Post on 22-Sep-2020
Wind Fields for Wave Simulations In the combined wind field model, predicted wind speeds
by mesoscale model UM and typhoon model UM are
combined by using following equations:
where r is the distance from the center of a typhoon and
RB is the typhoon outside boundary.
Characteristics of predicted wave heights Predicted wave heights by using wind field obtained from
mesoscale model are lower than those by typhoon model
at the center of the typhoon. On the other hand, predicted
wave heights using typhoon model become lower as the
distance from the center of the typhoon is farther.
Predicted wave heights by using combined wind field
show close values of those by typhoon model near the
center and those by mesoscale model for the region
outside. It is noticed that the wave heights predicted by
using combined wind field is a little higher than those by
typhoon model, since the wave energy from wind in the
center and outside of the typhoon is accumulated.
Extreme Wave Prediction by Using Combined Wind Field Jun TANEMOTO1 and Takeshi ISHIHARA2
1 Graduate Student, Department of Civil Engineering, The University of Tokyo, Japan, E-mail: tanemoto@bridge.t.u-tokyo.ac.jp 2 Professor, Department of Civil Engineering, The University of Tokyo, Japan, E-mail, ishihara@bridge.t.u-tokyo.ac.jp
Introduction
Wave simulations are commonly used for the
predictions of extreme wave height and associated
wave period, which are necessary for the design of
offshore wind turbines. Wind field used as sea surface
boundary conditions are the most important for wave
simulations, in which mesoscale model [1] and typhoon
model [2] have been used. As Tanemoto and Ishihara
[3] mentioned, predicted wind speeds by mesoscale
model and typhoon model underestimate observations
in the center of the typhoon and the region outside,
respectively. A combined wind field model [3] has been
proposed and these underestimations are improved. In
this study, the proposed combined wind field model is
used for wave simulations and the result is compared
with observations.
Predicted wind and wave fields
Wave simulation
WAVE WATCHI III (WW3) is used for wave simulations in this study. Typhoons induced annual
maximum wave heights observed at NOWPHAS Nakagusuku bay from 1992 to 2011 are simulated.
Configurations and domains used in the simulations are shown as follows.
Domain 2 Nakagusuku bay
Domain 4
Domain1 Domain2 Domain3 Domain4
Spin-up More than 10 days from the time peak wave height observed
Horizontal
resolution 0.5° × 0.5° 0.2°×0.2° 0.05° 0.02°
Bathymetry ETOPO2 ETOPO1
Sea surface
boundary
Case 1: Mesoscale model
NNRP (2.5°) WRF (18km) WRF (6km) WRF (2km)
Case 2: Typhoon model
Case 3: Combined wind field
Lateral boundary Open Nest down (2-way nestion)
Spectrum
resolution
36 directions and 36 frequencies
(0.0345~0.97Hz)
Source terms DIA nonlinear interaction
Tolman and Chalikov input and dissipation
Cavaleri and Malanotte-Rizzoli linear input
JONSWAP bottom friction
Bettjes-Jassen surf breaking
Mesoscale model Typhoon model Combined wind field
Mesoscale model Typhoon model Combined wind field
Validation
Predicted Time Series of the Waves During T0423 Wave height HS and period TS are As predicted as follows:
where E(f, θ) is energy spectrum for frequency f and direction θ. Significant wave
height H1/3 and period T1/3 are observed at the NOWPHAS Nakagusuku bay and
following formula are used in this study for the comparison:
Predicted wave heights by using wind field obtained from mesoscale model
underestimate observed wave height. These underestimations are improved by
using typhoon model and combined wind field, but predicted wave heights by
typhoon model underestimate observations before the typhoon attacked the site.
Predicted wave heights by combined wind field show good agreement with
observations.
Predicted wave periods by mesoscale model are also underestimated. In
contrast, those by typhoon model are overestimated before the typhoon
attacked the site. The reason is underestimations of wind-waves before the
typhoon attacked. As a result, waves are generated by the only the swell by the
typhoon far from the site. These overestimations are also improved by using
combined wind field.
Predicted Extreme Waves Predicted maximum wave height in the 20 years by mesoscale model is
underestimated. The underestimation is improved by typhoon model and
combined wind field. The wave period associated with extreme wave height is
calculated by the relation between wave heights and periods. In this study, these
relations are approximated by . Predicted extreme wave period
associated with these wave heights by mesoscale model also underestimate
observation. The underestimation is improved by typhoon model and combined
wind field model gives more accurate result.
0
2
4
6
8
10
12
14
10/16 10/17 10/18 10/19 10/20 10/21
H1
/3 (
m)
ObservationMesoscale modelTyphoon modelCombined wind field
0
5
10
15
20
10/16 10/17 10/18 10/19 10/20 10/21
T1
/3 (
s)
Significant wave height
Significant wave period.
2 5 10 20 50 100
0
5
10
15
20
-1 0 1 2 3 4 5
An
nu
al m
axim
um
H1
/3 (m
)
Reduced variate -ln(-ln(F))
Fitting for observationMesoscale modelTyphoon modelCombined wind field
0
5
10
15
20
0 5 10 15
Ass
oci
ated
T1
/3 (s
)
Annual maximum H1/3 (m)
Fitting for obserbation
Mesoscale model
Typhoon model
Combined wind field
Acknowledgment
This research is carried out as a part of a project
funded by The New Energy and Industrial
Technology Development Organization (NEDO),
Japan. The authors wish to express their deepest
gratitude to the concerned parties for their
assistance during this study.
Conclusions
Wind fields used as sea surface
boundary condition for wave simulation
are validated. Predicted extreme wave
height and period by using wind field
obtained from mesoscale model
underestimate observations. These
underestimations are improved by
using typhoon model and combined
wind field. The predicted extreme
wave period by typhoon model slightly
underestimate the observation, but
predicted extreme wave height and
period by combined wind field show
good agreement with observations.
References
[1] V. R. Swail and A. T. Cox, “On the Use of
NCEP–NCAR Reanalysis Surface Marine wind
Fields for a Long-Term North Atlantic Wave
Hindcast”, J. Atmos. Oceanic Technol., 17,
2000,pp. 532-545.
[2] S. H. Ou, J. M. Liau, T. W. Hsu and S. Y.
Tzang:“Simulating typhoon waves by SWAN
wave model in coastal waters of Taiwan”, Ocean
Eng., 29, 2002, pp.947-971.
[3] J. Tanemoto and T. Ishihara, “Prediction of
tropical cyclone induced wind field by using
mesoscale model and JMA best track”,
Proceedings of 8th Asia Pacific conference on
Wind Engineering, 2013, pp.1362-1370.
The relation between annual maximum wave heights and associated wave period
Extreme distributions for wave heights
Domain 3
1C T MU WU W U
1 0ST m m
1/3 0.956 SH H
,n
nm f E f dfd 04SH m
1/3 ST T
0.52 2
2 2
B
B
R rW
R r
1/3 1/3
bT aH
Observation Mesoscale model Typhoon model Combined wind field
H1/3, 50 11.2m 7.3m (-34.8%) 12.0m (+7.1%) 12.4m (+10.7%)
T1/3, 50 13.0s 10.1s (-20.1%) 12.0s (-7.5%) 13.0s (+0.2%)
Extreme wave height and
associated wave period
Return periods