Global Validation of SWIM/CFOSAT Wind Waves Against Voluntary Observing ship Data

The article presents a joint analysis of wind and wave characteristics derived from Voluntary Observing Ship (VOS) data and measurements of the innovative Ku-band radar SWIM (Surface Waves Investigation and Monitoring) carried by Chinese-French Ocean SATellite (CFOSAT). Global distributions of significant wave height and wind speed in both data sets demonstrate good qualitative and quantitative agreement, especially in regions with a high spatio-temporal density of visual observations. A particular focus is made on discriminating wave systems by joint analysis of separately observed wind sea and swell characteristics in VOS and the partitions of wave spectra measured by SWIM. It is shown that three wave partitions from SWIM cannot be clearly attributed to wind sea, first, and secondary swell systems. The first partition aggregates both wind sea and swell in the operational wavelength range of the SWIM radar, while the second and third partitions fit neither wind sea, nor swell. A comparison of VOS and SWIM data within a 50 km radius and a 30 min time lag shows a very close match for most parameters in terms of mean values, yet with relatively high dispersion of individual measurements. Plain Language Summary In this study, we analyze waves and winds derived from two independent sources: Voluntary Observing Ship (VOS) data and the innovative Ku-band radar SWIM (Surface Waves Investigation and Monitoring) carried by Chinese-French Ocean SATellite (CFOSAT). SWIM provides measurements of the directional spectra of ocean waves with further partitioning of multiple wave systems. In VOS, the separation is performed visually: wind seas are associated with local wind, and swells (first and secondary, if present) are not dependent on it. Significant wave height (measured by SWIM and calculated in VOS) is traditionally defined as the mean height of the highest one-third of the waves, or four times the square root of the zeroth-order moment of the wave spectrum. Satellite wind speed is evaluated using a parametric approach; VOS has high-precision anemometer measurements. Significant wave height and wind speed in VOS and SWIM are consistent in terms of average statistics, but show a large dispersion in individual measurements. The three wave partitions retrieved from SWIM measurements cannot be simply regarded as wind sea, first, and secondary swell systems. The first partition aggregates both wind sea and swell, while the second and third partitions look like residuals and fit neither wind sea, nor swell.

Automated summary

This article presents a joint analysis of wind and wave characteristics using data from Voluntary Observing Ship (VOS) and the Ku-band radar SWIM. The analysis shows good agreement in the global distributions of significant wave height and wind speed. The study also examines the partitions of wave spectra measured by SWIM and finds that they cannot be clearly attributed to wind sea or swell systems. A comparison of VOS and SWIM data shows close matches in average statistics but with some dispersion in individual measurements.