Jin Wu 
I, INTRODUCTION 
The wind-disturbed water surface consists of waves of va- 
rious lengths moving at various speeds. There has been ever-increa_— 
sing interest in determining the statistical properties of the micro- 
structure (wavelets) of such a surface. From a fluid-mechanics point 
of view, these wavelets are involved in the inception of wind waves 
and are believed to be related to the dissipation of the wave energy. 
From an oceanographic point of view, the quantitative measurement 
of the mean square surface slope provides the best determination of 
the coefficient involved in the equilibrium wind-wave spectrum for 
describing the directional energy-density distribution of ocean waves. 
From a meteorological point of view, the microstructure plays a ma- 
jor role on the radiation of thermal energy from the sea surface. 
Finally, from the remote-sensing point of view, ripples are impor- 
tant for interpreting reflection and back scattering of electromagne- 
tic waves from the ocean surface. 
A few optical methods have been adopted in the past for de- 
termining the microstructure of the wind disturbed water surface; 
these include the photographic method of Schooley (1954, 1955), and 
of Cox and Munk (1956), and the light refraction method of Cox 
(1958). However, the photographic method so far developed involves 
rather tedious data analysis and, moreover, is not completely apt for 
laboratory application, Owing to the limited fetch, the water surface 
structure in the laboratory wind-wave tank lacks spatial homogenei- 
ty, which is required for the photographic method. In the light refrac- 
tion method, the apparatus consists of submerged parts which offer 
obstruction to waves and are rather difficult to construct; in addition, 
the under tank illumination required is inconvenient for a deep wind 
wave tank which is appropriate to simulate the air-sea interface for 
more advanced studies, The present instrument, utilizing a light 
reflection principle, is capable of determining not only the surface 
slope but also surface curvature with high resolution. 
In the present study, the microstructure of the wind-distur- 
bed water surface, characterized by surface-slope and surface-cur- 
vature distributions, is measured in a laboratory tank under various 
wind and wave conditions. The features of these distributions are 
discussed, along with their variations with structures of dominant 
waves and the growth of slope and curvature statistics with the wind. 
The present measurement of surface curvatures is the only set of 
data of its kind. The mean-square sea-surface slope obtained by 
Cox and Munk (1956) are reanalyzed and compared with the present 
results. These two sets of data are shown in good agreement and to 
be complementary to the equilibrium wind-wave spectra, Finally, 
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