Recent satellite altimeter retracking and filtering methods have considerably reduced the noise level in estimates of the significant wave height (Hs), allowing to study processes with smaller spatial scales. In particular, previous studies have shown that wave-current interactions may explain most of the variability of Hs at scales 20 to 100 km. As the spatial scale of the measurement is reduced, random fluctuations emerge that should be associated to wave groups. Here we quantify the magnitude of this effect, and the contribution of wave groups to the uncertainty in Hs measurements by altimeters, with a particular focus on extreme extra-tropical storms. We take advantage of the low orbit altitude of the China-France Ocean Satellite (CFOSAT), and the low noise level of the nadir beam of the SWIM instrument. Our estimate of wave group effects uses directional wave spectra measured by off-nadir beams on SWIM, and signal processing theory that gives statistical properties of the wave envelope, and thus the local wave heights, from the shape of the wave spectrum. We find that the standard deviation of Hs associated to wave groups is a function of satellite altitude, wave height and spectral peakedness. For CFOSAT these fluctuations generally account for about 25% of the variance measured over a 80 km distance. This fraction is largest in storms and in the presence of long swells. When the estimated effect of wave groups is subtracted from the variance of Hs measurements, the remaining variability is higher in regions of strong currents.

Key Points

Wave groups contribute to small-scale fluctuations in altimeter wave heights, explaining 25% of the variance measured by CFOSAT in 80 km.

For the same wave height, fluctuations are larger in the presence of long and narrow-banded waves, typical of swell-dominated conditions.

Altimeters smooth out scales shorter than the square root of half the Hs times the altitude, and distort spatial patterns at that scale.

Plain Language Summary

Satellite altimeters routinely provide measurements of the height of ocean waves, and improved instruments or processing techniques have led to more precise and detailed measurements. Here we use a combination of simulations and data from the China France Ocean satellite (CFOSAT) to interpret the small scale fluctuations in wave height measurements as the effect of wave ”groups” which are fluctuations of the heights of consecutive waves associated to random waves. Due to spatial averaging within the radar footprint, we find that the fluctuations of significant wave heights (Hs) associated to wave group are a function of satellite altitude, wave height and other properties of the ocean waves. For CFOSAT, wave groups give a standard deviation of Hs that is of the order of 3 to 5% of Hs, typically half of the standard deviation in Hs measurements.