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