Development of an endtoend altimeter mission simulator

1
Development of an end-to-end altimeter mission
simulator
Alix Lombard - Juliette Lambin (CNES)Laurent
Roblou Julien Lamouroux (NOVELTIS)
2
Context

• Debates on future altimetry constellation design
• need for continuity and complementarity between
  missions
• variety of applications (climate, meso-scale,
  operational,) but all need multi-mission
• orbit sun-synchronous or not,
  cycle/repetitivity, existing tracks or not,
• payload bi-frequency or not, radiometer or
  not, platform stability (roll for wide-swath
  altimeter),
• data sampling, latency/availability,

• Need for a decision-making tool End-to-end
  mission simulator (RD CNES funding)
• objective examine the merits of various
  observing configurations / discriminate among
  them
• need for a simple, flexible, evolutive tool

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Status end-to-end altimeter mission simulator
for storm surge observations

• Possibility of studying multi-missions altimetry
  configurations, easy tuning of orbit
  configurations parameters

• Framework of Observing-Systems Simulation
  Experiments (OSSEs, Arnold and Dey, 1986)
  designed to evaluate the impact of observing
  system data in numerical analysis.
• Ensemble Twin Experiments method (Mourre et
  al., 2004)
• pseudo-observations generated from a control
  simulation (oceanic model)
• then assimilated in a free simulation
• The performance of the system is estimated in
  terms of model error (ensemble variance)
  reduction performed via a data assimilation
  system.

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Methodology

• Model configuration MOG-2D / T-UGO 2D (F.
  Lyard)
• barotropic, non linear, finite element
• zone well known / studied and representative /
  varied
• (open ocean, shelf and coastal seas)
• time period 15 days, typical / varied winter
  storm surges
• conditions (16/11 to 01/12/1999)
• atmospheric forcing surf. pressure / 10m-wind
  (ARPEGE)
• tidal forcing

• Generation of pseudo-observations
• Altimetry configuration set up by user (specify
  orbit parameters)
• pseudo-obs. (Sea Level Anomaly) extracted from
  the model reference simulation (non-perturbed
  run), at the space-time altimetry positions
• then noise-added (gaussian noise of 0-mean and
  standard deviation specified by instrument noise
  level)

Nadir
Wide swath

• Model errors computation (prior requirement for
  data assimilation)
• estimated from a 100 Ensemble simulations of the
  model in response to atmospheric forcing errors
  (surf. pressure and 10m-wind perturbed)
  Lamouroux, 2006
• error statistics thus estimated by the ensemble
  variance of the model at each analysis time step
  (daily) errors variable in time and space

11 cm²
20/11
0 cm²
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Methodology

• Data assimilation / Performance analysis method
• s-EnROOI (simplified Ensemble Reduced Order
  Optimal Interpolation) configuration
• simplified no sequential control of the
  model (ensemble error reduction only estimated at
  analysis time, not propagated in time via the
  model) ? quick execution / results obtained
• Possibility to implement EnROOI, ROEnKF, EnKF
  (higher performance but longer computational
  time) but idea to keep a simple / quick
  decision-making tool to discriminate between
  various observing scenario
• SEQUOIA MANTA codes used (De Mey, 2005)

Perturbed simulations
Model reference simulation
Ensemble variance reduction estimation at each
analysis time step
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Validation

• Ideal observing system
• regularly spaced grid
• pseudo-obs / analysis daily

• Results for Ta 20/11/1999
• (analysis time representative of model errors
  over the whole period)

• Time-averaged result

Ensemble variance of the model (before
correction)
Ensemble variance after pseudo-obs. assimilation
of ensemble variance reduction over the period
? Over the whole period (synthetic gain 78),
methodology validated
? Strong and uniform reduction of variance,
especially in the English Channel (gain Ta 94)
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Performance of various altimetry configuration

• Various performance diagnostics
• at each analysis time step, mapped
• synthetic over the period, space averaged

? Efficient tool to estimate the performances of
various altimetry configuration and to
discriminate among them.
? Allow to design orbit and assess performances
of multi-satellite altimetry systems
Reduction of ensemble variance time-averaged over
the period
NB the higher the percentage of variance
reduction, the more the altimeter mission will
provide helpful information to storm surges models
Lamouroux et al, OSTST meeting, Hobart, 2007
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Evolution end-to-end altimeter mission
simulator for the study of tide aliasing question

• Context of possible sun-synchronous orbit
  configurations (SWOT, Jason-3, Sentinel-3, ) ?
  tide aliasing problem

Existing module

• Extension work in progress

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Evolution end-to-end altimeter mission
simulator for the study of tide aliasing question

• Same methodology but some evolutions needed ?
  some work done, some in progress

• Ocean tide model configuration T-UGO 2D
• 28 ocean tide components, model validated
  through
• comparisons with FES2004 / GOT00b
• larger zone (long wave dynamics of ocean tides)
• 1-year simulation
• model dissipation parameters topography,
  bottom friction coefficient, transfer coefficient
  towards barocline modes

• Generation of pseudo-observations
• ocean tide model reference simulation
  (non-perturbed run) ? high frequency (HF)
• lower frequency (LF) ocean circulation
  simulation (daily reanalysis from PSY2V2 global
  ocean model computed by MERCATOR-Ocean)
• pseudo-obs. extracted from the sum of both
  simulations (ocean tide HF ocean circulation
  LF), at the space-time altimetry positions
• ? take into account the coupling HF aliased by
  altimetry sampling at LF / LF circulation

• Model errors computation
• estimated from ensemble simulations of the model
  in response to perturbed model dissipation
  parameters
• work in progress

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Conclusions and perspectives

• Work in progress for tidal analysis (end of RD
  funding SWOT PASO study)
• ensemble model error statistics (ensemble
  variance) computation
• implementation of specific tide aliasing
  diagnostics
• more realistic observation errors to be defined
  (especially for wide-swath altimeter)
• case studies (inferred from PASO SWOT instrument
  study)

• First prototype of the simulator (storm-surge
  model)
• efficient tool to estimate the performances of
  various altimetry configuration and discriminate
  among them
• simple, highly flexible and evolutive, first
  version of a powerful tool for designing orbit
  for multi-satellite altimetry systems (Jason-3,
  SWOT, Sentinel-3 )

• Work plan / Perspectives
• further tests of altimetry configurations case
  studies, ? realistic mission scenario tests
• implement other oceanic processes ocean
  circulation, waves,
• implement more complex data assimilation scheme
  e.g. for refined studies