Validation of CryoSat-2 in SAR Mode in the German Bight
Fenoglio-Marc, Luciana1; Dinardo, Salvatore2; Scharroo, Remko3; Roland, Aron4; Weiss, Robert5; Lucas, Bruno6; Becker, Matthias7; Benveniste, Jerome8
1Institute of Geodesy/Technische Universität Darmstadt, GERMANY; 2SERCO/ESRIN, EOP-SER Section, ITALY; 3Altimetric LLC/NOAA, UNITED STATES; 4Institute for Hydraulic and Water Resources Engineering, Technische Universität Darmstadt, GERMANY; 5BfG, GERMANY; 6DEIMOS/ESRIN, ITALY; 7Institute of Geodesy, Technische Universität Darmstadt, ITALY; 8ESA/ESRIN, ITALY
Level 2 Altimetry Data acquired by the CryoSat-2 in SAR Mode are validated in the German Bight during the period 2011-2012. Instantaneous sea level and wave height from altimetry are compared to in-situ measurements and to results from an operational circulation model.
The in-situ data are available from a network of stations having a good geographical distribution, which allows considering three relevant zones: (1) open sea, (2) coastal zone and (3) inland water. The network is maintained by the Waterway and Shipping Administration (WSV) and by the German Federal Institute of Hydrology (BFG, http://www.bafg.de). The relevant in-situ data are sea level, GPS coordinates and wave data. The operational model for the North Sea run by the German Federal Maritime and Hydrographic Agency (BSH) is expected to provide data with frequency of 15 minutes.
The CryoSat Data have been Delay-Doppler processed as from the FBR (Full Bit Rate) Level until the Level 1b and subsequently re-tracked using the SAMOSA's SAR Echo Model and a fitting scheme based on Levenberg-Marquard Least Square Minimization Algorithm. Sea Surface Height (SSH) and Significant Wave Height (SWH) at 20 and 1 Hz have been derived. The delay-Doppler Processing (L1b) and the Re-tracking Processing (L2) has been carried out at ESA/ESRIN by the EOP-SER Altimetry Team.
Pseudo pulse-limited (LRM) data derived from CryoSat in SAR mode and provided via the RADS database will be compared with parameters derived from the CryoSat SAR Echo Model data to estimate possible biases occurring in SAR mode with respect to the LRM Mode and tune up the SAR re-tracking scheme.
The following approach is adopted. We perform an absolute calibration/validation of the altimetry range by direct over-flight at three dedicated GNSS sites located offshore (Helgoland, FINO-1 and FINO-3). The absolute range bias will be derived in this case from the differences of the sea level heights recorded at the tide gauge/GNSS station and the CryoSat sea surface heights. We assess the quality of sea surface height (SSH), sea wave height (SWH), wind speed when approaching the coast and look for any degradation of the measurements. The low sea state conditions expected near coast are useful to assess the capacity of the SAR Altimetry to retrieve wave heights also at low sea state part of the sea spectrum.In particular, the validation in coastal zone is carried out at the coastal tide gauge at Langeoog to highlight the degradation in the behaviour of CryoSat passing from an open ocean regime to a coastal zone regime. The validation of SWH is made against in-situ data provided at platforms and buoys. For the validation of the SWH we use the hindcast results obtained in the IOWAGA project (http://wwz.ifremer.fr/iowaga/). The physics and numeric's are validated on global and local scale.
As performance metrics to measure the quality of the results, scatter plots, cross-correlations, standard deviations, and biases between the in-situ and the CryoSat-derived measurements (Sea Level and SWH) will be presented.
A preliminary comparison made at the offshore platform FINO3 between in-situ wave heights and the SWH derived from CryoSat data in SAR mode using the SAMOSA's SAR Echo Model shows a good agreement for sea wave heights higher than 1 meter (Figure 1). Differences between observed and derived from CryoSat SWHs are higher for SWH smaller than 1 meter suggesting a possible CryoSat’s inadequacy to measure in SAR mode low SWHs. This inadequacy might be due to the current operated multi-looking scheme that averages always almost all possible looks whereas over quasi-flat/specular surfaces it would be more sensible to limit the multi-looking average only to the near nadir looks.