Global Wave Forecasts Using Spacecraft Data 



would measure radar scattering cross section, a-^ versus the angle 4> of the 

 radar beam to the sea surface and present a graph of ctq versus <P. If is 90° 

 when the beam points straight down, then cr^ is high for low waves at = 90° 

 and low at small 0. Conversely for large waves cr^ is low for <P = 90° and high 

 for small 0. The variation of a^ for 4> defined in this way is shown in Fig. 12. 

 The difficulty is that present measurements are presented equally often in terms 

 of either wind speed or the ill-defined term, sea state. 



♦20 



+10 



o 



Ul 



Yi -10 



5 -20 



-30 



-40 



20 30 40 50 60 70 



DEPRESSION ANGLE. DEGREES 



80 



90 



Fig. 12 - Radar scattering cross section versus the 

 angle that the beam makes with a line parallel to the 

 sea surface for various sea states according toKatz. 

 (The convention here is opposite to the wording of 

 the text.) (From Ref. 27.) 



For a radar wavelength of 50 cm or so, it is not clear whether ^^ versus e 

 is predominantly a function of the entire wave spectrum, in which case the scat- 

 terometer measures wave height, or of some part of the high wave number 

 (k = a>Vg) portion of the wave spectrum, in which case wind speed is measured, 

 because the high-frequency part of the spectrum responds to the local wind very 

 quickly. 



These problems have been discussed in Ewing (27) and by Moore and Pier- 

 son (28). A program of measurement from a NASA aircraft along with simulta- 

 neous wave measurements is underway at the Spacecraft Oceanography Project 

 of the Naval Oceanographic Office. These problems should be resolved shortly. 



The purpose of the preparation of the above global wave forecasting and 

 hindcasting (or specification) computer-based procedure is to be ready to specify 

 the wave and wind fields over the world ocean in a way that will permit the use 



525 



