Impact of swell waves on atmospheric surface turbulence: wave–turbulence decomposition methods

Abstract

To study turbulence properties, specifically vertical momentum fluxes under swell wave conditions, I investigate the impact of waves on the power spectrum and spectral coherence of turbulent wind across various spatial and temporal scales. I propose and apply a wave–turbulence decomposition method to split high-frequency surface wind data into distinct wind and wave components. Under the assumption of frozen turbulence, this method substitutes an empirically fitted spectrum for the observed or modelled wind spectrum within the wave-affected frequency range. I proceed to estimate time series of waves and turbulence through this decomposition technique. Using a few days of sonic anemometer wind measurements at 15 m height from 20 to 26 June 2015, the upward momentum transfer could be observed under high–steady (∼7 m s−1) and decaying wind conditions. During the high and decaying winds, the atmospheric stability changes between unstable and stable conditions, blurring the wave signals due to the thermally and mechanically generated turbulence. The vertical wind spectra from selected episodes within the study period, acting as benchmarks, offer detailed insights into how waves affect energy elevation within the wave frequency band under low-wind, old-sea, and stable boundary layer conditions. These spectra also facilitate an effective performance assessment of the proposed decomposition method. Additionally, using a theoretical model derived from sonic anemometer measurements at heights of 15 and 20 m above the mean sea level, I parameterize the wave-contaminated coherence function, allowing for the synthetic generation of turbulent fluctuation spectra within the wave frequency band.