Period by the Wave-Group Method

Abstract

The wave-group period, T , is obtained by a eimple manual procedure from the periodic wave groupff'and sequences that appear in strip-chart records. A dozen or more values are commonly derived from a 20-minute record. When plotted as a time-series, T values form patterns that represent individual wave trains, each generated by a synoptic weather event. Swell trains and wind-wave trains can be distinguished by their characteristic period distributions. For swell trains, the mean wave-group frequency, f , obtained from the best-fit line to the time-series plot of f_ values (reciprocal T values), is equivalent to the frequency of maximum energy density, fm, obtained from spectral analysis. The distance and time of origin for a swell train can be determined from the time rate of change of f„, and with the use of weather maps the generating area can be identified and the deep-water arrival direction of the swell train at the wave gage determined. The zero-line crossing period, T , represents an integration of the waves from all wave trains present and has no synoptic significance.