4 Characterizing Parameters 



14 



To effect a summary description of the Harvest Platform database, frequency- 

 direction spectra are characterized with a set of parameters. These descriptors are 

 called bulk parameters because they are derived from extremal or integral proper- 

 ties of spectra, and so represent only part of the frequently more complicated di- 

 rectional structure of the wind wave field. A more exhaustive treatment of direc- 

 tional spectral structure at Harvest Platform is given by Long (1995b). For the 

 purposes of the present report, nine parameters are used. These parameters are: 

 characteristic wave height, peak frequency, two measures of characteristic direc- 

 tion, two measures of directional spread, two measures of asymmetry of direction- 

 ally distributed wave energy, and a measure of kurtosis of directional distri- 

 butions. This chapter contains the mathematical definitions of these parameters. 



Wave Height, Peak Frequency, and Peak Direction 



Characteristic wave height H mo is defined using the conventional definition of 

 four times the standard deviation of sea surface displacement. H no can be defined 

 in terms of the full frequency-direction spectrum, the frequency spectrum defined 

 by Equation 18, or the direction spectrum defined by Equation 19. A definition 

 that relates all of these entities is 



is 2 M N N M 



- = EE S(f n ,Q m )dfdQ = £ S[f n )df = £ S(Q m )de (20) 



16 m = i „ = i n = l m = l 



It should be noted that H mo reported herein is lower than what would be found in 

 conventional analysis because directional computations were truncated at 0.16 Hz 

 instead of the nominal 0.3-Hz limit for wind waves. Consequently, contributions 

 to H from high-frequency parts of wind wave spectra are not represented. 



Peak frequency / is defined as the discrete frequency at which the frequency 

 spectrum S{f n ) is maximum. This definition is conventional, in that it is the 

 usual characteristic frequency defined for nondirectional gauges. For conveni- 

 ence, Appendix A lists both f and its inverse, peak period T p ( = l/f p ). 



Chapter 4 Characterizing Parameters 



