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TECHNICAL REPORT

U.S. NAVY HYDROGRAPHIC OFFICE
SYNOPTIC AND PROGNOSTIC
WAVE CHARTS

JOHN J. SCHULE, JR.
and
JOHN F. ROPEK

Applied Oceanography Branch
Division of Oceanography

MAY 1955

ie ; ; U. S. NAVY HYDROGRAPHIC OFFICE
pw KI WASHINGTON, D. C.

ABSTRACT

This report describes a new method for presenting wave con-
ditions over a large area. Techniques for preparing both
synoptic and prognostic wave charts are presented, and some
of the problems encountered in analysis are discussed. Since
these wave charts can be transmitted by radio facsimile, they
are ideally suited for operational use.

In addition, a series of synoptic charts and 24-hour prognostic
charts as well as recommendations for further developmental
work are included.

M

FOREWORD

Up=to=date information on sea conditions over large areas
of the ocean would be of value to most types of naval operations.
Thus, in recent years, considerable research has been devoted

to the development of prediction techniques and methods of present-

ing wave forecasts to the operational forces.

Standard methods for observing, reporting, and predicting

wave conditions have been described in publications issued by the
But, as more is learned about the behavior

Hydrographic Office.
of the sea surface and as more data become available, new techniques

for applying wave forecasts to operational problems are necessary.
In order for this type of developmental work to contime, accurate

wave observations and evaluations of forecasting methods from ships
are extremely important. The methods of presentation described in
this report are not known to be in use at the present time, and the
wave charts are still in the developmental stage. The Hydrographic
Office welcomes comments as to their construction and operational

J. B. COCHRAN ~~

Captain, Ue. 5. Navy
Hydr ographer

value.

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DISTRIBUTION LIST (C)

ONO (Op-31, 314, 316, 533)
BUAER (2
BUORD (2
BUSHIPS (2)
BUDOCKS (2)
ONR on 416, 463, 465, 466)
NOL (2
NEL (2
NRL (2)
COMOPDEVFOR (2)
DTMB (2)
SUPNAVACAD (2)
NAVVARCOL (2)
NAVPOSTGRADSCOL, Monterey (2)
NDTS, Pasadena Annex (2)
AROWA
CIA (2)
BEB (2)
CE (2)
USFWS (2)
USC&GS (2)
DEPT COMMERCE (NBS) (2)
>—wHeP (2)
SIO (2)
UNIV WASH (2)
TEXAS A&M (2)
CBI (2)
UNIV CALIF (IER) (2)
NYU (2)
ASTIA (5)

iv

CONTENTS

Page
Foreword 444
Distribution List iv
Figures Vv
A. Introduction 1
B. Synoptic Wave Chart 1
C. Prognostic Wave Chart 2
D. Presented Chart Series 5
E. Results and Conclusions 6
F. Recommendations 7
Bibliography 22
FIGURES
1. Synoptic Wave Chart 12302 7 January 195) 9
2e Prognostic Wave Chart, Verifying Time 12302 8
January 195) 10
3- Synoptic Wave Chart 12302 8 January 195) ml
h. Prognostic Wave Chart, Verifying Time 12302 9
January 195) ie
5. Synoptic Wave Chart 1230Z 9 January 195h 13
6. Prognostic Wave Chart, Verifying Time 12302
10 January 195) 14
7. Synoptic Wave Chart 12302 10 January 195) 15
8. Prognostic Wave Chart, Verifying Time 1230Z 11
January 195 16
9, Synoptic Wave Chart 12302 11 Jamary 195) 17

Vv

10.
ll.
12.
136

le

FIGURES (Continued)
Prognostic Wave Chart, Verifying Time 1230Z 12 January 195)
Synoptic Wave Chart 12302 12 January 195),
Prognostic Wave Chart, Verifying Time 1230Z 13 January 195)

Synoptic Wave Chart 1230Z 13 Jamary 195)

TABLE

Observed and Forecast Wind and Wave Conditions (11 and 12
January 195),)

vi

Page

18
19
20
21

A. INTODUCTION

The Hydrographic Office Wave Charts are an attempt to present ocean
wave conditions synoptically over a large ocean areae Wave charts were
first drawn on an experimental basis in December 1952, and their develop-
ment has contimed on a part-time basis since then. The charts are now
being drawn dailye

The need for such a chart became apparent in planning the activities
of the Hydrographic Office Acoustic, Meteorological, Oceanographic Survey
(AMOS) cruises, and for special area surveys. With such forecasts, survey
work could be scheduled according to expected wave conditions, and locations
chosen in the most favorable areas for operating, thereby making survey time
most productive. :

Requests for forecasts of wave conditions for specific projects by
Fleet Commanders have increased during the past years. The wave charts be-=
came particularly valuable when the number of wave forecasts issued began
to increase, since the charts made possible immediate evaluation of wave
conditions for a large ocean srea.

This study includes a one-week series of synoptic wave charts and
2l<hour prognostic wave charts. This particular series of charts was selected
for presentation because of the high wave activity, making the relationships
between the predominant weve trains and the synoptic systems generating them
stand out much more clearly. Other than this, there are no features of this
series which differentiate its charts from those drawn daily at the Hydro-
praphic Office.

B. SYNOPTIC WAVE CHART

The synoptic wave charts presented in this study are based primarily
upon ship observations. A small percentage of plots consists of calculated
values, which are used only in 2reas where ship observations are sparse or
nonexistent. The plots consist of wind speed and direction, wave direction,
height, and periods; if the ship observation contains no wave group, wind
velocity end direction are plotted. The frontal systems and pressure centers
are drawn in, The plotter enters all values reported irrespective of whether
cr not they appear reasonable. The rejection or retention of data is de-
termined by the analyzer.

The synoptic wave charts ere drawn on the basis of the 1200Z ship ob-
servations. However, the 06002 ship cbservations are also included to com=
pensate for missing or garbled 1200Z rerorts and to provide an indication
of trend. When many ship observations are available for the same general
location, the most representative observations are plotted and the rest
rejected. The 06002 and 12007 ship observations are normally plotted in
different colors for easy analysis; this series, however, was plotted in
one color, according to the cade indicated in the lagend, for easier repro-
duction.

The plotted synoptic wave chart is used as an overlay on the 12302
synoptic weather chart. The analysis procedure is conducted on a light
table. First, the wave charts and associated weather charts are ex=
amined for the directionsof the predominant wave trains which are then
indicated by heavy arrows. An examination of all ship observations is
made, and those observations which appear to be unreasonable are re-
jected. A few spot calculations are made for wave heights and pericds
as a check on the order of magnitude of reported wave heights and periods;
this is especially important when very few ship observations are available
making it difficult to determine the reasonableness of a report. The cal-
culations are made using standard techniques i.e. Sverdrup-Munk (1951)
Bretschneider (1951), and Pierson, Neumann, and James(1955). The areas
which contain little or no ship observations must now be considered; for
these areas, wave heights and periods are calculated and plotted on the
wave charte

With the arrows indicating the predominant wave trains, and the iso-
baric pattern on the weather chart as a guide, contour lines of equal wave
height, based on the 1200Z ship observations, are sketched in;the 06002 ob-
servations are used as a puide. The calculated wave heights are now examined
to see if they fall reasonably well within the order of magnitude of the
contour lines based on reported ship observations. The calculated wave
heights will generally show the relative heights fairly well. If the abe
solute values appear to be off, the calculated heights are adjusted to fit
smoothly with the contours based on ship observations. There are no isolines
of wave period on the charts; this would add another set of lines to the chart
complicating it unnecessarily. In addition, it has been observed that re=
ported wave periods are not sufficiently reliable to draw reasonably good
isolines of wave period. Calculated wave heights are plotted for the points
which appear to be most necessary to fix the predominant features of the
wave chart and where ship observations are lacking.

No detailed calculations are made for the coastal areas since it would
require considerable time and effort to take into consideration the shelter=
ing effects and limited fetches; reported coastal observations, however, are
taken into consideration. Calculations of nearshore wave conditions are
made only for specific forecast requirements which are usually very much
localized. The indicated procedure, in its present state of development,
was designed to obtain a rapid evaluation of wave conditions over a large
ocean area with reasonably good accuracy.

C. PROGNOSTIC WAVE CHART
1. Principles

There are undoubtedly many ways in which the problem of producing a
prognostic wave chart can be approached. The method outlined below is one
of many compromises, with the major considerations being speed of analysis,
considered as firmly established since at present the charts are not pro=

uced as a standard item for operational use, but rather as a basis for
further developmental work.

Of prime importance, of course, is the availability of an accurate
prognostic surface weather chart. Assuming such a chart is available, the
calculation of wave height and period may not be a simple one since ef=
fective wind speed and direction, fetch, and duration must be determinede
In many cases it is almost impossible to determine all these quantities
with reasonable accuracy, and a sizeable error in any one may result in
considerable error in predicted wave heights. Further, the calculation
of wave heights for a large number of points would require considerable
time and effort, and, in the case of the 2li-hour wave prognostic chart,
could reduce the usable time of the chart to considerably less than 2h
hours. In this series, wave heights were calculated for points which
would appear to fix the predominant features of the charts. Intermediate
points were obtained by interpolation with the weather prognostic charts
as a guide. This again is done with the wave prognostic chart used as an
overlay on the weather prognostic chart. At present no attempt is made to
differentiate between sea and swell. The problem of crossed seas and con=
fused seas is handled on the basis of wave height, with wave direction fitting
into the predominant wave trains. Where extensive areas of confused sea are
reported, these areas are generally indicated as "Variable".

The actual drawing of the present wave prognostic chart is not done
with as straightforward a method as indicated above. The problem of moving
fetches is constant, and not an ©aSY one with which to cope. This problem
becomes especially important during winter conditions when fast-moving in-
tense systems are continually present in the North Atlantic. As a partial
solution to this problem, an attempt is made to maintain contimity from
chart to chart whenever possible. Consider the case of a system moving
at a reasonably constant speed and maintaining a reasonably constant size
and intensity. If the 2hhour prognostic weather chart indicates no ap-
preciable departure from the above mentioned conditions, it is reasonable
to assume that the wind field will generate waves closely approximating
those presently observed. With appropriate corrections for any changes in
the characteristics of the system it is possible to extrapolate into the
future the wave conditions associated with a particular weather system.
This is done whenever possible, even though it is realized that at present
considerable subjectivity enters this phase of the prognosis. However,
this same subjectivity is quite often met with in determining effective
wind speed, fetch, and duration when calculating wave neights for selected
points. The basis for this method is a knowledge of what a system is doing
insofar as waves are concerned.and corrections observed wave heights ac=
cording to expected changes in the generating system. Much more work needs
to be done in this respect. At the present time, such a prognosis is most
accurate when made by personnel who have observed a great number of cases of
synoptic weather systems and their associated wave systems.

Although the preparation of the synoptic wave chart involves the cal-=
culation of wave information, the bulk of the chart can be drawn on the basis
of ship observations. The calculated information and the observations can
both be corrected to fit the general wave pattern where necessary, result-
ing in a fairly accurate wave picture of the ocean area under consideration.
During the preparation of the prognostic wave chart, no observational guides
are available; every point on the chart must be predicted.

e300

2, Example

As an example of the steps necessiry in preparing a prognostic wave
chart, the prognostic wave chart, verifying time 1230Z% 12 Jamary, figure
10 is used. The basic principles described in C~-1 will be carried through.
The prognostic wave chart is based on the 30-hour weather prognostic chart
verifying time 1230Z, 12 January 195) and the synoptic wave chart of 12302,
11 Jamary 195). It should be noted that the prognostic weather chart pro#
vides the forecaster only wit! the expected conditions for a specific time
2, hours into the future. This is insufficient information with which to
establish the wave field for this time. The forecaster must establish the
complete wind history for the 2hehour period. In the present example this
is done in six-hourly intervals for the forecast period. The particular
interval of time chosen will depend upon the expected variability of the
wind field.

The following steps form a basic procedure for the construction of a
prognostic wave chart: (i) Selection of the points for which wave informa=
tion should be calculated in order to fix the predominant features of the
prognestic wave chart. (2) Evaluation of the wave field as related to the
synoptic weather systems and as indicated by the latest synoptic wave chart.
(3) Integration of (1) and (2).

A general analysis of the prognostic weather chart will indicate the
predominant fetch areas and those areas that show a minimum of wave activity.
The centers of these areas will fix the predominant features of the prognostic
wave chart. A sufficient mumber of points is now selected to provide the
detail necessary for drawing the prognostic wave chart. Wave plots are
calculated for these points using the form of table 1 as a guide. The prog-=
nosticated wave values are entered on the prognostic wave chart and the prog=
nosticated frontal positions are drawn in. Isopleths of wave height are
sketched in; a height interval of three feet is used at present.

In the evaluation of the synoptic wave chart, the following are noted:
(1) the positions and orientation of the major wave trains, (2) the position-
ing of the wave trains with respect to the frontal systems, (3) distribution
of heights within the trains, and {)) the history of the development of the
trains as indicated by previous synoptic wave charts. Such an evaluation of
the synoptic wave charts indicates the trend in the development and decay of
the predominant wave trains. The established trend forms the basis for
carrying the continuity into the forecast period,

The final form of the prognostic wave chart is determined by the inte-
gration of the calculated wave values and the established trends. ‘This pro=
cedure becomes especially important in areas where wind direction and velocity
show considerable variability. Under such conditions calculations are subject
to large ineccuracies. The isopleths of wave height which have been sketched
in are now modified to conform to the past and prognosticated trends. The
prognostic wave chart is completed with graphic symbols as indicated on figure

Table 1 represents a convenient working form and tabulation sheet for
data necessary to carry through the individual wave forecasts for the selected
points. The step of selection of points can be eliminated by establishing a

va) jo

permanent numbered grid covering the entire chart area. Column 2 lists

the wave information appearing on the latest synoptic wave chart, in the
present case, the 12302 11 January 195) chart. Columns 3, ), and 5 contain

the predicted wind history, at 6shourly intervals during the forecast period.
Column 6 contains the predicted wind values as calculated from the prognostic
weather charte The wind velocities are calculated on the basis of this geo-
strophic wind scale, with appropriate corrections made for isobaric curvature
and sea-air temperature differences. Column 6 also contains the predicted’
wave information which will be plotted on the prognostic wave charte

D. FRESENTED CHART SERIES

The series of charts presented in this study are typical of the Atlantic
Ocean during winter, and,show wave heights normally associated with moderately
intense storms. From a meteorological standpoint, the synoptic situations
in this series are characterized by blocking action over the extreme eastern
portion of the Atlantic Ocean (not shown) which resulted in the storms ap-=
pearing off the east coast of the United States being deflected toward the
north=northeast, and which also resulted in development of a strong pressure
gradient southward from Greenland. This condition prevailed until 11-12
January, after which time the blocking action relaxed, the Azores~Bermuda
high intensified, and the‘North Atlantic storms followed a more easterly
track. renee

One important part of the analysis of a wave chart and the preparation
of a prognostic wave chart is the association of the predominant wave .trains
with the synoptic weather systems generating thems therefore, a bried anal-
ysis of the presented series of charts will be made with this point in view.
The wave prognostic charts are based on the WBAN 308hour prognostic charts,
verifying time 12302.

Figure 1 is the wave chart for 1230Z, 7 January 195) and shows two pre-
dominant wave trains associated with the frontal system extending generally
southward from the vicinity of Nova Scotia. One train is associated with the
southerly flow ahead of the cold front and the other with the narthwesterly
flow behind the front. This positioning of wave trains is typical of all
such systems. Figure 2 is the 2h)ehour prognostic wave chart based on figure
1. The prognostic wave chart shows the continuation of both wave trains with
an increase in wave heights corresponding to the expected increase in the
intensity of the frontal system. To a large extent, the wave heights ap-
pearing on the prognostic chart are calculated using the wave heights ap=
pearing on figure 1 as a starting point. Figure 3 is the verification for
figure 2. It shows the existence of both wave trains with prognosticated
wave heights agreeing reasonably weil with those reported. The major error
is one of displacement of the axis of the wave trains; this is a common
error experienced in drawing the prognostic wave charts.

igure l} is the 2h-hour prognostic wave chart, verifying time 12307, 9
January 195i; figure 5 is the verifying chart for that time. It can be seen
that the prognosticated sea conditions ahead of the front asree satisfactorily
with the observed with respect to wave heights and direction and position=

esr

ing of the wave trains. Behind the front, however, the prognosticated axis
of the wave train is too far north and the area covered is too small. Under
any verification system, this section of the prognostic chart must be con=
sidered unsatisfactory. Following the same frontal system on figures 6 and
7 the prognosticated and observed sea conditions show satisfactory agreement.

Figures 6 and 7 also show the development of a new major wave train off
the east coast of the United States. Figures 8 through 13 show the continued
development of sea conditions associated with this system; the agreement be-
tween the prognostic wave charts and the wave charts can be considered satis=
" Fac COVY «

A brief scanning of figures 1 through 13 will undoubtedly give the ob= —
server af insight into the nature of the sea surface that would be difficult to
obtain by any other type of presentation for an ocean area of the size con=
sidered in these charts.

E. RESULTS AND CONCLUSIONS

The synoptic wave charts and wave prognostic charts are in their initial
stage of developments; their deficiencies are many and fully recognized. How-
ever, even in their present state they have been found to be of considerable
value. First, the charts give an excellent picture of wave heights and direc=
tions over an area as large as the North Atlantic Ocean; second, the charts
are a guide to the order of magnitude of wave heights for detailed forecasts
for specific locations; third, the charts make easier the forecasting of swell
Since the wave trains are outlined clearly as are conditions in the generating
area.

Shore ingtallatiens would benefit considerably from the synoptic and
wrognostic wave charts in that the directions and heights of the predominant
wave trains in the open ocean will be clearly indicated. This would be very
useful ss a tool for the prediction of swell approaching the shore installa-
tion and make possible more accurate prediction of surf conditions.

It should be pointed out here that the accuracy and general informa=
tion that can be obtained from such wave charts depends upon the effort
put into them. One major problem in this respect is the justification of
increased effort in terms of time, since any increase in time required to
produce a 2hehour prognostic wave chart will reduce its effective time and
its overall value.

From the stampoint of studying ocean waves and the systems generating
them, the wave charts are very revealing. A better understanding is being
obtained of wave characteristics in front of, in, and behind frontal zones$
the distribution of wave heights associated with synoptic systems; wave
heignts generated by wind fields of varying curvatures; relationships be-
tween swell and wave conditions in generating areas; and effects of chang-
ing wind direction on wave trains. Such problems are studied more easily
when a synoptic picture of wave conditions over a considerable area is
available, In addition, a picture presentation of wave conditions for
large areas of the ocean is much more useful and more easily interpreted

aba

than any word or number code message can ever be.

It is believed that this type of presentation of ocean wave conditions
would be of considerable value to both military and commercial interests

F. RECOMMENDATIONS

A synoptic presentation of wave conditions, as shown on the wave chart
and the wave prognostic chart, appears to be the best method of presenting
wave conditions over large ocean areas. This makes the preparation of in-
dividual wave forecasts unnecessary when numerous using agencies are in-
volved. This one feature of the wave charts makes them ideally suited
for facsimile transmission either as the entire chart or as sectionals.

The usefulness of areal wave intelligence to a military user transmitted
by facsimile or by canned map is obvious.

Although the coastal areas have, in general, been neglected in the
present wave charts, they need not be. There are several wave~-reporting
stations along the east coast of the United States whose reports will be
included on the wave charts as soon as arrangements, now underway,. are
completed. More such stations taking both visual and instrumental ob-
servations should be activated so that a reasonable coverage of the east
coast would be available. This information, if sufficient coverage can be
obtained, would be of considerable value to shore installations and activi-
ties operating near shoree

More work is necessary to decrease the time required to produce a
wave prognostic chart. This would involve further development of the
methods already discussed, application of the latest developments in the
forecasting field, and efficient design of new procedures and techniques
to make these charts a production item with acceptable accuracye

The synoptic wave charts and the prognostic wave charts already have
been proved to be of considerable value. With firther developmental work
they have the potential of becoming a useful addition to any ocean fare-

casting activity.

TABLE 1
OBSERVED AND FORECAST WIND AND WAVE CONDITIONS (11 and 12 January 195))
2 3 l 5

Observed Predicted Predicted Predicted Predicted Predicted
Wind and Waves Wind Wind Wind Wind Waves
1230Z 11 Jan. 18302 11 Jan. |00302 12 Jan. 12302 12 Jan.

Position Poff fafa Le feels feet fs taefs fe fuefaest a Lf ta

F a Fetch length in nautical miles H = Wave height in feet Vj = Wind speed effective for
i i duration
V «= Wind speed in knots T = Wave period in seconds # = Wind shifts during 2h<hour

period

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BIBLIOGRAPHY

BRETSCHNEIDER, C. L. Revised wave forecasting curves and procedure,
University of California. Institute of Engineering Research. Technical

Report no. 155-17, Series 29 =Tssue 7, 28 p., Berkeley: 1951.

PIERSON, W. Je, JR., NEUMANN, GERHARD, and JAMES, R.W. Practical
methods for observing and forecasting ocean waves by means of wave spectra
and statistics, New York University. College of Engineering. Research
Division. Department of Meteorology and Oceanography, Technical Report
TOS LE S22 Gp mss um aa,

U. S. HYDROGRAPHIC OFFICE. Techniques for forecasting wind waves and swell,
Hydrographic Office Publication no. 60). 37 pe Washington: 1951.

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