Altimeter data are used to compute wave height and wind velocity. The shape and intensity of the reflected radar signal depends on the sea state: a calm sea sends the signal back almost perfectly, like a flat mirror, whereas a rough sea scatters and deforms it (see How altimetry works: waveforms over the ocean).
Fishermen and other seafarers have an interest in knowing about ocean wave heights and wind velocities. Today, altimetry makes it possible to access such information in near-real time (within 3 to 48 hours) and to improve weather forecasting models. Altimetry cannot supply wave heights for any given location every quarter of an hour – at least not without many, many more satellites, but it is a good technique for identifying trends and computing climatologies, thus giving us information about seasonal and interannual variations, as well as about mean and maximum heights, which are of foremost importance to shipbuilding and offshore structures. Now that we have more than 25 years of continuous altimetry data, wave height and wind speed statistics can be used to achieve such goals with high precision. The link between wave heights in the North Atlantic and the NAO index was discovered in this way [Challenor, 2006]. Altimetry can also help study even very transient phenomena such as rogue waves: if no major significant wave height was observed by altimetry in a particular area, but some ships were nevertheless caught by a very high wave, it was likely to have been a rogue wave [Lefèvre, 2006].
Recent studies show that wave periods can be extracted from altimetry measurements [Caires, 2005, Quilfen, 2005]. This is also of major importance to shipbuilding. .
|Mean wave heights from altimetry off the British Isles in winter (December-February), spring (March-May), summer (June-August) and autumn (September-October) for the years 1993-97, The seasons are very well marked to the west of the Isles, with rough winters (identified by wave heights that can reach 14 metres or even as high as 20 metres), and calm summers.
(Credits Southampton OCeanography Centre)
Estimates of air-sea transfer rates of radiatively-active gases are needed for studying regional and global gas cycling and climate change. CO2 absorption increases with sea surface roughness. Absorption of CO2 by the ocean thus occurs more rapidly in winter (Northern or Southern) [Frew, 2006]. A rough sea surface also causes reflected radar waves to scatter more, meaning that the return signal received by the altimeter is weaker.
– Caires, S., A. Sterl, and C.P. Gommenginger. Global ocean mean wave period data: Validation and description. J. Geophys. Res., 110(C2), 1-12, 2005.
– Challenor, P., C. Gommenginger, D. Woolf, M. Srokosz, D. Carter, and D. Cotton, Satellite altimetry: A revolution in understanding the wave climate, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006
– Frew, N., D. Glover, and S. McCue, A Role for Altimeter Radars in Gas Exchange Studies, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006
– Lefèvre, J.M., L. Aouf, C. Skandrani and P. Queffeulou, Contribution of Satellite Altimetry to Wave Analysis and Forecasting, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006
– Quilfen, Y., B. Chapron, F. Collard, and M. Serre, Calibration/validation of an altimeter wave period model and application to Topex/Poseidon and Jason-1 altimeters, Marine Geodesy, 27, 535-550, 2005.
– Lefèvre, J.M. and P.D. Cotton, Ocean surface waves, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001