with either U2 or F2 whichever cornes first from the left side of the
figure. If U cornes first, Z/2 is added to the duration at this point,
and the U2 ordinate is followed to either this new duration or to the
F, whichever is first from the left side of the chart. (Compare with the
preceding paragraph.) At this point, HF2, TF2, t^2, and F^2 inclusive,
are read off. If the constant energy line had intersected F2 before U2,
it is only necessary to drop down along the F2 abscissa to its intersection with U2, and at this point read HF2, Tp2, t^ and Fzn2^Thls Pro'
cedure could be used for many cases in which U2 is greater than Up)
The major différences in technique are used when U2 is less than Uj
and the H2T2 = constant line from the intersection of Ui and twi + Z/2
does not intersect either U2 or F^. Forecasting theory used here predicts that waves due to a constant wind blowing over an unlimited fetch
for an unlimited duration will eventually reach limiting height and period
distributions beyond which growth will not continue. In Figure 3-16 the
limit of this State is delineated by the line labelled maximum condition.
To the right of this line, it is assumed that any energy transport to the
waves by the wind is compensated by wave breaking, hence no wave growth
occurs.
3.52 EFFECTS OF MOVING STORMS AND A VARIABLE WIND SPEED AND DIRECTION
In principle, it should be possible to extend the Inoue differential
équation for wave growth to highly irregular conditions, but no experimental vérification of this concept has been published. Kaplan (1953)
and Wilson (1955) hâve proposed techniques for applying the simplified
prédiction techniques to variablewind fields and changing fetches. The
procedures appear reasonable and these techniques are used, although no
statistics are available for vérification.
3.53 VERIFICATION OF SIMPLIFIED WAVE HINDCAST PROCEDURES
Comparisons of hindcast wave heights and observed wave heights,
similar to Figure 3-7, hâve been given by Jacobs (1965) for the PNJ waveprediction System, by Bâtes (1949) and Isaacs and Saville (1949) for the
early Sverdrup-Munk method, by Kaplan and Saville (1954), for the early
SMB method, and by Bretschneider (1965) for a later révision. The basic
data from which the prédiction curves were derived, summarized by
Bretschneider (1951), also indicate the range of variation that may be
expected.
It is generally believed that much of the discrepancy between observed
and predicted waves is random, and that statistical summaries of observations and predicted values will agréé much better than the individual
observations. Saville (1954) and Pierson, Neumann and James (1955) give
summaries of results from a systematic program for deepwater hindcasting
waves at the four locations shown in Figure 3-17. The U.S. Naval Weather
3-40
figure. If U cornes first, Z/2 is added to the duration at this point,
and the U2 ordinate is followed to either this new duration or to the
F, whichever is first from the left side of the chart. (Compare with the
preceding paragraph.) At this point, HF2, TF2, t^2, and F^2 inclusive,
are read off. If the constant energy line had intersected F2 before U2,
it is only necessary to drop down along the F2 abscissa to its intersection with U2, and at this point read HF2, Tp2, t^ and Fzn2^Thls Pro'
cedure could be used for many cases in which U2 is greater than Up)
The major différences in technique are used when U2 is less than Uj
and the H2T2 = constant line from the intersection of Ui and twi + Z/2
does not intersect either U2 or F^. Forecasting theory used here predicts that waves due to a constant wind blowing over an unlimited fetch
for an unlimited duration will eventually reach limiting height and period
distributions beyond which growth will not continue. In Figure 3-16 the
limit of this State is delineated by the line labelled maximum condition.
To the right of this line, it is assumed that any energy transport to the
waves by the wind is compensated by wave breaking, hence no wave growth
occurs.
3.52 EFFECTS OF MOVING STORMS AND A VARIABLE WIND SPEED AND DIRECTION
In principle, it should be possible to extend the Inoue differential
équation for wave growth to highly irregular conditions, but no experimental vérification of this concept has been published. Kaplan (1953)
and Wilson (1955) hâve proposed techniques for applying the simplified
prédiction techniques to variablewind fields and changing fetches. The
procedures appear reasonable and these techniques are used, although no
statistics are available for vérification.
3.53 VERIFICATION OF SIMPLIFIED WAVE HINDCAST PROCEDURES
Comparisons of hindcast wave heights and observed wave heights,
similar to Figure 3-7, hâve been given by Jacobs (1965) for the PNJ waveprediction System, by Bâtes (1949) and Isaacs and Saville (1949) for the
early Sverdrup-Munk method, by Kaplan and Saville (1954), for the early
SMB method, and by Bretschneider (1965) for a later révision. The basic
data from which the prédiction curves were derived, summarized by
Bretschneider (1951), also indicate the range of variation that may be
expected.
It is generally believed that much of the discrepancy between observed
and predicted waves is random, and that statistical summaries of observations and predicted values will agréé much better than the individual
observations. Saville (1954) and Pierson, Neumann and James (1955) give
summaries of results from a systematic program for deepwater hindcasting
waves at the four locations shown in Figure 3-17. The U.S. Naval Weather
3-40