GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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Figure 9.3 —
Percentage
exceedance of
significant wave
height, H s, and the
most likely highest
wave in three
hours, H max,3h,
from measurements
(12 520
valid observations,
including six
calms), at threehour
intervals at
OWS “Lima”,
December 1975 to
November 1981
(xx=H max,3h;
++=H s) (from
HMSO, 1985)
Percentage exceedance
provide useful indications of possible errors in the data.
Of course, no data should be totally discarded simply
because they do not meet some criteria without further
investigation.
Before progressing, it is important to examine the
results so far to see whether to analyse the entire data
set as a whole or whether to divide it in any way to
separate values arising from different physical realizations.
It is most important when analysing data from
sites affected by tropical storms to separate out those
caused by such storms — see Section 9.4.3. The occur-
GUIDE TO WAVE ANALYSIS AND FORECASTING
Wave height (m)
rence of mixed distributions has occasionally been
reported from some sites. For example, Resio (1978)
found that wave heights off Cape Hatteras, North
Carolina, hindcast from storm winds could be better
explained by fitting different distributions depending
upon the storm tracks.
In most parts of the world there is a marked
seasonal variation in wave conditions. In higher latitudes,
for example, generally much higher waves occur
in the winter than in the summer and Dattatri (1973)
reports higher waves off the west coast of India during
Figure 9.4 — Joint distribution (scatter plot) of significant wave height, H s, and zero-upcrossing period,
T – z, from measurements (12 520 valid observations, including six calms), at three-hour intervals
at OWS “Lima”, December 1975 to November 1981 (Key: n – parts per thousand (PPT),
n – number of occurrences (< 1 PPT) (from HMSO, 1985)