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CTOH regional altimetry products: examples of applications

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Mathilde Cancet
Mathilde Cancet
Mathilde Cancet
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Florence Birol at Université Toulouse III - Paul Sabatier
Florence Birol
Laurent Roblou
Laurent Roblou
Laurent Roblou
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C. Langlais at The Commonwealth Scientific and Industrial Research Organisation
C. Langlais
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Abstract and Figures

The Centre for Topographic studies of the Oceans and Hydrosphere (CTOH) is a French Observation Service dedicated to satellite altimetry studies, which are an important component of the ocean observing system. However, the use of this type of data for observing the coastal ocean is challenging because of instrumental limitations and some of the standard corrections are not adapted to marginal seas. In this context, a processing software (X-TRACK) dedicated to recovering altimetry data over marginal seas has been developed at LEGOS for some years. After a validation stage in several geographical areas, the X-TRACK software is now routinely operated at CTOH, providing data for coastal and regional applications. The question of how to interpret sea level anomalies observations in terms of coastal processes is still open. This study presents several examples of applications which start to address this issue and show that coastal altimetry data are able to capture characteristics of the coastal dynamics.
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CTOH REGIONAL ALTIMETRY PRODUCTS: EXAMPLES OF APPLICATIONS
Cancet, Mathilde
(1)
, Birol, Florence
(1)
, Roblou, Laurent
(1)
, Langlais, Clothilde
(2)
, Guihou, Karen
(2)
,
Bouffard, Jérôme
(3)
, Dussurget, Renaud
(1)
, Morrow, Rosemary
(1)
, Lyard, Florent
(1)
(1)
LEGOS/Univ. de Toulouse/CNRS, 14 Av. E. Belin, 31400 Toulouse (France), Mathilde.Cancet@legos.obs-mip.fr
(2)
CSIRO/MAR, Castray Esplanade, Hobart TAS 7000 (Australia), Clothilde.Langlais@csiro.au
(3)
IMEDEA (CSIC-UIB), Campus UIB, 07071 Palma de Mallorca (Spain), Jerome.Bouffard@uib.es
ABSTRACT:
The Centre for Topographic studies of the Oceans and
Hydrosphere (CTOH) is a French Observation Service
dedicated to satellite altimetry studies, which are an
important component of the ocean observing system.
However, the use of this type of data for observing the
coastal ocean is challenging because of instrumental
limitations and some of the standard corrections are not
adapted to marginal seas.
In this context, a processing software (X-TRACK)
dedicated to recovering altimetry data over marginal
seas has been developed at LEGOS for some years.
After a validation stage in several geographical areas,
the X-TRACK software is now routinely operated at
CTOH, providing data for coastal and regional
applications.
The question of how to interpret sea level anomalies
observations in terms of coastal processes is still open.
This study presents several examples of applications
which start to address this issue and show that coastal
altimetry data are able to capture characteristics of the
coastal dynamics.
1. CONTEXT
The objectives of the Centre for Topographic studies of
the Oceans and Hydrosphere (CTOH) are to 1) maintain
and distribute homogeneous altimetry databases for
applications over the oceans, the hydrosphere and
cryosphere, 2) help scientific users develop new
altimetry derived products and 3) contribute to the
development and validation of new processing
approaches of altimetry data in emerging research
domains.
Altimetry data are an important component of the ocean
observing system. They have been intensively used over
the last decade for observing and analyzing the large
scale circulation variability in many areas of the world
ocean. However, the use of this type of data for
observing the coastal ocean is challenging because of
three types of problems. First, and certainly the
strongest limitation, the radar echo itself interferes with
the surrounding land, which results in unreliable data
within ~5-10 km off the coastlines [1]. Second, the
standard corrections (e.g. wet troposphere correction,
wave height, high frequency and tide corrections from
global models, etc…) applied to the altimeter data are
not adapted to the intrinsic characteristics of the coastal
dynamics (shorter spatial and temporal wavelengths)
and add to the problem of poor quality altimeter data
over shallow waters. As a consequence the precision
decreases dramatically when approaching the coast and
the data are usually flagged within 50 km of the land in
products distributed by operational centres. Finally, the
space-time sampling of the altimeter satellites is
generally too low to capture a wide range of coastal
ocean processes. In the near future, the new generation
of altimeter missions will better fulfil the requirements
for coastal observations (AltiKa, SWOT, etc…). But in
parallel, we need to develop new approaches to address
the problems mentioned above, i.e. data processing
strategies dedicated to coastal purposes, to interpret the
coastal SSH data in term of underlying ocean dynamics,
to define their usefulness and limitations, and to analyse
how they could be complementary with other types of
coastal observing systems. Experience gained from
existing altimetry data is crucial to optimize both quality
and quantity of altimetry data records in the coastal
zones.
In this context, for some years, a dedicated data
processing system has been developed at LEGOS to
recover information from altimetry over marginal seas:
the X-TRACK software [2]. Starting from classical
Geophysical Data Record (GDR) products, it
incorporates the latest corrections available in the
CTOH database. A variable sampling rate processing
has been included (1Hz to 20 Hz) and the editing
strategy has been re-defined to recover a maximum of
useful information. Inversion algorithms have also then
been added to estimate a high resolution mean sea
surface directly from the improved altimeter data. The
post processing step is based on user’s defined criteria.
When available, improved geophysical corrections from
regional high-frequency models of tides and
atmospheric loadings are also applied. The result is a
processing tool which can be easily tuned to respond to
particular applications.
After a validation stage on dedicated oceanic regions
([3], [4], [5], [6]), where the improvement resulting
from this processing tool was shown (in terms of error
reduction and data availability near the shelf), the X-
TRACK software is now routinely operated by the
CTOH for regional or coastal applications. On request,
1Hz or higher frequency along-track data from different
altimeter missions are reprocessed on a regional basis.
Once they are validated [7], these data are made freely
available through the CTOH website:
http://ctoh.legos.obs-mip.fr.
Figure 1: Coastal products distribution via the CTOH website: blue rectangles show areas where data are available
and validated, red ones indicate regions where the data are still under validation.
2. APPLICATIONS
CTOH regional altimetry products have already been
used for various scientific applications (eg coastal and
shelf ocean dynamics, regional model validation, data
assimilation, …) in different areas: in the Mediterranean
Sea, the southwest and southeast Pacific Ocean, the
northern Indian Ocean, the Gulf of Biscay, and the
Great Australian Bight. The question of how to interpret
sea level anomalies observations in terms of coastal
processes is addressed here using different examples of
applications. It is shown that altimetry data offer the
opportunity to document coastal trapped wave activity,
processes associated with mesoscale variability near the
coasts as well as shelf circulation in different areas of
the world ocean.
As an example, in the case of the North-western
Mediterranean Sea boundary circulation, the seasonal
picture of the coastal circulation system emerging from
altimetry agrees fairly well with in-situ current data and
with the Sea Surface Temperature (SST) pattern [8].
Fig. 2.a. and 2.c. show that the structure of the Liguro-
Provençal-Catalan (LPC) Current is clearly visible on
the shelf break. The winter SST pattern shows a steep
thermal front over the shelf break that separates the
relatively warm waters of the LPC current from the
colder upwelled waters spreading from the Gulf of Lion.
Despite the large inter-track spacing of the
Topex/Poseidon and Jason-1 missions which limits the
spatial resolution of the whole shelf edge current
system, a consistent regional picture of the seasonal
evolution of the boundary flow appears: intensification
(decrease) of the boundary flow in winter (summer).
This regional picture can not be so clearly identified
when using AVISO data (Fig. 2.b. and 2.d.) because of
more missing data. The seasonal cycle dominates the
spectrum of the coastal circulation variability in the area
of interest, although a wide range of time scales is also
revealed (not shown here).
Figure 2: Variability of the Liguro-Provençal-Catalan current, in the Northwestern
Mediterranean Sea. Monthly climatology maps of cross-track geostrophic current
anomalies (derived from T/P and Jason-1 data; black arrows) and of SST anomalies
(AVHRR data) for August (2.a.: CTOH data and 2.b.: AVISO data) and December
(2.c.: CTOH data and 2.d.: AVISO data). Monthly SST anomalies have been
constructed by removing the average monthly mean over the domain. The SST dataset
covers the period Jan. 1998 – Dec. 2008. The altimetry dataset covers the period 1993-
2007. The coastal circulation scheme is shown for information.
3. CONCLUSION
Even if a lot remains to do, different studies have
already shown that altimetry data reprocessed with
coastally-oriented algorithms are able to capture
characteristics of the coastal dynamics, offering the
opportunity to document the variability of various
dynamical processes at different time scales. Therefore,
we believe that satellite altimetry, even with its intrinsic
limitations, is an important component of coastal
observing systems. The nearly global availability of
altimetry data holds considerable promise for advancing
our knowledge of the near shore ocean variability,
especially in numerous regions where the surface
boundary circulation is of great interest but is difficult
to observe with direct in situ measurements.
4. ACKNOWLEDGEMENTS
We would like to thank SHOM, REDMAR, APAT, and
SONEL for providing the tide gauge data used for the
validation stage, and the Centre for Topographic Studies
of the Oceans and Hydrosphere (CTOH) for the
processing of the altimeter observations.
5. REFERENCES
1. Deng, X., Featherstone, W., Hwang, C. & Berry, P.A.M.
(2002). Estimation of contamination of ERS-2 and
Poseidon satellite radar altimetry close to the coasts of
Australia. Marine Geodesy 25, 249-271.
2. Roblou, L., Lyard, F., Le Hénaff, M. & Maraldi, C. (2007).
X-TRACK, A new processing tool for altimetry in
coastal oceans. Proc. ENVISAT Symposium, Montreux,
Switzerland.
3. Vignudelli, S., Cipollini, P., Roblou, L., Lyard, F.,
Gasparini, G.P., Manzella, G. & Astraldi, M. (2005).
Improved satellite altimetry in coastal systems: Case
study of the Corsica Channel (Mediterranean Sea).
Geophys. Res. Let. 32, L07608, doi:1029/2005GL22602.
4. Bouffard, J., Vignudelli, S., Cipollini, P. & Ménard, Y.
(2008). Exploiting the potential of an improved
multimission altimetric data set over the coastal ocean.
Geophys. Res. Lett. 35, L10601,
doi:10.1029/2008GL033488.
5. Durand, F., Shankar, D., Birol, F. & Shenoi, S.S.C. (2008).
An algorithm to estimate coastal currents from satellite
altimetry: a case study for the East India Coastal
Current. Journal of Oceanography 64, 831-845.
6. Durand, F., Shankar, D., Birol, F. & Shenoi, S.S.C. (2009).
Spatio-temporal structure of the East India Coastal
Current from satellite altimetry. J. of Geophys Res. 114,
C02013, doi:10.1029/2008JC004807.
7. Cancet, M., Birol, F., Bouffard, J., Roblou, L., Lyard, F. &
Morrow, R. (2008): Toward coastal altimetry
applications. OSTST meeting, 10-12 November, Nice,
http://www.aviso.oceanobs.com/fileadmin/documents/O
STST/2008/birol_ctoh.pdf.
8. Birol, F., Cancet, M. & Estournel, C. (2009): Aspects of the
seasonal variability of the Northern Current (NW
Mediterranean Sea) observed by altimetry. J. of Mar.
Syst. (Under review).
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STUDY ON OCEAN SURFACE WAVES AND SEA LEVELS USING COASTAL ALTIMETRY - JASON-2 and SARAL ALTIKA DOCTOR OF PHILOSOPHY in PHYSICAL OCEANOGRAPHY Under the supervision of
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Most of the world's population lives close to the coast. These are the transition areas between land and sea. These are the vital regions for many offshore and marine activities, imports and exports, fishing, navigation, recreation etc. throughout history, humans have attempted to slow or alter the dynamic coastal zone. India has Coastline of about 7,517 km comprises States of Gujarat, Maharashtra, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Orissa and West Bengal. India, with most diverse ecosystem, high productivity and thickly populated over coastal region has gained its very own importance. Major and important cities of India were developed along the coastal regions. The Cities in the Coastline of India are notable for trading like heavy shipments through Ships from foreign and inside the coastal cities of India. A proper management of the coastal and near-shore zones is vital in the economic development of a nation. These Indian coastal regions have been subjected to several geomorphological changes under climate change scenario. Despite all of this Indian coasts are under threat due to multiple stresses like global climate change, human intervention. These stresses are driving vulnerabilities like sea-level-rise, coastal erosion, frequent extreme events, salt water encroachment etc. So, there is a need for the proper management of these coastal regions. The coastline has subjected to changes due to the action of waves, tides, currents and winds. Therefore, it is essential to better understand the physical processes, which are responsible for these changes. These coastal processes are complex and dynamic in nature as it involves to study and monitor especially the ocean surface waves and sea level variations (or tides) in the coastal regions. Remote sensing offers an excellent opportunity to study these ocean parameters using satellite radar altimeter. Preceding Jason-2, the altimeter data were too sparse and of poor quality to be of real use to study in coastal areas. Advanced instruments in Jason-2 enables us to study and monitor the coastal areas. In addition to this, Recently India in collaboration with French space agency launched an altimeter SARAL AltiKa, a first of kind to operate at high frequency with study of coastal areas as one of the main objectives. The main goal of the study is to observe, obtain retrieve and validate the wave height and sea level data from these two operationally reliable series of altimeters Jason-2 and SARAL AltiKa and to demonstrate the usefulness of high frequency data during extreme event cases like cyclone. In the present study, the Jason-2 PISTACH coastal products were observed and analyzed to identify the best re-tracker available to study and monitor coastal regions. A new processing method was applied to the re-trackers to obtain optimum results in the limited areas of the coastal region. The performance of these processed re-tracker was validated with In-situ observations. Also high frequency data from SARAL AltiKa was analyzed to observe and validate in the proximity of the coast. The validation was carried out in two methods. One at closest point analysis and at along the track. This study also presents an application of high frequency Jason-2 data for the study of extreme event cases like cyclone. A new way of approach (or) method was used for comparison of altimeter data with In-situ observations. The chapter wise contents of the thesis are briefly described below. This thesis consists of six chapters. Chapter 1 is an introductory chapter describing the importance of coastal regions and the importance of Ocean surface waves and sea level anomalies/ variations over coastal regions. It is followed by a brief introduction, history and development of satellite radar altimetry towards coastal regions and a brief description of Jason-2 and SARAL AltiKa altimeters. This chapter is completed with a description of the scope of the problem and specific objectives of the present study followed by the structure and outline of the thesis. Chapter 2 examines and identifies the best re-tracker available in Jason-2 PISTACH coastal products to obtain, retrieve and validate ocean wave parameters in the proximity to the coast. Chapter 3 contains information about SARAL AltiKa and to observe retrieve and validate SARAL along track wave data at different coastal regions along the east coast of India. SARAL validation is carried out at the point closest to the buoy and at all the points along the track close to the coast. Chapter 4 comprises of Sea level measurements from both missions to quality control, calibrate and validate individual altimeter measurements. Chapter 5 presents an application of high frequency satellite altimetry data to study wind and wave parameters during extreme event case. Chapter 6 is concluding chapter that summarizes the results of the study and opens several new scopes for further studies.
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Full-text available
  • Dec 2008
A methodology to derive surface geostrophic current from a newly released altimetric sea-level data set is presented. TOPEX/Poseidon data were first completely reprocessed from Geophysical Data Records using new algorithms accommodating marginal seas and coastal conditions. The methodology applied to the reprocessed data essentially consists of a smoothing of the raw along-track coastal altimetric data at scales at which the geostrophic equilibrium holds. This was reduced to a computational procedure using a set of objective criteria. The method to the East India Coastal Current (EICC) at the western boundary of the Bay of Bengal is applied. This paper first examines the quality of the new data set, which compares well with tide-gauge data; the current we derived is consistent with independent estimates. The methodology reveals the full spectrum of the along-shore current, ranging from intra-seasonal to inter-annual time scales, from the deep ocean to the shelf-break area where the EICC exists. The algorithm can be applied to any coastal region where an order of the Rossby radius can be defined, and it therefore opens up bright prospects for mapping the variability of other boundary-current systems in the world ocean from altimetry.
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Improved satellite altimetry in coastal systems: Case study of the Corsica Channel (Mediterranean Sea)
Article
  • Apr 2005
The fidelity of corrections and processing are critical for a realistic use of official altimetric products close to the coast. A new processing strategy, which starts from the TOPEX/Poseidon GDRs with the addition of improved corrective terms, is proposed and evaluated in the area of the Corsica Channel. Sea level anomalies agree with the coincident sea truth (bottom pressure and tide gauge) within 2-3 cm rms for seasonal and longer time scales. Analysis for almost ten years of coincident mooring and altimetric velocities shows that a substantial reduction of uncertainty to ~4 cm s-1 may be possible after reasonable filtering of the noise introduced by more variable coastal sea surface states. The conclusion is that the altimetry success is still limited to seasonal time scales, and provided that the oceanographic signal ensures an adequate signature to be isolated from background noise.
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Aspects of the seasonal variability of the Northern Current (NW Mediterranean Sea) observed by altimetry
Article
  • Jun 2010
  • J MARINE SYST
Altimetry has become a powerful tool to understand the dynamics of the deep-sea ocean circulation. Despite the technical problems encountered in the coastal zone by this observational technique, resulting in large data gaps in those areas, solutions already exist to mitigate this issue and to allow the retrieval of coastal information from existing altimetric data. Using some of these solutions, we have reprocessed a new set of 14.5 years of the TOPEX/Poseidon and Jason-1 satellite altimeter data over the Northwestern Mediterranean Sea, leading to a significant increase in the quantity of available data near coastlines. Time series of geostrophic surface velocity anomalies have been computed from the along-track altimeter sea level anomalies. In this paper, we evaluate the ability of these altimeter-derived currents to capture the main surface circulation features and the associated seasonal variability in the area of interest.
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Estimation of Contamination of ERS-2 and POSEIDON Satellite Radar Altimetry Close to the Coasts of Australia
Article
  • Oct 2002
It is broadly acknowledged that the precision of satellite-altimeter-measured instantaneoussea surface heights (SSH) is lower in coastal regions than in open oceans, duepartly to contamination of the radar return from the coastal sea-surface state and fromland topography. This study investigates the behavior of ERS-2 and POSEIDON altimeterwaveformdata in coastal regions and estimates a boundary around Australia's coastsin which the altimeter range may be poorly estimated by on-satellite tracking software. Over one million 20 Hz ERS-2 (March to April 1999) and POSEIDON (January 1998 toJanuary 1999) radar altimeter waveform data were used over an area extending 350 kmoffshore Australia. The DS759.2 (50 resolution) ocean depth model and the GSHHS(0.2 km resolution) shoreline model were used together to define the coastal regions. Using the 50% threshold retracking points as the estimates of expected tracking gate,we determined that the sea surface height is contaminated out to maximum distanceof between about 8 km and 22 km from the Australian shoreline for ERS-2, dependingpartly on coastal topography. Using the standard deviation of the mean waveforms as an indication of the general variability of the altimeter returns in the Australian coastalregion shows obvious coastal contamination out to about 4 km for both altimeters, andless obvious contamination out to about 8 km for POSEIDON and 10 km for ERS-2. Therefore, ERS-2 and POSEIDON satellite altimeter data should be treated with somecaution for distances less than about 22 km from the Australian coast and probablyignored altogether for distances less than 4 km.
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Toward coastal altimetry applications. OSTST meeting
  • Jan 2008
  • 10-12
  • M Cancet
  • F Birol
  • J Bouffard
  • L Roblou
  • F Lyard
  • R Morrow
Cancet, M., Birol, F., Bouffard, J., Roblou, L., Lyard, F. & Morrow, R. (2008): Toward coastal altimetry applications. OSTST meeting, 10-12 November, Nice, http://www.aviso.oceanobs.com/fileadmin/documents/O STST/2008/birol_ctoh.pdf.
Exploiting the potential of an improved multimission altimetric data set over the coastal ocean
  • Jan 2008
  • GEOPHYS RES LETT
  • 10601
  • J Bouffard
  • S Vignudelli
  • P Cipollini
  • Y Ménard
Bouffard, J., Vignudelli, S., Cipollini, P. & Ménard, Y. (2008). Exploiting the potential of an improved multimission altimetric data set over the coastal ocean. Geophys. Res. Lett. 35, L10601, doi:10.1029/2008GL033488.
Spatio-temporal structure of the East India Coastal Current from satellite altimetry
  • Jan 2009
  • 2013
  • F Durand
  • D Shankar
  • F Birol
  • S S C Shenoi
Durand, F., Shankar, D., Birol, F. & Shenoi, S.S.C. (2009). Spatio-temporal structure of the East India Coastal Current from satellite altimetry. J. of Geophys Res. 114, C02013, doi:10.1029/2008JC004807.