MODELING AND MEASUREMENTS OF MICROSCOPIC 
STRUCTURES OF WIND WAVES 
Jin Wu 
Hydronauttes Ine. 
Laurel, Maryland, U.S.A. 
ABSTRACT 
The microstructure of the wind-disturbed water 
surface, characterized by surface-slope and sur- 
face-curvature distributions, is measured in a labo- 
ratory tank under various wind and wave conditions, 
The relative frequencies of occurrence of various 
slopes generally follow a normal distribution. At 
low wind velocities, the formation of parasitic 
waves causes a skewed slope distribution; at high 
wind velocities, the wave breaking causes a peaked 
slope distribution. It is also shown that the mean- 
square slope rises suddenly at about the wind velo- 
city where the airflow boundary layer becomes 
turbulent. The curvature distribution of the wind 
disturbed water surface observed from different 
angles is generally skewed with greater radius of 
curvature at steeper viewing angles from the nor- 
mal to the mean water surface. As the wind velo- 
city increases, the average radius of curvature 
decreases rapidly at low wind velocities when 
waves are effectively excited by wind, and gradu- 
ally at high wind velocities when waves approach 
saturated state. The present measurements of 
surface curvatures are the only set of data of its 
kind. The mean-square surface slopes are com- 
pared favorably with those determined in the field; 
and both sets of data are consistent with the equi- 
librium wind-wave spectra. 
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