GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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(4) If da1da1 ≈ da2da2 and r1r1 > r2r2 , there is a single wave train in the direction given by the common
value of da1da1 and da2da2 .
(5) If the coded value of | da1da1 – da2da2 | > 2 and r1r1 < r2r2 , a confused sea exists and no
simple assumption can be made about the direction of the wave energy.
65.1.6 Use of Section 5
When used, this section shall contain the section identifier, an indicator (Ib) indicating whether
the section includes directional or non-directional data, pairs of data groups of spectral
estimates of the first to the nth frequencies or wave numbers and the direction from which waves
are coming in units of 4 degrees for spectral estimates (1) to (n) and their directional spread in
whole degrees.
NOTES:
(1) When non-directional spectra are transmitted, the group containing direction and directional spread
may be omitted.
(2) Complete directional spectra may be coded by repeating as many duplets as needed to define the entire
spectrum. A partial directional spectrum may be coded by selecting the largest spectral estimate for any
one frequency or wave number band over all directions and coding this for each frequency or wave
number band. Secondary peaks may not be coded unless the full directional spectrum is transmitted.
(3) For non-directional frequency spectra, the spectral estimates are in m2 Hz –1 . For non-directional wave
number spectra, the spectral estimates are in m3 . For a complete directional frequency spectrum,
spectral estimates are in m2 Hz –1 radian –1 . For a complete directional wave number spectrum, the
spectral estimates are in m4 . For incomplete directional spectra, whether in frequency or wave number,
the units of the spectral estimates should be m2 Hz –1 or m3 . That is, the total integrated energy within
a frequency band is given rather than just that of the peak. If the spectral estimate is less than 0.100 x
10 –5 , the value of 0 must be used. The exception to this occurs when all subsequent estimates at higher
frequencies are also 0, in which case only the zero immediately after the last non-zero spectral estimate
need be included; all others need not be coded.
(4) There may be cases when spectral estimates are given in integrated units, such as m2 , and it is necessary
to convert these to the units of the code. This is done by calculating the bandwidth at a frequency
by determining the frequency difference between midpoints on either side of the frequency in question.
The integrated spectral estimate is then divided by this computed bandwidth.
SPECIFICATIONS OF SYMBOLIC LETTERS USED IN WAVEOB (and not already defined)
A 1
WMO Regional Association area in which buoy, drilling rig or oil- or gas-production platform has
been deployed (1 – Region I; 2 – Region II, etc.). (Code table 0161)
(FM 13, FM 18, FM 22, FM 63, FM 64, FM 65)
A1A1A1 A2A2A2 Spectral estimates of the first to nth frequencies (or wave numbers if so indicated).
. . . (FM 65)
AnAnAn (1) The use of frequency or wave number is indicated by symbolic letter Ia. BB Number of bands described by the next two groups, except that BB = 00 indicates each of the
following groups represents only a centre frequency or wave number.
(FM 65)
B TB T
b w
C mC mC m
C smC smC sm
Total number of bands described.
(FM 65)
GUIDE TO WAVE ANALYSIS AND FORECASTING
Sub-area belonging to the area indicated by A 1. (Code table 0161)
(FM 13, FM 18, FM 22, FM 63, FM 64, FM 65)
Maximum non-directional spectral density derived from heave sensors, in m 2 Hz –1 for
frequencies and m 3 for wave numbers.
(FM 65)
Maximum non-directional spectral density derived from slope sensors, in m 2 Hz –1 for
frequencies and m 3 for wave numbers.
(FM 65)