PART V: DATA REDUCTION/PARAMETERIZATION 



135. Directional properties of the observed frequency- direction spectra 

 are found from a set of parameters which reduce the volume of data from the 

 entire spectrum (2,548 numbers in the analysis used here) to just a few 

 representative numbers for each case. Observed data are a signature of the 

 physical processes (local distribution of wave energy in frequency and 

 direction) represented by a geometric figure (the frequency-direction spec- 

 trum) . Characteristics of the physical processes are deduced from the shape 

 or shapes of all or part of the obseirved spectra. 



136. A conventional parameter (one that can be estimated reasonably 

 well with a low-resolution directional measurement system) is peak direction, 

 of which there are several measures. These were defined in Part II for the 

 full frequency-direction spectrum and for the integrated direction spectrum. 

 A peak direction can also be assigned for the distribution at each frequency 

 in a frequency-direction spectrum. 



137. If the full frequency-direction spectrum is known, there are 

 several other important characteristics that can be determined. A primary 

 parameter of interest is one that characterizes directional spread. This is 

 broadly defined as an arc of directions containing a significant amount of 

 wave energy. A second parameter is directional asymmetry. This indicates how 

 evenly energy is distributed on either side of a peak in a directional 

 distribution. Depending on the level of detail desired, these two parameters 

 can be determined from the frequency-direction spectrum as a whole (bulk 

 characterization) , from distributions at individual frequencies (frequency 

 characterization) , or from individual modes (well -separated peaks) within the 

 distribution at a given frequency (modal characterization) . Once computed, 

 these parameters can be ordered and correlated with other parameters to 

 characterize the variety and variability of observed sea states. 



Frequency Characterization of Spread. Asymmetry, and Position 



138. A conventional treatment of directional distributions of wave 

 energy (see, e.g., texts by Coda 1985 or Horikawa 1988) is to consider a slice 

 through the frequency- direction spectrum S(fj,,^^) along a line of constant 

 frequency (e.g., f = f j, , one of the frequencies from discrete Fourier 



54 



