1088 
the spectrum is the important one. In the problem of 
storm tracking we are most interested in the longest 
periods present, as those travel most rapidly.® It is 
therefore desirable to divorce from the problem of wave 
forecasting the selection of that particular portion of 
the spectrum which happens to be important to a defi- 
nite application. 
The derivation of the physical laws governing the 
growth of the component wave trains under wind ac- 
tion remains the outstanding theoretical problem in the 
development of a method for forecasting the wave spec- 
trum. The problem promises to be a difficult one for at 
least two reasons: (1) the limitations on the growth of 
the shorter waves imposed by stability considerations 
introduce a nonlinearity into the problem; (2) to the 
extent to which the lower, shorter waves are sheltered 
from the wind by the large waves, the energy budgets of 
the component wave trains are not mutually inde- 
pendent. 
The problem of forecasting the transformation of the 
swell traveling through the region of decay also resolves 
itself into the problem of forecasting the change in the 
spectrum of the swell. The observed increase in the 
period of the significant swell is the result of the rela- 
tively rapid attenuation of the short-period components. 
One possible mechanism is provided by the viscous 
effects of the water, according to which the dissipation 
of energy is inversely proportional to the fourth power 
of the wave period. It can, however, easily be demon- 
strated that the effect of viscosity is several orders of 
magnitude too small to explain the observed trans- 
formation. Sverdrup [21] has, however, demonstrated 
that the selective attenuation resulting from the air 
resistance on waves traveling through regions of calm 
(equation (8) with U = 0) provides a possible mech- 
anism for selective attenuation. Groen and Dorrestein 
suggest that selective damping by eddy viscosity is 
the mechanism. Further study is indicated. 
It should also be noted that laws governing the gen- 
eration of waves at low wind speed can be expected to 
differ from those governing the generation at moderate 
and high speeds. The reason lies in the character of the 
sea surface, which appears to be hydrodynamically 
smooth at wind speeds less than 6 m sec, and hydro- 
dynamically rough at wind speeds exceeding 7 m sec7! 
[16]. Equation (9) and all subsequent development is 
based on the assumption of a rough sea surface. 
In addition to the more fundamental problems men- 
tioned above, there remain many problems of practical 
applications. Among the more important ones is the 
prediction of waves and swell from a tropical storm. 
The rules of thumb which have been established® do 
not in all cases lead to adequate forecasts. 
Most of the actual testing of wave forecasting meth- 
5. Consult “Ocean Waves as a Meteorological Tool’’ by 
W. H. Munk, pp. 1090-1100 in this Compendium. 
6. W. J. Francis, Jr., Commander, USN, ‘‘Waves and Swell 
from a Tropical Storm.’ Scripps Institution of Oceanography 
Wave Report No. 29, Nov. 1944 (unpublished). 
MARINE METEOROLOGY 
ods has been carried out for west-coast situations. - 
Forecasting for an east coast offers some special prob- 
lems, and these are being investigated by the Depart- 
ment of Meteorology, New York University, under the 
sponsorship of the Beach Erosion Board, and by the 
Woods Hole Oceanographic Institution [6]. 
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