Altimetry: POC & CTOH develop Coastal Applications...
altimetric data are generally used in the frame of offshore
circulation studies. Indeed in coastal zones the use of altimetric
data is restricted because of technical problems such as the
continental influence (that makes the radiometric correction
tricky), resolution of geophysical corrections (high frequency
sea level variations associated to tides and atmospheric pressure
forcing), uncertainties on the mean sea level and last, spatial
and temporal resolution of altimetric data generally to low
in regard to the variability of coastal ocean processes. However,
on one hand data post-treatment lines for coastal purposes are
now improving and on the other hand new satellite projects will
provide a better spatial description of the sea surface.
The CTOH (observation
team of LEGOS) has recently engaged a permanent researcher,
Florence Birol, who
will be involved in the development of new post-treatment lines
that will highlight the scientific interest of coastal altimetric
data. One of the three "test" areas is the Gulf of
Lions. This choice should benefit from numerous in situ data
and mature modeling background developed at POC. The scientific
objective is to determine to what extents coastal processes
can be observed with satellite altimeters. We will be particularly
interested by the Liguro Provencal variability, ROFI induced
by Rhone river discharge, upwelling. First we will use numerical
simulations to estimate whether the sea level variations produced
by coastal dynamics can be evidenced by altimetric data or not,
before altimetric data studies, strictly speaking, begin.
At the same time Jerome
Bouffard's PhD (POC) is considering the multi-satellite
data set (with ERS-1/2 and Topex/Poseidon), especially in coastal
zones where alimeter's signal is usually discarded, in order
to improve it and to integrate it within a system of data
assimilation for a coastal hydrodynamic model.
open-ocean deep convection from space
Herrmann, M., J. Bouffard, and
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
the potential of an improved multi mission altimetric dataset
over the coastal ocean
Vignudelli, P. Cipollini, and Y. Menard, (GRL 2008)
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 20022005. 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.
of ocean dynamics with a regional circulation model and improved altimetry
in the North-western Mediterranean
Bouffard, J., S. Vignudelli, M.
Herrmann, F. Lyard, P. Marsaleix, Y. Ménard, and
P. Cipollini, (TAO 2008)
spatial and temporal resolution of satellite altimetry is usually
sufficient for monitoring the changes of sea surface topography in
the open ocean. However, coastal ocean dynamics are much more complex,
being characterized by smaller spatial and temporal scales of variability.
The quality and availability of satellite-derived products along the
coasts have to be improved, with a strategy optimized for coastal
targets. Therefore a coastal multi-satellite altimetry dataset (TOPEX/Poseidon,
Jason-1; Envisat; GFO) at a 10 - 20 Hz sampling rate has been derived
from routine geophysical data products using a new processing software
dedicated to coastal zone applications. Improved along-track sea level
variations with fine space scales are available in the North-western
Mediterranean Sea from 2001 to 2003, and are compared with high-resolution
numerical model elevations from the eddy-resolving model SYMPHONIE.
This preparatory work emphasizes the potential of improved multi-satellite
altimetry for validating coastal hydro-dynamical models and could
contribute in the future to a better tuning of the boundary conditions
of the simulations.