TPB No. 494
Subject: Ocean Surface Waves
SIGNIFICANT CHANGES FROM LAST BULLETIN ON THIS SUBJECT NO. 453
Created 05/25/2003; revised 04/07/2004.
W/NP21:HSC, LDB
This bulletin, prepared by H. S. Chen, L. D. Burroughs, and H. L. Tolman of the Marine Modeling and Analysis
Branch (MMAB), Environmental Modeling Center (EMC), National Centers for Environmental Prediction
(NCEP), describes automated global ocean wave guidance provided in graphic, alphanumeric, and GRIB
formats.
The NOAA WAVEWATCH III (NWW3) was implemented in March 2000. It is a third generation model which
accounts for wave dispersion within discrete spectral bins by adding diffusion terms to the
propagation equation (Booij and Holthuijsen 1987); it uses the Chalikov and Belevich (1993)
formulation for wave generation and the Tolman and Chalikov (1996) formulation for wave dissipation;
it employs a third order finite difference method by utilizing a split-mode scheme with a Total
Variance Diminishing limiter to solve wave propagation; its computer code has been optimized to
fully utilize the MPP structure of the IBM mainframe computer and all the power of FORTRAN 90; it uses a spatial
resolution of 1.25^{o} x 1.00^{o}on a lon./lat. grid, a domain from 78^{o}N to 78^{o}S, and a directional
resolution of 24 directions.
The bulletins and graphics of the new guidance follow the same formats shown in TPB No. 453 (Chen et
al., 1999), except for changes to the spectral text bulletins now being sent to AWIPS and the
following model improvements :
- The model has been re-coded in FORTRAN 90 to utilize modular concepts and allocatable data
structures. No noticeable changes have resulted in the guidance.
- Improved source term integration schemes have been used with no perceptible changes to the
guidance.
- A new propagation scheme to eliminate the Garden Sprinkler Effect more efficiently and to
account for unresolved islands and sea ice.
- Re-tuning to eliminate model biases induced by changes above.
- Spectral text bulletins for the NWW3 are available at
http://polar.ncep.noaa.gov/waves .
These files are in ASCII and are available by anonymous ftp at
ftp://polar.ncep.noaa.gov/pub/waves/date.cycle,
where date represents the date in yyyymmdd format and cycle represents the run cycle
identifier (t00z, t06z, t12z or t18z, respectively). These bulletins have been implemented on AWIPS, but
with a condensed format necessitated by the capabilities of the communications gateway and
display capabilities of AWIPS.
The ocean wave guidance is generated four times daily out to 168 hours based on the 0000, 0600, 1200 and 1800 UTC
cycles of the of the Global Forecast System (GFS).
Technical Procedures Bulletin No. 453 is now operationally obsolete.
OCEAN SURFACE WAVES^{(1)}
by H. S. Chen, L. D. Burroughs, and H. L. Tolman^{(2)}
1. INTRODUCTION
During the last five decades, wind wave forecasts have improved significantly from the empirical
approaches based on Sverdrup and Munk (1947) and Bretschneider (1958) to the spectral approaches
based on the radiative transport equation (e.g. SWAMP Group 1985). At present, the most advanced
spectral model for research and forecast is the so-called third generation wave^{(3)} model (WAMDI
Group 1988) of which the NWW3 is an example (Tolman 2002). The Marine Modeling and Analysis Branch (MMAB)
has made systematic efforts to test and develop models based on prediction accuracy, computational
efficiency and sound wave dynamics and to employ them to produce operational forecasts.
The NWW3, as noted above, is a third generation model; it accounts for wave dispersion within
discrete spectral bins by adding diffusion terms to the propagation equation (Booij and Holthuijsen
1987); it uses the Chalikov and Belevich (1993) formulation for wave generation and the Tolman and
Chalikov (1996) formulation for wave dissipation; it employs a third order finite difference
method by utilizing a split-mode scheme with a Total Variance Diminishing limiter to solve wave
propagation; its computer code has been optimized to fully utilize the Massively Parallel Processing
(MPP) structure of the IBM computer; it uses a spacial resolution of 1.25^{o} x 1.00^{o} lon./lat.
grid, a domain north-south from 78^{o}N to 78^{o}S, and a directional resolution of 24 directions.
This TPB briefly describes the NWW3 and the wave guidance products which are being disseminated.
This guidance consists of significant wave height (H_{s}), which combines sea and swell; mean wave
direction (D_{m}); mean wave period (T_{m}); and directional wave spectra at selected grid points.
Guidance is available in graphic, alphanumeric, and GRIB formats. Note that other wave and wind
parameters are also available in GRIB format, i.e., peak wave period and direction, wind sea peak
wave period and direction, wind speed and direction, and u and v wind components, and are posted at
http://polar.ncep.noaa.gov/waves on the web. The reader is referred to World Meteorological
Organization (WMO) Report No.702 (second edition; 1998) for wave definitions, measurements and
modeling.
The bulletins and graphics of the new guidance follow the same formats shown in TPB No. 453 (Chen et
al., 1999), except for changes to the spectral text bulletins now being sent to AWIPS and the
following model improvements :
- The model was originally coded in FORTRAN 77 to assure portability in the early 1990s. It has
been re-coded in FORTRAN 90 to utilize modular concepts and allocatable data structures. The
conversion greatly simplifies the maintenance of the NWW3 family of wave models at NCEP. To
simplify the code further, some minor changes of operations were adopted. No noticeable
changes have resulted in the guidance.
- The source term integration scheme has been changed to forward in time since the time scales
are comparable to the time step (Hargreaves and Annan 2001). This results in a smoother
spectra with little impact on guidance. The parameters of dynamic time stepping have been
reset to get slightly faster initial growth again with no noticeable changes in the guidance.
- A new cheaper propagation scheme has been included in the model to eliminate the 'Garden
Sprinkler' Effect (see figs. 1 , 2 , 3, 4, and 5). A new way to account for unresolved
islands and sea ice has also been included in the model (see figs. 6, 7, and 8). Dramatic
improvements in model guidance have occurred in the vicinity of island groups world wide (see
figs. 9, 10, 11, and 12).
- Re-tuning to eliminate model biases induced by changes above has also been done.
- Spectral text bulletins for the NWW3 are available at
http://polar.ncep.noaa.gov/waves .
These files are in ASCII and are available by anonymous ftp at
ftp://polar.ncep.noaa.gov/pub/waves/date.cycle,
where date represents the date in yyyymmdd format and cycle represents the run cycle
identifier (t00z, t06z, t12z or t18z, respectively). These bulletins have been implemented on AWIPS, but
with a condensed format necessitated by the capabilities of the communications gateway and
display capabilities of AWIPS. See fig. 13 for a sample bulletin and Table 1 for the list of
points having spectral wave bulletins, their locations, and their bulletin headers.
The ocean wave guidance is generated four times daily out to 168 hours based on the 0000, 0600, 1200 and 1800 UTC
cycles of the of the Global Forecast System (GFS).
2. NOAA WAVEWATCH III (NWW3) OCEAN WAVE FORECAST MODEL
Global ocean wave forecasts are operationally generated at the NCEP by using the NWW3 model. Fields
of directional frequency spectra in 24 directions and 25 frequencies are generated at hourly
intervals up to 168 hours. The 24 directions begin at 90 degrees to the east and have a directional
resolution of 15 degrees. The 25 frequencies used by the NWW3 are given by bin in Table 2.
Wave spectral data are computed on a 1.25 by 1.00 degree longitude/latitude grid for ocean points
between latitude 78.0 degrees North to 78.0 degrees South. Wind fields are the only driving force
used in the model. They are constructed from spectral coefficients of the lowest sigma layer winds
from the NCEP analysis and forecasts of the GFS (Kanamitsu
et al. 1991; Caplan et al. 1997) with no interpolation to the model grid required. The winds are
then adjusted to a height of 10 m by using a logarithmic profile corrected for stability with air-
sea temperature differences. Analyzed wind fields from the previous 12 hours at 3-h intervals are
used for a 12-h wave hindcast. Winds from the GAFS at 3-h intervals out to 168 hours are used to
produce wave forecasts out to 168-h which are produced four times daily from the 0000, 0600, 1200 and 1800 UTC cycles.
3. AVAILABLE PRODUCTS AND DISSEMINATION
The ocean surface waves are calculated for grid points covering the whole globe, excluding land, the
North and South pole areas, and inland water bodies, such as Great Lakes, Chesapeake Bay,
Mediterranean Sea,etc. The calculated waves are disseminated graphically via AWIPS and NAWIPS,
in alphanumeric format via AWIPS
for selected grid points, and in GRIB format via AWIPS.
a. Spectral text bulletins on the web
Spectral text bulletins are presented for numerous points of NWW3. These bulletins are in ASCII and
are available on the INTERNET at present. The line length of the table is 130 characters by 160
lines (see Fig. 14). The header of the table identifies the output location, the generating model
and the run date and cycle of the data presented. At the bottom of the table, a legend is printed.
The table consists of 8 columns. The first column gives the time of the model results with a day
and hour (the corresponding month and year can be deduced from the header information). The
second column presents the overall significant wave height (H_{s}, the number of individual wave fields
identified with a wave height greater that 0.05 m (n), and the number of such fields with a wave
height over 0.15 m that could not be tracked in the remainder of the table (x). Individual wave
fields in the spectrum are identified using a partitioning scheme similar to that of Gerling (1992).
In the remaining six columns individual wave fields are tracked with their height (H_{s}), peak wave
period (T_{p}) and mean wave direction (dir, direction in which waves travel relative to North).
Generally, each separate wave field is tracked in its own column. Such tracking, however, is not
guaranteed to work all the time. An asterisk in a column identifies that the wave field is at least
partially under the influence of the local wind, and, therefore, most likely part of the local wind
sea. All other individual wave fields are pure swell.
b. Spectral text bulletins for AWIPS
The format for the spectral text bulletins sent to AWIPS is generally the same as that for the web,
except that the period is to the nearest second, the wave heights are to the nearest foot, the
direction is from (meteorological, rather than oceanographic), the number of fields that couldn't
be tracked is not given, and the asterisk indicating when a wave field is, at least, partially
under the influence of the local wind is not shown. The bulletin width is 69 characters, which is a
legacy of the teletype era and the display capability of AWIPS. A sample bulletin is shown in fig.
13, and the list of points for the NWW3 is given in Table 1.
d. GRIB bulletins
GRIB bulletins are available for use in AWIPS. Table 3 gives the bulletin headers and their meaning.
Bulletins are available at 6-h intervals from 00- through 72-h and at 12-h intervals from 72- through
168-h. Available parameters are H_{s}, D_{m}, T_{m}, peak wave direction and period, wind sea peak wave direction
and period, and u and v components of the wind velocity. A 1.25 ^{o} x 1.00^{o} lon./lat. grid is used with
a domain from 0 ^{o} - 360^{o}E and 78^{o}N to 78^{o}S.
4. EVALUATION
Extensive evaluation of the NWW3 model has been carried out by comparing with buoy data and ERS2
altimeter data. These results are available at http://polar.ncep.noaa.gov/waves/ .
5. REFERENCES
Booij, N. and L. H. Holthuijsen, 1987: Propagation of ocean waves in discrete spectral wave models. J.
Comput. Phys., 68, 307-326.
Bretschneider, C. L., 1958: Revisions in wave forecasting: Deep and shallow water. Proc. 6th Int. Conf.
Coastal Eng., ASCE, 30-67.
Caplan, P., J. Derber, W. Gemmill, S.-Y. Hong, H.-L. Pan and D. Parish, 1997: Changes to the NCEP
operational medium-range forecast model analysis/forecast system. Wea. Forecasting, 12, 581-594.
Chalikov, D. V. and Belevich, M. Y., 1993: One-dimensional theory of the boundary layer. Boundary-Layer
Meteor., 63 , 65-96.
Chen, H. S., L. D. Burroughs and H. L. Tolman, 1999: Ocean surface waves. NWS Technical Procedures
Bulletin No. 453, NOAA, U.S. Department of Commerce, 17 pp. [OBSOLETE; available at http://polar.ncep.noaa.gov/mmab/tpbs/obsolete.tpbs/nww3tpb/nww3tpb.html]
Gerling, T. W., 1992: Partitioning sequences and arrays of directional wave spectra into component wave
systems. J. Atmos. Ocean. Techn., 9, 444-458.
Hargreaves, J. C. and J. D. Annan, 2001: Comments on Improvement of the short fetch behavior in the WAM
model. J. Atmos. and Oceanic Tech., 18, 711-715.
Kanamitsu, M., J. C. Alpert, K. A. Campana, P. M. Caplan, D. G. Deaven, M. Iredell, B. Katz, H.-L. Pan,
J. E. Sela and G. H. White, 1991: Recent changes implemented into the global forecast system at NMC.
Wea. Forecasting, 6, 425-435.
Neumann, G. and W. J. Pierson, 1966: Principles of physical oceanography. Prentice-Hall, Inc., New
York, 571 pp.
Sverdrup, H. U. and W. H. Munk, 1947: Wind, sea and swell: Theory of relations for forecasting.
Publication 601, Hydrographic Office, U.S. Navy, 50 pp.
The SWAMP Group, 1985: Ocean Wave Modeling. Plenum Press, New York, 256 pp.
Tolman, H. L. and Chalikov, D., 1996: Source terms in a third-generation wind-wave model. J. Phys.
Oceanogr., 26, 2497-2518.
Tolman, H. L., 2002: User manual and system documentation of WAVEWATCH-III version 2.22. Technical Note
No. 222, Marine Modeling and Analysis Branch, NCEP, National Weather Service, NOAA, Department of Commerce, 139 pp.
[Available at http://polar.ncep.noaa.gov/mmab/papers/tn222/MMAB_222.pdf ].
The WAMDI Group, 1988: The WAM model - A third generation ocean wave prediction model. J. Phys.
Oceanogr., 18, 1775-1810.
World Meteorological Organization, 1998: Guide to wave analysis and forecasting (second edition). WMO
No. 702, Secretariat of the World Meteorological Organization, Geneva, Switzerland, 224 pp.