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- Title
Along‐Track Resolution and Uncertainty of Altimeter‐Derived Wave Height and Sea Level: Re‐Defining the Significant Wave Height in Extreme Storms.
- Authors
De Carlo, Marine; Ardhuin, Fabrice
- Abstract
Satellite altimeters are the most common source of wave measurement in phenomenal sea states, with significant wave heights exceeding 14 m. Unfortunately their data is still considered with skepticism, because there is usually no other data to verify the accuracy of the largest values. Here we investigate the self‐consistency of the measurement, and their small scale variability, in order to define an estimate of satellite altimeter precision. Using numerical simulations of ocean surfaces and the processing involved in satellite retracking, we find that wave groups are responsible for a variance in estimated altimeter wave heights that is proportional to the square of the spectral peakedness parameter and the significant wave height. Additional variance induced by speckle noise is proportional to the wave height. The effect of wave groups generally dominates in the most severe storms. This variability requires a relatively large scale smoothing or filtering to yield accurate wave height estimates. For example, the largest ever reported 1 s average significant wave height from altimeters sampled by Jason‐2 in the North Atlantic in 2011, at H‾s=20.1 ${\overline{H}}_{s}=20.1$m, is now interpreted to correspond to a true wave height Hs = 18.5 ± 0.3 m. The difference between 20.1 and 18.5 m is mostly due to wave group contributions to the raw measurement. We argue that wave group effects should not be included in the definition of the significant wave height, just like the maximum wave height differs from the significant wave height. Plain Language Summary: Over most of the past 30 years, satellite altimeters have been the only means to measure wave properties in the most severe ocean storms. How do we know that these data are trustworthy, and how can we define uncertainties? Here we show that as a satellite flies along its orbit, it reports wave height that fluctuate because of the random nature of the wavefield that can be organized in groups at the scale of a few kilometers. We are able to simulate the precision of the measurements, as a function of the wave height and the degree of organization of the wavefield, measured by a "spectral peakedness" parameter. This novel understanding can be used to define the precision of the measurements. For example, as far as we know, the largest reported value for a 1 s averaged satellite measurement of the significant wave height was H‾s=20.1 ${\overline{H}}_{s}=20.1$m in a 2011 North Atlantic Storm, with no precision given. We can now re‐interpret this data as evidence of a true significant wave height Hs = 18.5 ± 0.3 m. The local fluctuations up to 20 m are caused by wave groups and should not be counted in the significant wave height. Key Points: Retracking of altimeter waveforms yields fluctuations in wave height and sea level, correlated at the scale of the effective footprintA good approximation for the effective footprint diameter is the square root of the product of wave height and satellite altitudeEstimating phenomenal wave heights precise to better than 2% requires filtering over wave groups, typically over a distance of 20–50 km
- Subjects
ROGUE waves; SEA level; OCEAN waves; ARTIFICIAL satellite tracking; SPECKLE interference; OCEAN conditions (Weather); SEVERE storms
- Publication
Journal of Geophysical Research. Oceans, 2024, Vol 129, Issue 6, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2023JC020832