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Thèse
Matthieu Le Hénaff:
Evaluation objective de réseaux d'observation
en domaine côtier par la modélisation
d’ensemble.
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About
POC:
The
POC group has been created in 2001 by scientists from LEGOS
and LA. One of the main objectives of POC is to study the oceanic
circulation at regional and coastal scales. The approach is
essentially... read
the following
g
ated.
Last 10 publications (full
list):
Herrmann, M., J. Bouffard,
and K. Beranger, 2009 Monitoring
open-ocean deep convection from space, Geophysical Research
Letters, doi:10.1029/2008GL036422
Durand, F., D. Shankar, F.
Birol, and S.S.C. Shenoi, 2009,
Spatio-temporal structure of the East India Coastal Current
from satellite altimetry, Journal of Geophysical Research,
doi:10.1029/2008JC004807
Le
Hénaff M., De Mey P., Marsaleix P., 2009
Assessment of observational networks with the Representer Matrix
Spectra method-application to a 3D coastal model of the Bay
of Biscay.
Ocean Dynamics, 59, 3-20
http://dx.doi.org/10.1007/s10236-008-0144-7
Marsaleix P., Auclair
F., Floor J. W., Herrmann M. J., Estournel C., Pairaud I., Ulses
C., 2008. Energy conservation
issues in sigma-coordinate free-surface ocean models. Ocean
Modelling, 20,
61-89. http://dx.doi.org/10.1016/j.ocemod.2007.07.005
Le Hénaff, M., P. De Mey, B. Mourre, and P.-Y.
Le Traon, 2008: Contribution
of a wide-swath altimeter in a shelf seas assimilation system
– Impact of the satellite roll errors, J. Atmos. Oceanic.
Technol., vol 25, issue 11, pp 2133-2144, http://dx.doi.org/10.1175/2008JTECHO576.1
Bouffard, J., S. Vignudelli,
P. Cipollini, and Y. Menard, 2008,
Exploiting the potential of an improved multi mission altimetric
dataset over the coastal ocean, Geophysical Research Letters,
doi:10.1029/2008GL033488
Bouffard, J., S. Vignudelli, M.
Herrmann, F. Lyard, P. Marsaleix, Y. Ménard,
and P. Cipollini, 2008,
Comparison of ocean dynamics with a regional circulation model
and improved altimetry in the North-western Mediterranean. Terrestrial,
Atmospheric and Oceanic Sciences, 19, 1-XXX, doi:
10.3319/TAO.2008.19.1-2.117(SA)
Herrmann M., Estournel C.,
Déqué M. , Marsaleix
P., Sevault F., Somot S., 2008,
Dense water formation in the Gulf of Lions shelf: Impact of
atmospheric interannual variability and climate change, Continental
shelf research,
28, 2092-2112 doi:10.1016/j.csr.2008.03.003
Ulses, C.,
C. Estournel, J. Bonnin, X.
Durrieu de Madron, and P. Marsaleix
2008. Impact of storms and dense water cascading on shelf-slope
exchanges in the Gulf of Lion (NW Mediterranean). Journal
of Geophysical Research 113, C02010, doi:10.1029/2006JC003795
Herrmann, M. J.,
S. Somot, F. Sevault, C. Estournel,
and M. Deque, 2008. Modeling
the deep convection in the northwestern Mediterranean Sea using
an eddy-permitting and an eddy-resolving model: Case study of
winter 1986-1987. Journal of Geophysical Research, 113,
C04011 doi:10.1029/2006JC003991
POC
Publications from 2000
SYMPHONIE
Non-Hydrostatic:
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NH |
Francis
Auclair Cyril
Nguyen
A
non-hydrostatic version of the POC 3D free surface model Symphonie
has been developed. A prognostic equation for the vertical momentum
has been implemented resulting in the inversion of a classical
Poisson equation for the non-hydrostatic component of the pressure.
The animations show a comparison ofthe hydrostatic
and non-hydrostatic versions of Symphonie on the classical and
simple test case of the oscillations of a 10m-wide and 10m-deep
basin. The hydrostatic modeling shows long non-dispersive waves
while the non-hydrostatic version exhibits short, dispersive
waves as expected based on the dimension of the basin. In perfect
agreement with theory, the resulting period of the oscillations
is 1.5 larger in the non-hydrostatic version.
 Video:
Vertical velocity in a 2D vertical section.
Toulouse, April 2007.
Toulouse, january
2008. © Claudine Marsaleix
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Contribution
of a wide-swath altimeter in a shelf seas assimilation
system – Impact of the satellite roll errors
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| 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
Photographs:
POC linux
cluster
Assessment
of observational networks with the Representer Matrix
Spectra method-application to a 3D coastal model of the
Bay of Biscay
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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
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| Monitoring
open-ocean deep convection from space |
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| Herrmann,
M., J. Bouffard, and
K. Beranger
(GRL 2009) |
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Deep convection
(DC) is a key-process of the oceanic circulation, costly to
monitor in-situ and under the influence of climate change. Our
study is a first step toward monitoring DC from space: we investigate
the feasibility of observing its variability using improved
satellite altimetry. An oceanic simulation of the Mediterranean
circulation was performed for the 1999-2007 period. DC interannual
variability is realistically modelled, and the sea surface elevation
(SSE) is in agreement with altimetry data. Numerical results
show a strong correlation between the annual DC characteristics
and the winter SSE. From that, we propose a method to monitor
DC interannual variability and long term evolution using altimetry
data. Our method, applied to the longest available altimetry
series, represents correctly the interannual variability of
DC in the Northwestern Mediterranean between 1994 and 2007.
doi:10.1029/2008GL036422
Exploiting
the potential of an improved multi mission altimetric
dataset over the coastal ocean
|
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| Bouffard,
J., S.
Vignudelli, P. Cipollini, and Y. Menard, (GRL 2008) |
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Until
now, most satellite altimetry studies of the coastal ocean have
been based on along-track data from a single mission, whereas
up to four missions were operative in 2002–2005. Here,
to monitor the coastal ocean we have applied specialized corrections
and dedicated processing strategies to compute a multimission
data set at a mean distance of 32 km of the coast. The resulting
altimetric data set is compared with sea level data from three
in situ stations over a coastal zone of the northwestern Mediterranean.
The mean rms difference between this data set and the sea level
stations is 2.9 cm against 3.7 cm when using the AVISO altimetric
product. Comparison of altimeter-derived geostrophic velocities
with a mooring also shows that the spatial and temporal variability
of the surface current field is well reproduced. The agreement
with in situ measurements extends to intraseasonal time scales
showing a significant improvement compared to previous studies
in the 50 km coastal-band.
doi:10.1029/2008GL033488
Modeling
the deep convection in the northwestern Mediterranean
Sea using an eddy-permitting and an eddy-resolving model:
Case study of winter 1986-1987
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Herrmann,
M. J.,
S. Somot, F. Sevault, C. Estournel, and M. Deque (JGR 2008)
In
the Northwestern Mediterranean sea, winter 1986-87 was particularly
cold, inducing a strong open-ocean convection event. In order
to investigate the impact of numerical models spatial resolution
on the convection representation and the effects of deep convection
on the Northwestern Mediterranean circulation, we perform
two numerical three-dimensional simulations (eddy-permitting
vs. eddy-resolving). Models are forced at the surface by the
ERA40 atmospheric fluxes, with a simple heat flux correction
to better mimic the observed value. We examine the characteristics
of the deep convection (mixed layer, water masses characteristics
, convection zone and mesoscale structures), and perform temporal
analysis of this event in terms of kinetic energy, buoyancy
equilibrium and deep water (DW) evolution. The convection
characteristics are represented similarly on a global scale
by both models and are in good agreement with observations,
except for the size of the convection region. However, the
eddy-resolving model reproduces better the mesoscale structures,
whose role in the DW formation, mixing and transport is shown
to be essential. The boundary circulation and the overturning
are enhanced during the convection event. 66% of the DW spreading
is due to the bleeding effect into the Catalan sea during
the convection event, whereas 33% is due to the mesoscale
structures southwestward advection after the event. 60% of
the restratification with respect of the water column initial
structure occurs before July 1987 and is due to light water
advection. Afterwards, restratification is due to the mixing,
and is not complete before next year convection.
http://dx.doi.org/10.1029/2006JC003991
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