-20 



-30 



XANDL 



(VERT. AND HOR. POL.) 



-X(HOR.POL) 

 -L'.HOR.POL.: 

 -220MHz(H0R.P0L.) 

 -50MHz (HOR. POL.) 



30 40 bO GO 



GRAZING ANGLE (decrees) 



Figure 27. 



Composite of o° data for a 

 "medium" sea (Skolnik, 1970). 



so small that many waves which are more than 1.5 kilometers (4,920 feet) 

 from the radar, will not be seen because of shadowing by nearer waves 

 (see Fig. 28), and also because of the small radar cross section at small 

 grazing angle. At elevations of about 20 meters, the radar begins to see 

 some return from wave troughs which makes the lines of the wave crests 

 harder to distinguish (Fig. 29). 



Another factor is due to the modulation of the radar-scattering cap- 

 illary waves by the longer gravity waves and swell. This modulation, 

 due to hydrodynamic interactions, results in a concentration of capillary 

 wave energy near the wave crests giving increased capillary amplitude and 

 stronger radar return there. Phillips' (1966) description on the strain- 

 ing of small, short waves by the larger waves is one of the hydrodynamic 

 mechanisms contributing to this concentration. The effect of modulation 

 of capillary waves on radar return has been investigated by Yeshchenko 

 and Lande (1972), Keller and Wright (1975, 1976), Plant, Keller, and 

 Wright (1978), Reece (1978), and Wright (1978). 



The rotating display sweep registers an image of the sea scatterers 

 from the wave crests on the PPI display which in turn can be recorded by 

 time-lapse photography. The waves usually exhibit a long-crested charac- 

 ter (also appears in aerial photos) and appear as light and dark strips 

 across the PPI scope. It is often assumed that the light areas repre- 

 sent the front face or crest of the waves, and that the dark strips repre- 

 sent the backface or trough. The correctness of this intrepretation is 



30 



