Contribution
of a wide-swath altimeter in a shelf seas assimilation
system – Impact of the satellite roll errors
Le Hénaff, M., P. De Mey, B. Mourre,
and P.-Y. Le Traon (JAOT, 2008)
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The authors
investigate the potential qualitative improvement brought by
wide-swath, interferometry-based ocean altimetry measurements
with respect to classical nadir altimeters in a coastal/shelf
data assimilation system. In addition, particular attention
is paid to roll errors, which could significantly reduce the
expected benefits of wide-swath altimetry. A barotropic, nonlinear
free-surface model is set up over the European shelf as part
of an ensemble Kalman filter. Experiments assimilating simulated
data are performed over the North Sea to test the ability of
altimeter configurations to reduce model errors due to the action
of meteorological forcing in the presence of bathymetric uncertainties.
A simplified wide-swath observation scheme is used, composed
of nadir altimeter height plus a nadir-centered cross-track
sea level slope measurement. The simplified wide-swath measurements
are found to be able to constrain events unsampled by a single
nadir altimeter owing to a wider domain of influence in the
cross-track direction and the ability to detect cross-track
gradients.
Since the satellite-borne interferometer is highly sensitive
to the platform behavior, especially satellite roll, experiments
taking roll errors into account are then carried out. Whereas
observational errors are considered independent in most data
assimilation studies, the roll of the platform correlates
those errors along the path of the satellite. Despite the
large amplitude of the roll errors, the contribution of the
wide-swath altimeter in coastal zones remains valuable as
long as the roll frequency is known (within Gaussian error)
and the assimilation scheme is designed to take observational
error correlations into account. http://dx.doi.org/10.1175/2008JTECHO576.1
Assessment
of observational networks with the Representer Matrix
Spectra method-application to a 3D coastal model of the
Bay of Biscay
Le
Hénaff M., De Mey P.,
Marsaleix P.
(Ocean Dynamics) |
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The
development of coastal ocean modeling in the recent years
has allowed an improved representation of the associated complex
physics. Such models have become more realistic, to the point
that they can now be used to design observation networks in
coastal areas, with the idea that a "good" network
is a network that controls model state error. To test this
ability without performing data assimilation, we set up a
technique called Representer Matrix Spectra (RMS) technique
that combines the model state and observation error covariance
matrices into a single scaled representer matrix. Examination
of the spectrum and the eigenvectors of that matrix informs
us on which model state error modes a network can detect and
constrain amidst the observation error background. We applied
our technique to a 3D coastal model in the Bay of Biscay,
with a focus on mesoscale activity, and tested the performance
of various altimetry networks and an in situ array deployment
strategy. It appears that a single nadir altimeter is not
efficient enough at capturing coastal mesoscale physics, while
a wide swath altimeter would do a much better job. Testing
various local in situ array configurations confirms that adding
a current meter to a vertical temperature measurement array
improves the detection of secondary variability modes, while
shifting the array higher on the shelf break would obviously
enhance the model constraint along the coast. The RMS technique
is easily set up and used as a "black box," but
the utility of its results is maximized by previous knowledge
of model state error physics. The technique provides both
quantitative (eigenvalues) and qualitative (eigenvectors)
tools to study and compare various network options. The qualitative
approach is essential to discard possibly inconsistent modes.
http://dx.doi.org/10.1007/s10236-008-0144-7
Relative performance of
future altimeter systems and tide gauges in constraining a model
of North Sea high-frequency barotropic dynamics
We
evaluate in this paper the ability of several altimeter systems,
considered separately as well as together with tide gauges,
to control the time evolution of a barotropic model of the North
Sea shelf. This evaluation is performed in the framework of
the particular model errors due to uncertainties in bathymetry.
An Ensemble Kalman Filter (EnKF) data assimilation approach
is adopted, and observing-systems simulation experiments (OSSEs)
are carried out using ensemble spread statistics. The skill
criterion for the comparison of observing networks is, therefore,
not based on the misfit between two simulations, as done in
classic twin experiments, but on the reduction of ensemble variance
occurring as a consequence of the assimilation. Future altimeter
systems, such as the Wide Swath Ocean Altimeter (WSOA) and satellite
constellations, are considered in this work. A single WSOA exhibits,
for instance, similar performance as two-nadir satellites in
terms of sea-level correction, and is better than three satellites
in terms of model velocity control. Generally speaking, the
temporal resolution of observations is shown to be of major
importance for controlling the model error in these experiments.
This result is clearly related to the focus adopted in this
study on the specific high-frequency response of the ocean to
meteorological forcing. Altimeter systems lack adequate temporal
sampling for properly correcting the major part of model error
in this context, whereas tide gauges, which provide a much finer
time resolution, lead to better global statistical performance.
When looking into further detail, tide gauges and altimetry
are demonstrated to exhibit an interesting complementary character
over the whole shelf, as tide gauge networks make it possible
to properly control model error in a ~100-km coastal band, while
high-resolution altimeter systems are more efficient farther
from the coast.
More details in:
Mourre
B., De Mey P., Ménard
Y., Lyard F. and
Le Provost C. 2006. Relative performances of future altimeter
systems and tide gauges in controlling a model of North Sea
high frequency barotropic dynamics. Ocean Dynamics
DOI: 10.1007/s10236-006-0081-2
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Real
time sea level gauge observations and operational oceanography
 The
contribution of tide-gauge data, which provide a unique monitoring
of sea-level variability along the coasts of the world ocean,
to operational oceanography is discussed in this paper. Two
distinct applications that both demonstrate tide-gauge data
utility when delivered in real-time are illustrated. The first
case details basin-scale operational model validation of the
French Mercator operational system applied to the North Atlantic.
The accuracy of model outputs in the South Atlantic Bight both
at coastal and offshore locations is evaluated using tide-gauge
observations. These data enable one to assess the model's nowcasts
and forecasts reliability which is needed in order for the model
boundary conditions to be delivered to other coastal prediction
systems. Such real-time validation is possible as long as data
are delivered within a delay of a week. In the second application,
tide-gauge data are assimilated in a storm surge model of the
North Sea and used to control model trajectories in real-time.
Using an advanced assimilation scheme that takes into account
the swift evolution of model error statistics, these observations
are shown to be very efficient to control model error, provided
that they can be assimilated very frequently (i.e. available
within a few hours).
More details in:
Mourre B., Crosnier L., Le Provost C., 2006. Real time sea
level gauge of observations and operational oceanography. Philosophical
Transactions of the Royal Society. 364, 867 - 884. DOI:
10.1098/rsta.2006.1743
An
ensemble method for the exploration of model errors due to uncertainties
in bathymetry
 Sea
level model error statistics due to uncertainties in bathymetry
in shallow seas are investigated through an original approach
based on an ensemble method. The model is free-surface, barotropic
and implemented over the entire European continental shelf.
The main focus is the specific sea level response of the ocean
to meteorological forcing in the presence of bathymetric errors.
The introduction of such errors is generally unavoidable considering
our insufficient knowledge of bottom topography in most shallow
regions of the world. These errors are likely to impact on ocean
modeling. In particular, coastal gravity waves, which are responsible
for an important part of sea level variability due to atmospheric
forcing over shelves, are sensitive to bathymetric errors.
An ensemble of perturbed bathymetric solutions
is generated by first examining differences between selected
existing bathymetric databases and then randomly combining
perturbations generated from these differences. The objective
is that the ensemble of simulations obtained by running the
model over these bathymetries best represents the probability
density of model states due to this particular source of errors.
Interesting space-time characteristics of
sea level error covariances in the perspective of sea-level
data assimilation are pointed out. These statistics are shown
to be neither homogeneous over shelves, nor isotropic when
approaching the coast. They are not even stationary, since
they are very dependent on the meteorological regime. These
features are of crucial importance since they impose heavy
constraints on the choice of the scheme necessary to get dynamically
consistent results from data assimilation over continental
shelves.
The question of the ability of this method
to provide sensible bathymetry corrections is addressed in
the last part of this paper. The correction is obtained by
assimilating sea-level data generated by a simulated Wide
Swath Ocean Altimeter system via a global inverse formulation
using ensemble statistics. Twin experiments are carried out
in that perspective. Results are encouraging, even if a significant
part of bathymetric errors remains uncorrected after the analysis.
More details in: Mourre
B., De Mey P., Lyard
F. and
Le Provost C., 2004 Assimilation of sea level data
over continental shelves : an ensemble method for the exploration
of model errors due to uncertainties in bathymetry, Dynamics
of Atmospheres and Oceans. 38, 93-121. doi:10.1016/j.dynatmoce.2004.09.001
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