Constraining coastal ocean models with altimetry

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Constraining coastal ocean models with altimetry

• Pierre De Mey, LEGOS/POC

WATER-HM/SWOT meeting CNES HQ, Paris, February
2008
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Coastal ocean domains and scales
North Celtic Seas
Oregon CTZ
Chl
Sea-level variance Jan 1999
SST 24 Aug 2003
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Constraining the small-scale coastal EKE
Surface EPE and EKE in POLCOMS
Surface EKE (J/m3)
Surface EPE (J/m2)

• Small-scale EKE patches and filaments
• Fast time scales as critical as short space
  scales to constrain EKE on shelves

(after Michel Huthnance, 2008)
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1. Downscaling to the coastal scale

• Message Downscaling of large-scale models to the
  coastal scale is shaping into a community. New
  science issues are being addressed.

WATER-HM/SWOT meeting CNES HQ, Paris, February
2008
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GODAE Coastal and Shelf Seas Working Group
www.godae.org
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GODAE Coastal and Shelf Seas Working Group
(bold new or updated in 2007)
P19
P18
P20
P21-22
P23-24-25-26
P29-37
P27
P16
P28-31-38
P12
P17
P7
P30
P32-33-34-35
P6
P9-10-11
P8
P10-13-14-15
P4
P36
P31
P5
P3
P2
P1

• 38 projects
• Varied in objectives and methods
• Geographically clustered
• Africa 1
• Australia 2
• China Seas 2
• Japan Seas 3
• Indian Ocean 1

• North Pacific 1
• North America West Coast 4
• Gulf of Mexico 4
• North America East Coast 2
• Arctic and Nordic Seas 3
• Norwegian, Baltic, and North Seas 7
• Northeast Atlantic 6
• Mediterranean 4

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GODAE CSSWG Science issues

• White Paper available on GODAE web site
  www.godae.org
• Main interests of downscaling from GODAE
  solutions (WP 2.2)
• Better local estimates, extend predictability
• Enhance representativeness of high-resolution
  observations, opening the door for better data
  assimilation in Coastal and Shelf Seas
• Critical issues and requirements (WP 3.2)
• Access to adequate large-scale solutions
• Importance of an adequate fine-scale vorticity
  field gt
• Jump in ocean physics and in forcing functions
• Initialization
• Tides and high-frequency barotropic dynamics
• How do we measure impact and value in CSS models?
• MERSEA/GODAE legacy on metrics impact
  assessment procedures in coastal models
• Additional issues data assimilation, two-way
  coupling, unstructured grid modelling
• Community in need of routine, adequate
  observations

Bay of Biscay vorticity
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2. What should we constrain?

• Message We are beginning to get a quantitative
  idea of the errors we must aim at correcting in
  coastal models with the help of sea-level
  measurements.

WATER-HM/SWOT meeting CNES HQ, Paris, February
2008
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Instantaneous ensemble variance a proxy of
model error variance
Bay of Biscay 3D model 30m temperature ensemble
variance, July 30, 2004
What is this? Near-surface temperature ensemble
variance in response to atmospheric forcing
errors. It is a proxy of the actual model
errors. Exhibits the fine time/space scales of a
tracer and is a mix of shelf, shelf-break,
upwelling, and mesoscale responses. Relevance to
WATER-HM? We need to be able to constrain these
fine error scales.
(after Le Hénaff De Mey, 2008)
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Non-local, structured errors in coastal current
SLA
Depth-averaged velocity
Surface salinity
Temperature
EOF-1 79.8
What is this? Ensemble multivariate EOFs in the
Catalan Sea coastal current in response to
coastal current inflow perturbations (mimicking
downscaling errors). Relevance to WATER-HM? SLA
errors are small-scale (O(40km)) and strongly
correlated to fine-scale (u,v,T,S) 3-dimensional
errors which we can then expect to correct if SLA
is observed at sufficiently fine scales.
EOF-2 11.0
EOF-3 4.9
(Jordà et al., 2006)
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Non-local, structured errors in coastal current
SLA
Depth-averaged velocity
Surface salinity
Temperature
EOF-1 79.8
EOF-2 11.0
EOF-3 4.9
(Jordà et al., 2006)
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Activation of coherent error features by storms
What is this? The SLA component of a particular
ensemble EOF in response to atmospheric forcing
errors. It is a proxy of the actual model
errors. As the time series shows, it is activated
during the July 7-8 storm and is characterized by
a shelf-wide response, a surge response, and a
mesoscale response with O(1day) time
scale. Relevance to WATER-HM? Questions 1
(mesoscale), 2 (coastal) and 3 (storm-related).
We expect a wide-swath altimeter to consistently
constrain the fine-scale, multivariate ocean
response to those fast events, and hopefully help
better predict the associated phenomena.
Ensemble EOF-3 SLA, 3D BoB model
July 1
August 31
2004
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Constraining shelf-break exchanges
What is this? The BT transport, high-frequency
SLA, and inverted-barometer SLA components of a
particular ensemble EOF in response to
atmospheric forcing errors. It is a proxy of the
actual model errors. As the bottom right panel
shows, the situation is that of a southwesterly
wind blowing towards the English Channel. The top
right panel shows water piling up in the channel.
The left panel shows the corresponding
fine-scale exchanges through shelf-break canyons
and around capes. Relevance to WATER-HM?
Questions 2 (coastal) and 3 (storm-related). We
expect a wide-swath altimeter to resolve fast
time scales in straits and semi-enclosed seas to
constrain the variability of exchanges between
shelf and deep ocean.
Ensemble-time EOF-1 16-Nov-1999 12Z00 -- State
vector (SLAHF, Ub, SLAIB, t)
scaled transport
SLAHF
SLAIB
(Lamouroux and De Mey, 2007)
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Constraining shelf-break exchanges
Ensemble-time EOF-1 16-Nov-1999 12Z00 -- State
vector (SLAHF, Ub, SLAIB, t)
scaled transport
SLAHF
SLAIB
(Lamouroux and De Mey, 2007)
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3. How would a single wide-swath instrument do?

• Message A single wide-swath instrument on a
  JASON-type orbit would significantly contribute
  to the constraint of the coastal ocean mesoscale,
  coastal current variability and shelf-wide
  sea-level changes.

WATER-HM/SWOT meeting CNES HQ, Paris, February
2008
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Wide-swath vs. nadir in Bay of BiscayStochastic
modelling with atm. forcing perturbations in 3D
BoB
(left panel) What is this? The RM spectra plot on
the left shows the number of degrees of freedom
of model (forecast) error which can be detected
by a particular array amidst observational noise.
This is done by counting eigenvalues above 1.
This is shown for three arrays (legend).
Representer matrices are calculated by stochastic
modelling with atmospheric forcing
errors. Relevance to WATER-HM? Questions 1
(mesoscale) and 2 (coastal). One Wide-Swath
altimeter on a JASON orbit detects 4 degrees of
freedom, while one nadir instrument (JASON)
detects only one. The more d.o.f.s are
detected, the more critical ocean processes will
be constrained.
Array Modes -- SLA
Scaled RM spectra
Top Wide Swath (4 dofs) Mid WS over deep ocean
(2 dofs) Bottom JASON (1 dof)
Slosh
Meso1
Meso2
(after Le Hénaff De Mey, 2008)
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Wide-swath vs. nadir in Bay of BiscayStochastic
modelling with atm. forcing perturbations in 3D
BoB
(right panel) What is this? The array modes of
model error corresponding to the spectra to the
left. For each array, mode 1 is mostly water
sloshing around between shelf and deep-ocean
domains modes 2 and 3 are a mix of mesoscale
submeso response, slope current variability and
shelf processes. Relevance to WATER-HM? Questions
1 (mesoscale) and 2 (coastal). As was seen on
the left panel, JASON can only detect (and
constrain) the slosh mode. One needs a
wide-swath instrument to detect (constrain) all
three modes a 4th one not shown. In this way,
one can objectively demonstrate that a wide-swath
instrument is needed to constrain the coastal
ocean mesoscale and coastal current variability.
(A collaboration between LEGOS and OSUs OST
proposals has been proposed on this topic)
Array Modes -- SLA
Scaled RM spectra
Top Wide Swath (4 dofs) Mid WS on deep ocean
(2 dofs) Bottom JASON (1 dof)
Slosh
Meso1
Meso2
(after Le Hénaff De Mey, 2008)
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Wide swath degradation due to roll errors
What is this? Scaled RM spectra for one
wide-swath track in the Bay of Biscay without,
and with, along-track correlated (50sec) roll
errors. Relevance to WATER-HM? Questions 1
(mesoscale) and 2 (coastal). As long as we have
a good idea of the along-track period of the roll
errors, the effect of those errors is limited (1
detected eigenvalue drop). However the effects
of (1) shorter-period roll errors and of (2) yaw
errors would have to be further quantified.
(after Le Hénaff De Mey, 2008)
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Summary

• Downscaling of large-scale models to the coastal
  ocean is shaping into a community. New science
  issues are being addressed, some of them similar
  to SWOTs.
• The knowledge of coastal circulation is critical
  to many applications (ecosystem management, water
  quality, disaster prevention, transportation).
  These applications by themselves are beyond the
  scope of this mission (see Washington meeting
  summary), but a wide-swath altimeter would
  provide solid scientific foundations towards
  those objectives.
• We are beginning to get a quantitative idea of
  the errors we must aim at correcting in coastal
  models with the help of sea-level measurements.
• A single wide-swath instrument on a JASON-type
  orbit would significantly contribute to the
  constraint of the coastal ocean mesoscale,
  coastal current variability and shelf-wide
  sea-level changes.
• Other comments
• Long-wavelength roll errors are probably easier
  to separate from coastal processes than other
  errors such as shorter-wavelength roll and yaw
  errors.
• Swath overlap is a desired feature to maximize
  revisit time and better constrain the
  higher-frequency part of the spectrum (shelf
  seas).