Near nadir microwave specular returns from the sea surface-measurements of winds from breeze to hurricane

Wind velocity over the sea surface is the most important parameter governing atmospheric and oceanic systems. Two satellite-borne sensors: scatterometer and altimeter have been developed. The former has been specifically designated for mapping sea-surface winds, but the latter which can also detect the surface elevation has attracted a great deal of attention lately. The scatterometer, operated on the basis of Bragg scattering from ocean ripples, are found to be operative at only intermediate winds with speeds between 4 and 10 m/s; it is quite incentive at both low and high winds. Low wind speeds, however, prevail over the world's oceans, while measurements over stormy conditions are most needed as in situ instruments often fail. The remote sensing of sea-surface winds with an altimeter is based on specular reflections from the surface roughness. At low and intermediate winds, the altimeter return is shown to be inversely proportional with the mean-square slope contributed mainly by ocean ripples, having their lengths longer than the radar wavelength. An algorithm is justifiably constructed, and is tuned with reported buoy-measured winds. This algorithm is most sensitive at low winds. At high winds, the sea-surface roughness produced by breaking waves starts to influence the altimeter return. Reported data obtained under hurricanes are used to show that additional altimeter returns at various wind speeds are consistent with the results of sea-surface whitecap coverages. With these understandings of backscattering mechanisms at different wind regions, a continuous function is proposed to determine the sea-surface winds, from breeze to hurricane, from the altimeter returns.