wave train. Waves in the surf zone are usually clearer with the CERC 

 unit. Reproduction has unavoidably reduced the definition of the radar 

 photos for this report. 



Radar can also image multiple wave train conditions. Figure 7 from 

 the CERC radar in California shows two wave trains (a swell from the 

 southwest and a second wave train from west-northwest) . Three wave 

 trains can be seen in Figure 8 (also from the CERC radar). Single or 

 multiple wave trains were seen in 80 to 90 percent of the PPI photos 

 taken with the two radar systems; surf zone waves were seen in all the 

 photos. 



In the sequence of radar images in Figures 9 to 12, the radar return 

 for the same location and sea conditions is shown for four different 

 radar ranges. These images were all taken within a 2-hour period dur- 

 ing the Cape Cod test. Good return was seen even on the 5.6-kilometer 

 (3 nautical miles) range with waves obvious to at least 3.7 kilometers 

 (2 nautical miles) from the radar. 



2. Possible Measurements with Radar . 



Since the CERC system takes an image of the wave field every radar 

 sweep and the time between sweeps is recorded with LED's, the wave speed 

 can be measured by noting the distance traveled by a particular wave be- 

 tween two frames. Figures 13, 14, and 15 show how this measurement of 

 wave speed is determined. The figures show the radar return for identi- 

 cal conditions and settings, except that each image was taken 5.5 seconds 

 later than the preceding one. In comparing these figures, the distance 

 traveled by a particular wave can be measured (marked by arrows) . A 

 rough estimate of period can also be made since both Figures 13 and 15 

 show a wave just reaching the breakwater. The times on the photos indi- 

 cate that the wave period is approximately 11 seconds. 



By viewing a series of radar images taken over a longer time period, 

 the evolution of the wave field can be documented (see Figs. 16 to 19 

 which were taken at approximately 4-hour intervals, and show that the 

 wavelengths increase with time) . 



Refraction phenomena as imaged in radar photos can be compared with 

 that predicted by refraction theory. Figures 20 and 21 show two cases 

 of refraction as seen in radar images. Figure 20 taken at Torrey Pines, 

 San Diego, California, shows strong bending of wave crests in the left 

 of the image due to an underwater feature (Scripps Canyon). Figure 21 

 shows the bending of a wave train which occurs when deepwater wave crests 

 approach the shore at an angle of approximately 45°. A short wave train 

 due to a local wind from the northeast and a longer swell from the south- 

 east also appear in the figure. Refraction is shown by arrows drawn per- 

 pendicular to the waves on each wave train at two different distances 

 from shore. If the bathymetry is known, and the waves are not too high, 

 it may also be possible to separate refraction due to the bottom from 

 that due to currents. 



