Near‐nadir, quasi‐specular backscatter data obtained with a 14‐GHz airborne radar altimeter are analyzed in terms of the surface mean square slope (mss) parameter. The raw mss data, derived from a least squares fitting of a ray optical scattering model to the return waveform, show an approximately linear wind speed dependence over the wind speed range of 7–15 m s−1, with a slope of about one half that of the optically determined mss. Further analysis based on a simple two‐scale scattering model indicates that, at the higher wind speeds, ∼20% of this apparent slope signal can be attributed to diffraction from waves shorter than the estimated diffraction limit of ∼0.10 m. The present slope data, as well as slope and other data from a variety of sources, are used to draw inferences on the structure of the high wavenumber portion of the wave spectrum. The data support a directionally integrated model height spectrum consisting of wind speed dependent k−5/2 and classical Phillips' k−3 power law subranges in the range of gravity waves, with a transition between the two subranges occurring around 10 times the peak wavenumber, and a Durden and Vesecky wind speed dependent spectrum in the gravity‐capillary wave range. With a nominal value of the spectral constant Au = 0.002 in the first k−5/2 subrange, this equilibrium spectrum model predicts a mss wind speed dependence that accords with much of the available data at both microwave and optical frequencies.

A microwave radar technique for remotely measuring the vector wave number spectrum of the ocean surface is described. The technique, which employs short‐pulse, noncoherent radars in a conical scan mode near vertical incidence, is shown to be suitable for both aircraft and satellite application. The technique has been validated at 10 km aircraft altitude, where we have found excellent agreement between buoy and radar‐inferred absolute wave height spectra.

The directional spectrum of a fully arisen ∼3 m sea as measured by an experimental airborne radar, the NASA Ku‐band radar ocean wave spectrometer (ROWS), is compared to reference pitch‐roll buoy data and to the classical SWOP (stereo wave observation project) spectrum for fully developed conditions. The ROWS spectrum, inferred indirectly from backscattered power measurements at 5‐km altitude, is shown to be in excellent agreement with the buoy spectrum. Specifically, excellent agreement is found between the two nondirectional height spectra, and mean wave directions and directional spreads as functions of frequency. This agreement is found despite certain discrepancies between the radar and buoy angular harmonics which are believed to be due to buoy instrumental effects. A comparison of the ROWS and SWOP spectra shows the two spectra to be very similar, in detailed shape as well as in terms of the gross spreading characteristics. Both spectra are seen to exhibit bimodal structures which accord with the Phillips' resonance mechanism. This observation is thus seen to support Phillips' contention that the SWOP modes were indeed resonance modes, not statistical artifacts.

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A b s t r a c tThe Goddard Space Flight Center 13.9