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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 Hs is derived from the total area beneath the curve.


Definition of the maximum wave height.

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 (Hmax).  These values do not correspond exactly but differences are normally small.


Image Added

Fig2.2.1-6: Schematic showing difference between representation of maximum wave height.

The black line shows a record of sea surface elevation as measured by a buoy (η). The maximum crest height above zero-crossing is shown green.  The crest to trough height (Hmax), is shown blue.

The forecast envelope of the varying sea surface wave crests is shown by the grey line (ρ). The maximum of the the envelope of crest heights is at a slightly different time and elevation.  The maximum envelope crest to trough height is taken as twice the maximum of the envelope above zero-crossing (shown red).