Coupling atmospheric and ocean wave models for storm simulation

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. The thesis is submitted to the Danish Technical University in partial fulfillment of the requirements for the PhD degree. DTU PhD-0074(EN) April 2017 DOI 10.11581/DTU:00000020 Sponsorship: Danish Forskel (PSO-12020) “X-WiWa” project Pages: 129 References: 173 Figures: 67 Tables: 9 Wind Energy Department Technical University of Denmark P.O. Box 49 DK-4000 Roskilde Denmark Telephone +45 93511127 jitd@dtu.dk www.dtu.dk