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Other parameters are defined to characterise the sea state as prescribed by the wave spectrum.  In particular, the reciprocal of the frequency corresponding to the peak of the spectrum is the wave peak period.  Different mean periods are calculated by spectrally averaging the spectrum and similarly for mean wave direction (see IFS documentation part VII, chapter10).

Wind waves and Swell

Very often, the sea state is composed of different wave systems.  If there is any sufficient wind, there will always be a wave system associated with it, referred to to "wind-wave" or "wind-sea".  The part of the spectrum that is not associated with the local wind is normally called "swell".

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Fig2.2.1-5: Wave Energy associated with a given frequency E(f) plotted against wave frequency (f).  The Equivalent Wave Height (EWH) associated with a given wave frequency is derived from the area under the curve for that frequency bin. The significant wave height H_s is derived from the total area beneath the curve.

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Maximum wave height

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from forecasts and observations

Currently ECMWF output shows the expectation value of the maximum height of the envelope of waves (ρ).  However, buoy observations of sea elevation (η) tend to report the maximum wave height estimated from the zero crossing (H_max).  These values do not correspond exactly but differences are normally small.

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