Hasselmann and Sahieler 



I. INTRODUCTION 



The development of numerical wave prediction methods in the 

 past years [l, 2, 17] has increased the need for wave data on a 

 synoptic scale, both as a reference for testing and improving the 

 models and as real-time input for the computations. Synoptic wave 

 data would also be of value for numerical weather forecasting by 

 providing indirect information on surface winds in otherwise poorly 

 covered areas of the oceans. The growing interest in electromag- 

 netic backscatter from the sea surface stems largely from the 

 potentiality of the method for furnishing sea- state data of this kind. 

 Radar scatterometers in satellites could scan most of the world 

 oceans in a few hours. Alternatively, large areas of the ocean can 

 be sampled using HF stations on land. Following the pioneering 

 work of Crombie [ 6] and others , Ward [ 22] has recently detected 

 the backs cattered return of ionospheric HF modes from relatively 

 small, 100 km square patches of the sea surface at distances up to 

 3000 km. 



Unfortunately, both techniques suffer from wave length limi- 

 tations. Cloud absorption and finite atenna size define an effective 

 transmis sion window for satellite scatterometers in the conventional 

 radar wave length range between a few fractions of a cm and about 

 50 cms. Backscatter measurements over long horizontal ranges 

 are similarly restricted to ionospheric modes in the decameter 

 band. In both cases, the electromagnetic wave lengths are consider- 

 ably shorter than the principal components of the surface-wave 

 spectrum, which normally lie in the range between 50 and 500 m. 

 The bad wave length matching creates difficulties in relating the 

 backscattered signals obtained by these methods to significant sea- 

 state parameters. 



Scattering experiments in both the centimeter- decimeter and 

 decameter bands have now clearly established the basic validity of 

 the first-order (Bragg) wave-wave interaction theory. According to 

 this model, the backscattered radiation arises from interactions with 

 two gravity- wave components whose wavenumbers k^ are determined 

 by the Bragg (resonance interaction) condition for constructive inter- 

 ference, k^= ± 2k' , where k' represents the horizontal wavenumber 

 component of the incident radiation. For non-normal incidence, 

 the wave lengths of the scattering and incident components are then 

 of the same order, which implies that the scattering surface waves 

 normally lie in the high- wavenumber , equilibrium range of the 

 surface-wave spectrum. It appears therefore from first-order 

 theory that backscatter measurements may yield a useful independent 

 determination of Phillips' constant [15, 22] , but do not contain sig- 

 nificant information on the more interesting low -wave number part 

 of the wave spectrum which contains most of the wave energy. 



Fortunately, the scattering measurements , while supporting 

 the Bragg theory, also indicate that it should be regarded only as a 

 first approximiation. The Doppler spectra, in particular , exhibit 



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