Hasselmann and Sohieler 



10- 



CD 

 T3 



u>20 



b 



30 - 



40- 



10° 20° 



DEPRESSION 



30^40'' 

 ANGLE 



60" 90" 



Fig. 2. Theoretical and observed Bragg backscatter cross sections 

 for vertically polarised 24 cnn (L band) waves (from Wright 

 [23]) 



Figure 2 shows a comparison by Wright [23] of experimental 

 and theoreticcd Bragg cross sections for vertically polarised cm-dm 

 waves. The surface waves were represented by a Phillips' spectrum 

 F (k) = (Q'/2)k" S(4j) » with a uniform half-plane angular spreading 

 function, S(i|j) = tt-" for < |4j| < Tr/2, S{^) =0 for - ir/2 < | i|j | :^ ir. 

 The constant a was chosen to fit the observed cross sections, but 

 is not inconsistent with other estimates from direct measurements of 

 gravity- wave spectra (cf. also [ 15] ). Shown in Fig, 3 are theoreti- 

 cal and experimental cross section ratios f^/cr^^. The agreement 

 here is also very good, except for the shortest wave length (3.4 cm, 

 8910 MHz), where scattering by spray may be beginning to mask the 



t 



The theoretical cross sections shown in Figs. 2 and 3 were, in fact, 

 computed for the wave-facet interaction model considered in the 

 next section. However, the deviations from the first-order Bragg 

 model are negligible. 



368 



