Operational Wave Forecasting System for the Portuguese - e-Geo

Journal of Coastal Research
SI 56
1055 - 1059
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Operational Wave Forecasting System for the Portuguese Coast
F. Sansana Silva, J. P. Pinto and S. Almeida
Divisão de Oceanografia
Instituto Hidrográfico
Rua das Trinas, nº49, 1249 – 093 Lisboa , Portugal
[email protected]
ABSTRACT
SANSANA SILVA, F., PINTO, J.P. and ALMEIDA, S., 2009. Operational wave forecast system for the Portuguese
coast. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 1055 –
1059. Lisbon, Portugal, ISSN 0749-0258.
In order to predict the wave conditions for the Portuguese Coast, a wave forecast system has been implemented
at the Instituto Hidrográfico. This system is based on two third-generation, phase-average models,
WAVEWATCH III (WW3) and Simulating Waves Nearshore (SWAN). The WW3 simulations are divided into
two separate areas, the North Atlantic and the South Atlantic. The results obtained from these simulations are
used as boundary conditions for the SWAN simulations. This nearshore prediction system is based on the nesting
methodology, improving the spatial resolutions shoreward. The nearshore simulations are divided into eight
different areas, which cover the Iberian Portuguese Coast and the Madeira Archipelago. The results are presented
daily in the Institute’s web page and allow, beyond other possibilities, the spectral analysis of the incident wave
field for several points along the shore. A monthly validation is performed by comparison to nearshore wave
buoys measurements.
ADITIONAL INDEX WORDS: Spectral Wave Models, Wave Forecast, SWAN, WAVEWATCH III
evaluate the wave conditions in other areas with a larger spatial
coverage, and predict the wave conditions in future scenarios,
impelled the development of these numerical wave models.
Numerous wave models with distinct formulations and
applications have been created recently. The third generation
wave models, such as WW3 (TOLMAN, 1999, 2002) and
SWAN (BOOIJ et al., 1999), are the state of the art models for
basin and coastal scale applications.
The variability of the wave conditions in the Portuguese
Coast is due to atmospheric processes in the mid-Atlantic
region. The formation of wind waves in these areas which
propagate to coastal regions as swell, generate most of the
waves observed nearshore. In order to properly forecast waves
nearshore, it is necessary to accurately predict
INTRODUCTION
The forecast and assessment of the wave conditions in
coastal areas is of extreme importance in our present days.
With the intensification of economical activities in these
regions, it is vital to ensure the safety of the economical
operators and the practicability of their activities. The wave
forecast and hindcast allows the evaluation of coastal
engineering projects and may prevent accidents or
environmental disasters( RUSU et al., 2008a).
The Portuguese Coast is comprised of many different coastal
systems with distinct environmental conditions. The wave data
obtained from the waverider directional buoys can be used to
characterize a specific area, however it is not possible to extend
these observations to the entire territory. The necessity to
Table 1: Computational grids for the simulations
Grid
Grid Description
Northern Limit
Southern Limit
Western Limit
Eastern Limit
x  y (º)
WWG1
WWG2
SWG1
SWG2
SWG3
SWG4
SWGNZ
SWGCS
SWGM1
SWGM2
North Atlantic
South Atlantic
Portuguese Coast
Portuguese Coast - Northwest
Portuguese Coast - Southwest
Portuguese Coast - South
Nazaré Area
Cascais Area
Madeira’s Archipelago
Madeira’s Island
70º N
25º N
44,5º N
42º N
39,5º N
37,5º N
39,66º N
39º N
33,4º N
33º N
5º N
70º S
35,5º N
39º N
36,5º N
36º N
39,56º N
38,5º N
32,1º N
32,46º N
85º W
60º W
12º W
10,5º W
10,5º W
9,5º W
9,16º W
10º W
17,6º W
17,46º W
40º E
90º E
7º W
8º W
8º W
6,5º W
9,04º W
9º W
16º W
16,51º W
0,5 º  0,5 º
1 º 1 º
0,067 º  0,067 º
0,01 º  0,01 º
0,01 º  0,01 º
0,01 º  0,01 º
0,001 º  0,001 º
0,005 º  0,005 º
0,01 º  0,01 º
0,005 º  0,005 º
Journal of Coastal Research, Special Issue 56, 2009
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Wave Forecast System
model do not address conditions where the waves are strongly
depth-limited. This constraint implies that the model is
generally applied on spatial scales between 20 to 100 km, and
outside the surf zone (TOLMAN, 1999).
The application of WW3 to coastal areas with horizontal
scales less than 20 km and with water depths less than 20 m is
not realistic, as the shallow water effects on waves (depthinduced breaking, triad wave-wave interactions) were not
included in the formulations. Moreover, the numerical
techniques used are extremely time-consuming when applied to
coastal areas with a complex environment (barrier islands,
estuaries, tidal flats, etc) (BOOIJ et al., 1999, RIS et al., 1999).
This is the starting point to the SWAN wave model. Using the
same base formulations, but with a more applicable numerical
scheme when applied to coastal areas (implicit upwind
propagation scheme) and an extension of the base formulation
to coastal areas, it is possible to predict the wave conditions in
lakes, estuaries and coastal areas.
SWAN is able to compute several wave generation and
dissipation processes such as wind generation, whitecapping,
triad and quadruplet wave-wave interactions, bottom friction
and depth-induced breaking.
the wave conditions in open waters, to obtain suitable boundary
conditions for the nearshore simulations. The nesting
methodology is used in this forecast system to increase the
spatial resolution from open ocean to coastal regions.
Figure 1. Snapshot of significant wave height (Hs) for the
Atlantic Ocean, illustrating the area covered by the WW3
forecast (North and South Atlantic).
MODELLING SYSTEM
The wave forecasting system implemented at Instituto
Hidrográfico was initiated in 2002, within the framework of the
national funded project MOCASSIM (PARLE/POCTI-QCA
III). Currently, the system provides detailed wave forecast to
the Atlantic Ocean area and the Portuguese Coast.
The Atlantic Ocean forecast is performed with the WW3,
with a temporal extension of six days. In Figure 1, a snapshot
of the wave height for the Atlantic Ocean is presented, showing
the spatial coverage of the Atlantic forecast.
The coastal forecasts for the different areas in the Iberian
Portuguese Coast and in Madeira’s Archipelago are performed
with SWAN, with a temporal extension of three days.
This work describes the current configuration of the
operational wave forecasting system for the Portuguese Coast
implemented at Instituto Hidrográfico, and summarizes the
results acquired since December 2007.
The Portuguese Coast forecast system, ranges from the basin
scale to coastal scale. It is not possible to develop a coastal
forecast system without first understanding and identifying the
areas of wave generation within the study regions. For this
purpose, the implemented system is based in a three-level
nesting scheme. The main features of the computational grids
are described in Table 1.
THE WAVE MODELS
The application of a coupled system with SWAN and WW3,
or with other third generation spectral models, is a common
procedure employed in forecasting systems (DYKES et al.,2002,
ROGERS et al., 2007, RUSU et al., 2008a, RUSU et al., 2008b), in
the sense that both models are based on the same physical
principles.
The WW3 model was developed at NOAA/NCEP, following
the work developed by KOMEN (1994) in the WAM model
(GARCIA et al., 2005). However, WW3 has several key
differences from its predecessor such as the governing
equations, the model structure, the numerical methods and the
physical parametrizations (TOLMAN,1999).
WW3 is a phase-averaging model that solves the spectral
action density balance equation for wavenumber-direction
spectra. The governing equations include refraction, due to
temporal and spatial variations of the mean water depth and
current. The source terms include non linear interactions,
dissipation due to whitecapping, bottom friction, wind wave
growth and decay (TOLMAN, 1999).
An important constraint to the formulation of WW3 is that
the parameterizations of physical processes included in the
Figure 2. North and South Atlantic bathymetric chart and wind
forcing, used for the WW3 forecasts.
Journal of Coastal Research, Special Issue 56, 2009
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Sansana Silva et al.
Figure 3. SWAN’s outer nest boundary forcing transmission from
WW3 and respective wind forcing.
The first area covered is the Atlantic basin, with a
meteorological forcing of the U. S. Navy's Operational Global
Atmospheric Prediction System Model (NOGAPS 4.0), with a
spatial resolution of 1º and temporal extension of 144 hours
(Figure 2). The WW3 applied for the North Atlantic region,
provides the boundary conditions required to perform the outer
nest forecasts with the SWAN wave model (Figure 3).
Wind forcing for the SWAN model is provided by the
Portuguese Instituto de Metereologia, based on the
meteorological model ALADIN, with a 48 hours temporal
extension and with a spatial resolution of 12 km, covering the
Iberian Peninsula. In order to achieve a 72 hours wind forcing,
the data available from the NOGAPS model is used to
complete the last 24 hours of the simulation.
The simulations of the Madeira’s Archipelago have a wind
forcing provided by the University of Madeira (UMa), using
the meteorological model MM5, with a spatial resolution up to
2 km.
The forcing conditions for the high resolution areas are
provided by the outer nest run, following the nesting
methodology (Figure 4).
Table 2 summarizes the wind and boundary forcing used in
the forecast system.
Figure 4. Bathymetric grids and boundary conditions for the
high resolution areas.
FORECAST OUTPUTS
The complete series of outputs of the models are updated
daily and are presented in the Institute’s Operational Forecast
webpage (www.hidrografico.pt). These results are divided into
three main sections, covering the areas presented in the section
MODELLING SYSTEM:
- Graphical display of wave height, direction and period of
all the areas presented in the MODELLING SYSTEM section
(Figure 5(a)).
- Graphical representation of a 2D (frequency-direction)
spectrum for a predefined number of points, including the
locations of the waverider directional buoys of the Instituto’s
coastal network. These figures are accompanied by a table in
which the different wave systems are analyzed individually
(Figure 5(b)).
- A monthly graphical comparison between the wave
parameters predicted by SWAN and the data obtained from the
waverider buoys (Figure 5(c)).
In order to validate the system, the data collected from the
waverider directional buoys has been compared with the results
of the model. These results present a preliminary analysis of
the behavior of the system and allow a first educated guess of
the limitations and modifications needed in the system.
Figure 7 shows the time evolution of the significant wave
height, compared with the observations of the coastal buoys
network, located in Leixões (8,98º W 41,32º N), Sines (8,93º
W 37,92º N) and Faro (7,90º W 36,91º N).
Table 2: Details of the boundary conditions and wind forcing
for the operational wave forecasting system
Grid
Meteorological Forcing
Boundary Conditions
WWG1
NOGAPS
-
WWG2
NOGAPS
-
SWG1
ALADIN + NOGAPS
WWG1
SWG2
ALADIN + NOGAPS
SWG1
SWG3
ALADIN + NOGAPS
SWG1
SWG4
ALADIN + NOGAPS
SWG1
SWGNZ
No wind forcing
SWG2
SWGCS
ALADIN + NOGAPS
SWG3
SWGM1
MM5 – Madeira
WWG1
SWGM2
MM5 – Madeira
WWG1
Journal of Coastal Research, Special Issue 56, 2009
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Wave Forecast System
During all the periods analyzed, the results show a tendency
of the wave forecasting system to underestimate the significant
wave height as indicated by the scatter plots of significant
height for the Leixões (Figure 6 (a)), Sines (Figure 6 (b)) and
Faro (Figure 6 (c)) Stations.
The statistical results presented reveal that the accuracy of
the system is similar to the results presented in other works
(RIS et al., 1999, RUSU et al., 2008a) with similar conditions,
however, the system presents distinct behavior in different
areas. The Leixões Station presents the larger RMSE and larger
discrepancies in the results, since this area is more exposed to
severe sea conditions, with frequent storm events, situations
which are more difficult to forecast with accuracy.
In Table 3, the mean error statistics for the significant wave
height (Hs) is presented in order to evaluate the forecast results
in the year 2008.
Table 3: Mean error statistics for the significant height (Hs)
Station
RMS (m)
Bias (m)
Leixões
0.58
–0.38
Sines
0.40
–0.24
Faro
0.25
–0.02
(a)
(b)
(a)
(b)
(c)
(c)
Figure 5. Model outputs examples: Snapshot of significant
height for the North Atlantic (a); Spectral output for the
location of the waverider buoy (b); Monthly comparison
between the model results and the data obtained from the
waverider buoy (c).
Figure 6. Scatter plots of significant height of the Leixões (a),
Sines (b) and Faro (c) buoys against model data.
Journal of Coastal Research, Special Issue 56, 2009
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Sansana Silva et al.
Figure 7. Observed and computed significant wave height (Hs) during the year of 2008.
ROGERS , W.E.; KAIHATU, J.M.; HSU , L.; JENSEN , R.E.; DYKES ,
J.D., and HOLLAND, K.T., 2007. Forecasting and
CONCLUSIONS AND FUTURE WORK
hindcasting
waves with the SWAN model in the Southern
A wave forecast system for the Portuguese Coast has been
California Bight, Coastal Engineering, 54, 1–15.
presented in this work. The main goal of the forecast system is
RUSU, E.; PILAR, P., and SOARES, C.G., 2008a, Evaluation of the
to provide a coastal wave forecast, covering from the basin
wave conditions in Madeira Archipelago with spectral
level to the regional scale in such a manner that only the wind
models. Coastal Engineering. 35, 1357–1371
forcing information is needed to execute the forecast.
USU, L.; PILAR, P., and SOARES, C.G., 2008b, Hindcast of the
R
Statistical analysis performed show a good agreement
wave conditions along the west Iberian coast. Coastal
between the model results and the coastal observation network
Engineering. 55, 906–919.
buoys.
KOMEN, G.J.; CAVALERI, L.; DONELAN, M.; HASSELMANN, K.;
Currently, a new system architecture is under preparation,
HASSELMANN, S., and JANSSEN, P.A.E.M., (eds.), 1994,
with a large increase in the computation capacity. This new
Dynamics
and Modelling of Ocean Waves. Cambridge:
system will allow us to increase the number of high resolution
Cambridge University Press, 532p.
areas, apart from increase the models resolutions and timeTOLMAN, H.L., 1999. User manual and system documentation
scales.
of WAVEWATCH-III version 1.18. NOAA/NWS/NCEP/
A more extensive validation and calibration study for coastal
OMB Technical Note 166, 110p.
areas is under preparation, with the objective of improving the
T
OLMAN, H.L., 2002. User manual and system documentation
current forecast system. A correct calibration of the models and
of WAVEWATCH-III version 2.22. NOAA/NWS/NCEP/
an increase in spatial and temporal resolution, complemented
OMB Technical Note 222, 133p.
with a better wind forcing, can improve significantly the
system results. The intent of extending the system, with the
increase of the number of high resolution areas, can improve
the forecast to specific areas, with economical or social
interest.
ACKNOWLEDGEMENTS
Instituto de Metereologia, University of Madeira and the U.
S. Navy Global Ocean Data Assimilation Experiment are
gratefully acknowledged for providing the wind data.
LITERATURE CITED
BOOIJ, N.; RIS, R.C., and HOLTHUIJSEN, L.H., 1999. A thirdgeneration wave model for coastal regions, 1. Model
description and validation. Journal of Geophysical
Research, 104, 7649–7666.
DYKES, J.D.; HSU, Y.L., and ROGERS, W.E., 2002, The
Development of an Operational SWAN Model for NGLI,
Oceans '02 MTS/IEEE, 2, 859–866.
GARCIA, P.C.; BALSEIRO, C.F.; PENABAD, E.; GÓMEZ, B., and
PÉREZ-MUÑUZURI, V., 2005. One year validation of wave
forecasting at Galician coast. Journal of Atmospheric and
Ocean Science, 10(4), 407–419.
RIS, R.C.; HOLTHUIJSEN, L.H., and BOOIJ, N., 1999. A thirdgeneration wave model for coastal regions, 2. Verification.
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