[Paper Review] Validation of the multi-mission altimeter data for the Baltic Sea region

N. A. Kudryavtseva, Tarmo Soomere|arXiv (Cornell University)|Mar 29, 2016

Ocean Waves and Remote Sensing3 citations

TL;DR

This study validates multi-mission altimeter data against in situ wave measurements in the Baltic Sea, identifying reliable satellite data subsets by filtering out high backscatter, land-proximate, and icy conditions. It finds excellent agreement (RMSD: 0.23–0.37 m, bias: ±0.23 m) between validated altimetry and in situ SWH, with CRYOSAT-2, ERS-2, JASON-1/2, and SARAL showing bias below 0.06 m, while GEOSAT Phase 1 data remains unvalidated due to inconsistencies.

ABSTRACT

We present a complete cross-validation of Significant Wave Heights (SWH) extracted from altimetry data from all ten existing satellites with available in situ (buoy and echosounder) wave measurements for the Baltic Sea basin. The main purpose is to select an adequate altimetry data subset for a subsequent evaluation of the wave climate. The satellite measurements with the backscatter coefficients >13.5, errors in the SWH normalized standard deviation >0.5 m and snapshots with centroids closer than 0.2 degrees to the land are not reliable. The ice flag usually denotes the ice concentration of >50%. The presence of ice affects the SWH data starting from concentrations 10%, but substantial effects are only evident for concentrations >30%. The altimetry data selected based on these criteria have very good correspondence with in situ data, except for GEOSAT Phase 1 data (1985-1989) that could not be validated. The root-mean-square difference and bias of altimetry and in situ data are in the range of 0.23-0.37 and +-0.23 m, respectively. The bias for CRYOSAT-2, ERS-2, JASON-1/2 and SARAL is below 0.06 m. The SWH time series from several satellite pairs (ENVISAT/JASON-1, SARAL/JASON-2, ERS-1/TOPEX) exhibit substantial mutual temporal drift and part of them evidently are not homogeneous in time. A new high-resolution SWH data set from the SARAL satellite reveals a very good correspondence with the in-situ data and with the data stream from previous satellites.

Motivation & Objective

• To evaluate the reliability of significant wave height (SWH) data from ten operational altimetry satellites in the Baltic Sea region.
• To identify and filter out unreliable altimeter measurements based on backscatter, proximity to land, and ice presence.
• To assess temporal homogeneity and drift between satellite data pairs for long-term wave climate analysis.
• To validate the performance of the SARAL satellite mission against in situ observations and prior missions.
• To provide a robust, validated subset of altimetry data suitable for long-term wave climate studies in the Baltic Sea.

Proposed method

• Cross-validation of SWH from ten satellite missions against in situ buoy and echosounder measurements across the Baltic Sea.
• Application of filtering criteria: backscatter >13.5 dB, normalized standard deviation error >0.5 m, and centroid distance to land <0.2°.
• Use of ice flag data to assess the impact of ice concentration on SWH accuracy, with thresholds at 10%, 30%, and 50% ice coverage.
• Calculation of root-mean-square difference (RMSD) and bias between altimetry and in situ SWH for each satellite mission.
• Assessment of temporal consistency between satellite pairs (e.g., ENVISAT/JASON-1, SARAL/JASON-2) using time series analysis.
• Creation of a high-resolution SWH data set from SARAL, compared to in situ and previous satellite data streams.

Experimental results

Research questions

• RQ1 Which altimetry data subsets are reliable for wave climate studies in the Baltic Sea, based on in situ validation?
• RQ2 How do backscatter, proximity to land, and ice presence affect the accuracy of satellite-derived SWH?
• RQ3 What is the temporal stability and homogeneity of SWH time series across different satellite pairs?
• RQ4 How does the performance of the SARAL satellite mission compare to previous missions and in situ measurements?
• RQ5 Why is GEOSAT Phase 1 data not validated despite being part of the multi-mission altimeter dataset?

Key findings

• Satellite measurements with backscatter >13.5 dB, RMSD error >0.5 m, or land proximity <0.2° are deemed unreliable for SWH estimation.
• Ice presence affects SWH data starting at 10% concentration, with substantial degradation evident at >30% ice coverage.
• The validated altimetry data show a root-mean-square difference (RMSD) of 0.23–0.37 m and a bias of ±0.23 m when compared to in situ measurements.
• CRYOSAT-2, ERS-2, JASON-1/2, and SARAL exhibit a bias below 0.06 m, indicating high accuracy and consistency.
• Significant temporal drift is observed between several satellite pairs (e.g., ENVISAT/JASON-1, SARAL/JASON-2, ERS-1/TOPEX), indicating non-homogeneous time series.
• The SARAL satellite data set shows excellent agreement with in situ observations and with data from previous missions, confirming its reliability for long-term wave climate analysis.