFACS 3D-Var Assimilation (ENACT)
  Stream 2
• OPA-ORCA2, ERA-40, 3D-Var of T and S profiles,
  1962-2001
• Exp. CFASA CERFACS Assimilation (ENACT) Stream
  A
• OPA-ORCA2, ERA-40, upgraded 3D-Var of T and S
  profiles,1993-2001
• Contributions made to intercomparison Items 3,
  4, 5, 6 and 7
• 5-year spin-up prior to start of each
  reanalysis stream

CLIVAR/GODAE Ocean reanalysis workshop, ECMWF, 31
Aug. 1 Sept.
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Future

• ENSEMBLES project production of ensembles of
  variational analyses
• Climate forecast initialization
• Ocean reanalysis
• Online background-error estimation
• Collaboration with ECMWF and MERCATOR on
  development of future versions of OPAVAR
  (NEMOVAR).

CLIVAR/GODAE Ocean reanalysis workshop, ECMWF, 31
Aug. 1 Sept.