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ALES: A multi-mission adaptive subwaveform retracker for coastal and open ocean altimetry

ALES: A multi-mission adaptive subwaveform retracker for coastal and open ocean altimetry
ALES: A multi-mission adaptive subwaveform retracker for coastal and open ocean altimetry
Satellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sampling and global coverage. Nevertheless, coastal data have been flagged as unreliable due to land and calm water interference in the altimeter and radiometer footprint and uncertainty in the modelling of high-frequency tidal and atmospheric forcing.

Our study addresses the first issue, i.e. altimeter footprint contamination, via retracking, presenting ALES, the Adaptive Leading Edge Subwaveform retracker. ALES is potentially applicable to all the pulse-limited altimetry missions and its aim is to retrack both open ocean and coastal data with the same accuracy using just one algorithm.

ALES selects part of each returned echo and models it with a classic “open ocean” Brown functional form, by means of least square estimation whose convergence is found through the Nelder–Mead nonlinear optimisation technique. By avoiding echoes from bright targets along the trailing edge, it is capable of retrieving more coastal waveforms than the standard processing. By adapting the width of the estimation window according to the significant wave height, it aims at maintaining the accuracy of the standard processing in both the open ocean and the coastal strip.

This innovative retracker is validated against tide gauges in the Adriatic Sea and in the Greater Agulhas System for three different missions: Envisat, Jason-1 and Jason-2. Considerations of noise and biases provide a further verification of the strategy. The results show that ALES is able to provide more reliable 20-Hz data for all three missions in areas where even 1-Hz averages are flagged as unreliable in standard products. Application of the ALES retracker led to roughly a half of the analysed tracks showing a marked improvement in correlation with the tide gauge records, with the rms difference being reduced by a factor of 1.5 for Jason-1 and Jason-2 and over 4 for Envisat in the Adriatic Sea (at the closest point to the tide gauge).
coastal altimetry, retracking, subwaveform retracker, validation, tide gauge, satellite altimetry
0034-4257
173-189
Passaro, Marcello
08b0a0ca-17a8-453e-b466-a1c37076f45f
Cipollini, Paolo
276e356a-f29e-4192-98b3-9340b491dab8
Vignudelli, Stefano
2f848cc5-1811-4b8d-9f27-fc933d12a2b4
Quartly, Graham D.
3d1e4e87-f001-4d18-b95f-9bca4db6ff9d
Snaith, Helen M.
40f759ed-8c90-4d76-8e9c-7d7a4c264adf
Passaro, Marcello
08b0a0ca-17a8-453e-b466-a1c37076f45f
Cipollini, Paolo
276e356a-f29e-4192-98b3-9340b491dab8
Vignudelli, Stefano
2f848cc5-1811-4b8d-9f27-fc933d12a2b4
Quartly, Graham D.
3d1e4e87-f001-4d18-b95f-9bca4db6ff9d
Snaith, Helen M.
40f759ed-8c90-4d76-8e9c-7d7a4c264adf

Passaro, Marcello, Cipollini, Paolo, Vignudelli, Stefano, Quartly, Graham D. and Snaith, Helen M. (2014) ALES: A multi-mission adaptive subwaveform retracker for coastal and open ocean altimetry. Remote Sensing of Environment, 145, 173-189. (doi:10.1016/j.rse.2014.02.008).

Record type: Article

Abstract

Satellite altimetry has revolutionised our understanding of ocean dynamics thanks to frequent sampling and global coverage. Nevertheless, coastal data have been flagged as unreliable due to land and calm water interference in the altimeter and radiometer footprint and uncertainty in the modelling of high-frequency tidal and atmospheric forcing.

Our study addresses the first issue, i.e. altimeter footprint contamination, via retracking, presenting ALES, the Adaptive Leading Edge Subwaveform retracker. ALES is potentially applicable to all the pulse-limited altimetry missions and its aim is to retrack both open ocean and coastal data with the same accuracy using just one algorithm.

ALES selects part of each returned echo and models it with a classic “open ocean” Brown functional form, by means of least square estimation whose convergence is found through the Nelder–Mead nonlinear optimisation technique. By avoiding echoes from bright targets along the trailing edge, it is capable of retrieving more coastal waveforms than the standard processing. By adapting the width of the estimation window according to the significant wave height, it aims at maintaining the accuracy of the standard processing in both the open ocean and the coastal strip.

This innovative retracker is validated against tide gauges in the Adriatic Sea and in the Greater Agulhas System for three different missions: Envisat, Jason-1 and Jason-2. Considerations of noise and biases provide a further verification of the strategy. The results show that ALES is able to provide more reliable 20-Hz data for all three missions in areas where even 1-Hz averages are flagged as unreliable in standard products. Application of the ALES retracker led to roughly a half of the analysed tracks showing a marked improvement in correlation with the tide gauge records, with the rms difference being reduced by a factor of 1.5 for Jason-1 and Jason-2 and over 4 for Envisat in the Adriatic Sea (at the closest point to the tide gauge).

Other
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More information

e-pub ahead of print date: 1 April 2014
Published date: 5 April 2014
Keywords: coastal altimetry, retracking, subwaveform retracker, validation, tide gauge, satellite altimetry
Organisations: Marine Biogeochemistry, Physical Oceanography

Identifiers

Local EPrints ID: 363764
URI: http://eprints.soton.ac.uk/id/eprint/363764
ISSN: 0034-4257
PURE UUID: f7a75d47-d59f-4a9d-82cc-45cdd8fd18f0

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Date deposited: 02 Apr 2014 13:42
Last modified: 14 Mar 2024 16:29

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Contributors

Author: Marcello Passaro
Author: Paolo Cipollini
Author: Stefano Vignudelli
Author: Graham D. Quartly
Author: Helen M. Snaith

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