Abstract's details
Coastal Sea Level along the North Eastern Atlantic Shelf from SAR altimetry
CoAuthors
Event: 2017 Ocean Surface Topography Science Team Meeting
Session: Advances in coastal altimetry: measurement techniques, science applications and synergy with in situ and models
Presentation type: Type Poster
Contribution: not provided
Abstract:
As quality of Synthetic Aperture Radar (SAR) altimetry data is at least as good as conventional altimetry over open ocean and higher in coastal zone up to 2-4 km from coast, SAR data allow meaningful application to oceanography and sea level monitoring. This study investigates the impacts of CryoSat-2 and Sentinel-3 data processed in SAR mode in the 10 Kilometre stripe along the North-Eastern Atlantic shelf.
We use CryoSat-2 SAR altimeter products from the ESA G-POD processor and the in-house reduced SAR altimetry RDSAR/TALES and RDSAR/STAR products. All are derived by processing using coastal dedicated retracking. Additionally an enhanced methodology generates the SAR waveforms applying Hamming weighting window on burst data prior to the along-track FFT, zero-padding prior to the range FFT and doubling of the extension for the radar range swath. The Sentinel-3 SAR and RDSAR data used are the official products distributed by Copernicus. CryoSat-2 data cover the 6-year interval from January 2011 to December 2016 and Sentinel-3 data cover one year from June 2016 to Mai 2017. Conventional altimetry data are from the sea level CCI database.
First we analyse the impact of these SAR altimeter data on the estimation of sea level near coast and its rise. Data quality is analysed as function of distance to coast and orientation of the tracks. We consider various methodologies for building, selecting and absolute comparing CryoSat-2 time-series of ellipsoidal heights to tide gauge records at GPS stations. The methodology is replicated in the various coastal sub-regions. The VLM along the North-Eastern Atlantic shelf is small (+/- 2 cm/yr) compared to the North-Western Atlantic Coast VLM (+/- 6 mmm/yr), which makes the analysis more challenging. We find that VLM rates derived from the altimeter minus tide gauge differences slightly depend on the selection of the altimeter data and are in good agreement with the rates derived from Global Positional System (GPS) analysis. Rates are comparable to the VLM derived from the SLCCI datasets over the longer time span 1993-2015 using a similar methodology.
Second we investigate the impact of the new data in the estimation of an improved mean dynamic topography. A mean surface is derived from the SAR altimeter data and combined to state of the art global geoid EIGEN-6C4 and local geoids as the german local geoid CGC2016 to derive the mean dynamic topography. We compare the results to existing oceanographic and geodetic mean dynamic topography solutions, both on grid and pointwise at the tide gauge stations.
Finally, the SAR data are used for the analysis of extreme sea levels. Individual measurements collected into boxes along the coast are compared to the tide gauge extremes. We investigate the contribution of the present SAR missions to extreme sea level analysis, accounting that the registered extreme levels depend on the repeat period and therefore on the mission characteristics.
We use CryoSat-2 SAR altimeter products from the ESA G-POD processor and the in-house reduced SAR altimetry RDSAR/TALES and RDSAR/STAR products. All are derived by processing using coastal dedicated retracking. Additionally an enhanced methodology generates the SAR waveforms applying Hamming weighting window on burst data prior to the along-track FFT, zero-padding prior to the range FFT and doubling of the extension for the radar range swath. The Sentinel-3 SAR and RDSAR data used are the official products distributed by Copernicus. CryoSat-2 data cover the 6-year interval from January 2011 to December 2016 and Sentinel-3 data cover one year from June 2016 to Mai 2017. Conventional altimetry data are from the sea level CCI database.
First we analyse the impact of these SAR altimeter data on the estimation of sea level near coast and its rise. Data quality is analysed as function of distance to coast and orientation of the tracks. We consider various methodologies for building, selecting and absolute comparing CryoSat-2 time-series of ellipsoidal heights to tide gauge records at GPS stations. The methodology is replicated in the various coastal sub-regions. The VLM along the North-Eastern Atlantic shelf is small (+/- 2 cm/yr) compared to the North-Western Atlantic Coast VLM (+/- 6 mmm/yr), which makes the analysis more challenging. We find that VLM rates derived from the altimeter minus tide gauge differences slightly depend on the selection of the altimeter data and are in good agreement with the rates derived from Global Positional System (GPS) analysis. Rates are comparable to the VLM derived from the SLCCI datasets over the longer time span 1993-2015 using a similar methodology.
Second we investigate the impact of the new data in the estimation of an improved mean dynamic topography. A mean surface is derived from the SAR altimeter data and combined to state of the art global geoid EIGEN-6C4 and local geoids as the german local geoid CGC2016 to derive the mean dynamic topography. We compare the results to existing oceanographic and geodetic mean dynamic topography solutions, both on grid and pointwise at the tide gauge stations.
Finally, the SAR data are used for the analysis of extreme sea levels. Individual measurements collected into boxes along the coast are compared to the tide gauge extremes. We investigate the contribution of the present SAR missions to extreme sea level analysis, accounting that the registered extreme levels depend on the repeat period and therefore on the mission characteristics.