Coupling atmospheric and ocean wave models for storm simulation

Jianting Du

Research output: Book/ReportPh.D. thesis

646 Downloads (Pure)

Abstract

This thesis studies the wind-wave interactions through the coupling between the
atmospheric model and ocean surface wave models. Special attention is put on
storm simulations in the North Sea for wind energy applications in the coastal zones. The two aspects, namely storm conditions and coastal areas, are challenging for the wind-wave coupling system because: in storm cases, the wave field is constantly modified by the fast varying wind field; in coastal zones, the wave field is strongly influenced by the bathymetry and currents. Both conditions have complex, unsteady sea state varying with time and space that challenge the current coupled modeling system.

The conventional approach of estimating the momentum exchange is through parameterizing the aerodynamic roughness length (z0) with wave parameters such as wave age, steepness, significant wave height, etc. However, it is found in storm and coastal conditions, z0 parameterization method often fails in reproducing z0 because the complexity of the sea state cannot be represented by a few selected wave parameters. Different from the parameterization method, physics-based methods take the idea that the loss of momentum and kinetic energy from the atmosphere must, by conservation, result in the generation of the surface waves and currents. The physics-based methods are sensitive to
the choice of wind-input source function (Sin), parameterization of high-frequency wave spectra tail, and numerical cut-off frequencies. Unfortunately, literature survey shows that in most wind-wave coupling systems, either the Sin in the wave model is different from the one used for the momentum flux estimation in the atmospheric model, or the methods are too sensitive to the parameterization of high-frequency spectra tail and numerical cut-off frequencies.

To confront the above mentioned challenges, a wave boundary layer model (WBLM) is implemented in the wave model SWAN as a new Sin. The WBLM Sin is based on the momentum and kinetic energy conservation. The wave-induced mean wind profile changes at all vertical levels within the wave boundary layer, and the spectral sheltering effect at each frequency within the wave spectrum are explicitly considered. The WBLM Sin is used for both the calculation of the wave growth and the estimation of the air-sea momentum flux. Moreover, the WBLM Sin extended the model ability in high-frequency ranges so that the issue of high-frequency spectra tail and numerical cut-off frequencies are automatically solved. The new WBLM method is proved to be able to improve both the wave simulation and stress estimation in idealized fetch-limited wind-wave evolution studies.

To apply the WBLM method in real cases, proper setup of the dissipation source function, numerical stability and model efficiency are needed to be considered. Therefore, a revised dissipation source function for the wave model and a refinement of the numerical algorithm of WBLM Sin is done. The new pair of wind-input and dissipation source functions are evaluated with point measurements through wave simulations during offshore and onshore storms in the west coast of Denmark. The WBLM method is proved to provide significant wave height and mean wave period that outperforms the other approaches in SWAN when compared with measurements.

The WBLM method is further applied in the wind-wave coupling system during a number of North Sea storms. In comparison, six other coupling method have also been used for one of the storms. Results of wind, wave, and stress have been validated with point measurements at a coastal, shallow water site. In particular, the spatial distribution of z0 from WBLM is found to have similar spatial patterns as the Advanced Synthetic Aperture Radar (ASAR) radar backscatter; both show features of the bathymetry. Analysis of the wind field from the non-coupled and WBLM coupled experiments show that the wind-wave coupling is important in strong wind conditions, varying wind conditions
(e.g. front system, open cellular convections during a storm), and coastal areas.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages136
DOIs
Publication statusPublished - 2017
SeriesDTU Wind Energy PhD

Cite this

Du, J. (2017). Coupling atmospheric and ocean wave models for storm simulation. Kgs. Lyngby: Technical University of Denmark. DTU Wind Energy PhD https://doi.org/10.11581/DTU:00000020
Du, Jianting. / Coupling atmospheric and ocean wave models for storm simulation. Kgs. Lyngby : Technical University of Denmark, 2017. 136 p. (DTU Wind Energy PhD).
@phdthesis{b5e7c6466c5a4f758e09ee04443db72b,
title = "Coupling atmospheric and ocean wave models for storm simulation",
abstract = "This thesis studies the wind-wave interactions through the coupling between theatmospheric model and ocean surface wave models. Special attention is put onstorm simulations in the North Sea for wind energy applications in the coastal zones. The two aspects, namely storm conditions and coastal areas, are challenging for the wind-wave coupling system because: in storm cases, the wave field is constantly modified by the fast varying wind field; in coastal zones, the wave field is strongly influenced by the bathymetry and currents. Both conditions have complex, unsteady sea state varying with time and space that challenge the current coupled modeling system.The conventional approach of estimating the momentum exchange is through parameterizing the aerodynamic roughness length (z0) with wave parameters such as wave age, steepness, significant wave height, etc. However, it is found in storm and coastal conditions, z0 parameterization method often fails in reproducing z0 because the complexity of the sea state cannot be represented by a few selected wave parameters. Different from the parameterization method, physics-based methods take the idea that the loss of momentum and kinetic energy from the atmosphere must, by conservation, result in the generation of the surface waves and currents. The physics-based methods are sensitive tothe choice of wind-input source function (Sin), parameterization of high-frequency wave spectra tail, and numerical cut-off frequencies. Unfortunately, literature survey shows that in most wind-wave coupling systems, either the Sin in the wave model is different from the one used for the momentum flux estimation in the atmospheric model, or the methods are too sensitive to the parameterization of high-frequency spectra tail and numerical cut-off frequencies.To confront the above mentioned challenges, a wave boundary layer model (WBLM) is implemented in the wave model SWAN as a new Sin. The WBLM Sin is based on the momentum and kinetic energy conservation. The wave-induced mean wind profile changes at all vertical levels within the wave boundary layer, and the spectral sheltering effect at each frequency within the wave spectrum are explicitly considered. The WBLM Sin is used for both the calculation of the wave growth and the estimation of the air-sea momentum flux. Moreover, the WBLM Sin extended the model ability in high-frequency ranges so that the issue of high-frequency spectra tail and numerical cut-off frequencies are automatically solved. The new WBLM method is proved to be able to improve both the wave simulation and stress estimation in idealized fetch-limited wind-wave evolution studies.To apply the WBLM method in real cases, proper setup of the dissipation source function, numerical stability and model efficiency are needed to be considered. Therefore, a revised dissipation source function for the wave model and a refinement of the numerical algorithm of WBLM Sin is done. The new pair of wind-input and dissipation source functions are evaluated with point measurements through wave simulations during offshore and onshore storms in the west coast of Denmark. The WBLM method is proved to provide significant wave height and mean wave period that outperforms the other approaches in SWAN when compared with measurements. The WBLM method is further applied in the wind-wave coupling system during a number of North Sea storms. In comparison, six other coupling method have also been used for one of the storms. Results of wind, wave, and stress have been validated with point measurements at a coastal, shallow water site. In particular, the spatial distribution of z0 from WBLM is found to have similar spatial patterns as the Advanced Synthetic Aperture Radar (ASAR) radar backscatter; both show features of the bathymetry. Analysis of the wind field from the non-coupled and WBLM coupled experiments show that the wind-wave coupling is important in strong wind conditions, varying wind conditions(e.g. front system, open cellular convections during a storm), and coastal areas.",
author = "Jianting Du",
year = "2017",
doi = "10.11581/DTU:00000020",
language = "English",
publisher = "Technical University of Denmark",

}

Du, J 2017, Coupling atmospheric and ocean wave models for storm simulation. DTU Wind Energy PhD, Technical University of Denmark, Kgs. Lyngby. https://doi.org/10.11581/DTU:00000020

Coupling atmospheric and ocean wave models for storm simulation. / Du, Jianting.

Kgs. Lyngby : Technical University of Denmark, 2017. 136 p. (DTU Wind Energy PhD).

Research output: Book/ReportPh.D. thesis

TY - BOOK

T1 - Coupling atmospheric and ocean wave models for storm simulation

AU - Du, Jianting

PY - 2017

Y1 - 2017

N2 - This thesis studies the wind-wave interactions through the coupling between theatmospheric model and ocean surface wave models. Special attention is put onstorm simulations in the North Sea for wind energy applications in the coastal zones. The two aspects, namely storm conditions and coastal areas, are challenging for the wind-wave coupling system because: in storm cases, the wave field is constantly modified by the fast varying wind field; in coastal zones, the wave field is strongly influenced by the bathymetry and currents. Both conditions have complex, unsteady sea state varying with time and space that challenge the current coupled modeling system.The conventional approach of estimating the momentum exchange is through parameterizing the aerodynamic roughness length (z0) with wave parameters such as wave age, steepness, significant wave height, etc. However, it is found in storm and coastal conditions, z0 parameterization method often fails in reproducing z0 because the complexity of the sea state cannot be represented by a few selected wave parameters. Different from the parameterization method, physics-based methods take the idea that the loss of momentum and kinetic energy from the atmosphere must, by conservation, result in the generation of the surface waves and currents. The physics-based methods are sensitive tothe choice of wind-input source function (Sin), parameterization of high-frequency wave spectra tail, and numerical cut-off frequencies. Unfortunately, literature survey shows that in most wind-wave coupling systems, either the Sin in the wave model is different from the one used for the momentum flux estimation in the atmospheric model, or the methods are too sensitive to the parameterization of high-frequency spectra tail and numerical cut-off frequencies.To confront the above mentioned challenges, a wave boundary layer model (WBLM) is implemented in the wave model SWAN as a new Sin. The WBLM Sin is based on the momentum and kinetic energy conservation. The wave-induced mean wind profile changes at all vertical levels within the wave boundary layer, and the spectral sheltering effect at each frequency within the wave spectrum are explicitly considered. The WBLM Sin is used for both the calculation of the wave growth and the estimation of the air-sea momentum flux. Moreover, the WBLM Sin extended the model ability in high-frequency ranges so that the issue of high-frequency spectra tail and numerical cut-off frequencies are automatically solved. The new WBLM method is proved to be able to improve both the wave simulation and stress estimation in idealized fetch-limited wind-wave evolution studies.To apply the WBLM method in real cases, proper setup of the dissipation source function, numerical stability and model efficiency are needed to be considered. Therefore, a revised dissipation source function for the wave model and a refinement of the numerical algorithm of WBLM Sin is done. The new pair of wind-input and dissipation source functions are evaluated with point measurements through wave simulations during offshore and onshore storms in the west coast of Denmark. The WBLM method is proved to provide significant wave height and mean wave period that outperforms the other approaches in SWAN when compared with measurements. The WBLM method is further applied in the wind-wave coupling system during a number of North Sea storms. In comparison, six other coupling method have also been used for one of the storms. Results of wind, wave, and stress have been validated with point measurements at a coastal, shallow water site. In particular, the spatial distribution of z0 from WBLM is found to have similar spatial patterns as the Advanced Synthetic Aperture Radar (ASAR) radar backscatter; both show features of the bathymetry. Analysis of the wind field from the non-coupled and WBLM coupled experiments show that the wind-wave coupling is important in strong wind conditions, varying wind conditions(e.g. front system, open cellular convections during a storm), and coastal areas.

AB - This thesis studies the wind-wave interactions through the coupling between theatmospheric model and ocean surface wave models. Special attention is put onstorm simulations in the North Sea for wind energy applications in the coastal zones. The two aspects, namely storm conditions and coastal areas, are challenging for the wind-wave coupling system because: in storm cases, the wave field is constantly modified by the fast varying wind field; in coastal zones, the wave field is strongly influenced by the bathymetry and currents. Both conditions have complex, unsteady sea state varying with time and space that challenge the current coupled modeling system.The conventional approach of estimating the momentum exchange is through parameterizing the aerodynamic roughness length (z0) with wave parameters such as wave age, steepness, significant wave height, etc. However, it is found in storm and coastal conditions, z0 parameterization method often fails in reproducing z0 because the complexity of the sea state cannot be represented by a few selected wave parameters. Different from the parameterization method, physics-based methods take the idea that the loss of momentum and kinetic energy from the atmosphere must, by conservation, result in the generation of the surface waves and currents. The physics-based methods are sensitive tothe choice of wind-input source function (Sin), parameterization of high-frequency wave spectra tail, and numerical cut-off frequencies. Unfortunately, literature survey shows that in most wind-wave coupling systems, either the Sin in the wave model is different from the one used for the momentum flux estimation in the atmospheric model, or the methods are too sensitive to the parameterization of high-frequency spectra tail and numerical cut-off frequencies.To confront the above mentioned challenges, a wave boundary layer model (WBLM) is implemented in the wave model SWAN as a new Sin. The WBLM Sin is based on the momentum and kinetic energy conservation. The wave-induced mean wind profile changes at all vertical levels within the wave boundary layer, and the spectral sheltering effect at each frequency within the wave spectrum are explicitly considered. The WBLM Sin is used for both the calculation of the wave growth and the estimation of the air-sea momentum flux. Moreover, the WBLM Sin extended the model ability in high-frequency ranges so that the issue of high-frequency spectra tail and numerical cut-off frequencies are automatically solved. The new WBLM method is proved to be able to improve both the wave simulation and stress estimation in idealized fetch-limited wind-wave evolution studies.To apply the WBLM method in real cases, proper setup of the dissipation source function, numerical stability and model efficiency are needed to be considered. Therefore, a revised dissipation source function for the wave model and a refinement of the numerical algorithm of WBLM Sin is done. The new pair of wind-input and dissipation source functions are evaluated with point measurements through wave simulations during offshore and onshore storms in the west coast of Denmark. The WBLM method is proved to provide significant wave height and mean wave period that outperforms the other approaches in SWAN when compared with measurements. The WBLM method is further applied in the wind-wave coupling system during a number of North Sea storms. In comparison, six other coupling method have also been used for one of the storms. Results of wind, wave, and stress have been validated with point measurements at a coastal, shallow water site. In particular, the spatial distribution of z0 from WBLM is found to have similar spatial patterns as the Advanced Synthetic Aperture Radar (ASAR) radar backscatter; both show features of the bathymetry. Analysis of the wind field from the non-coupled and WBLM coupled experiments show that the wind-wave coupling is important in strong wind conditions, varying wind conditions(e.g. front system, open cellular convections during a storm), and coastal areas.

U2 - 10.11581/DTU:00000020

DO - 10.11581/DTU:00000020

M3 - Ph.D. thesis

BT - Coupling atmospheric and ocean wave models for storm simulation

PB - Technical University of Denmark

CY - Kgs. Lyngby

ER -

Du J. Coupling atmospheric and ocean wave models for storm simulation. Kgs. Lyngby: Technical University of Denmark, 2017. 136 p. (DTU Wind Energy PhD). https://doi.org/10.11581/DTU:00000020