The group line in wavenumber-frequency spectra of water waves

Abstract

For a long time, the radar community has been wondering what the causes for this feature called the group line are. The group line is a feature that appears in the frequency area close to zero in the \((k,\omega)\) spectrum. In this thesis, we aim to do hydrodynamic experiments of long-crested, breaking and non-breaking waves and non-linear simulations of long-crested, non-breaking waves in order to find out more about this feature and its properties. We will primarily be using the JONSWAP spectrum and the Pierson-Moskowitz spectrum to make the waves in the experiments and the simulations. Including the non-breaking runs done, the results show the group line in all of our results. This indicates that wave breaking alone is not the cause of the group line. In addition, we recognise that there are two group lines in some cases and that they are seemingly dependent on the steepness of the waves. This double group line appears when the waves seem to almost be breaking which makes us inclined to believe that wave breaking might be the factor that changes the slope of the group line.