Abstract
This paper addresses the issue of how the selection of buoys and the calculation of altimeter averages affect the metrics characterising the errors of altimetric wave height estimates. The use of a 51-point median reduces the sensitivity to occasional outliers, but the quality of this measure can be improved by demanding that there is a minimum number of valid measurements. This had a marked impact in both the open ocean and the coastal zone. It also affected the relative ordering of algorithms’ performances, as some fared poorly when a representative value was gleaned from a single waveform inversion, but had a much better ranking when a minimum of 20 values were used. Validation procedures could also be improved by choosing altimeter-buoy pairings that showed a good consistency. This paper demonstrated an innovative procedure using the median of the different retrackers analysed, which can be easily extended to other data validation exercises. This led to improved comparison statistics for all algorithms in the open ocean, with many showing errors less than 0.2 m, but there was only one strong change in the relative performance of the 11 Jason-3 retrackers. For Sentinel-3A, removing the inconsistent coastal buoys showed that all of the new algorithms had similar errors of just over 0.2 m. Thus, although improvements were found in the procedure used for the Sea State Round Robin exercise, the relative rankings for the buoy calibrations are mostly unaffected.
Highlights
Significant wave height (SWH) is listed by GCOS (Global Climate Observing System [1]) as an “essential climate variable”, since the study of its geographical and temporal variation is an important component in understanding the evolving environment of our planet
It is tempting to compare the results for Jason-3 and Sentinel-3A e.g., noting that the MLE-4, STARv2, and TALES algorithms applied to the best buoys for S3A give better results in the coastal zone than those algorithms applied to Jason-3, whereas the converse is true for the best open ocean buoys
An intercomparison of the results stated in these papers is challenging because of how much the chosen metrics depend upon the particular selection of buoys and the flagging of the altimeter data
Summary
Significant wave height (SWH) is listed by GCOS (Global Climate Observing System [1]) as an “essential climate variable”, since the study of its geographical and temporal variation is an important component in understanding the evolving environment of our planet. Satellite-borne radar altimeters are the primary means of providing near-global measurements of SWH, with datasets being collated from multiple instruments being combined to give 33 years of near-continuous coverage that has been used in various climatological studies [3]. One focus of this work has been to instigate a Round Robin process to evaluate a number of candidate algorithms, and assess their reliability in providing data, with low noise levels and minimal outliers, which compare well with model forecasts and the limited coastal buoy network [5]. The metrics for the assessment (bias, standard deviation, correlation coefficient) vary with the data selection criteria for the altimetry and the choice of buoys used, according to their degree of exposure, proximity to coast, and wave regime encountered, with many previous studies concentrating the evaluation in open ocean conditions [6,7,8].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
