T4: SurfaceWave-Driven Energy Fluxes at the Air-Sea Interface

Principal investigators: Dr. Marc Buckley (Helmholtz-Zentrum Hereon), Lars Czeschel (Universität Hamburg), Prof. Thomas Rung (Hamburg University of Technology)

The transfer of energy by the free-surface waves is important for the coupled atmosphere-ocean system. In this project, we propose three work packages to experimentally (WP1) as well as numerically (WP2) examine the unknown details of the energy transfer mechanisms in the vicinity of the ocean surface, and to prepare the supplementary modelling of this energy transfer in general circulation models (WP3). Experimental and numerical efforts will exploit the techniques developed during the first phase of this CRC (M6) to identify the physics controlling air-sea energy fluxes and to quantify the mechanical energy budget within the coupled atmospheric and oceanic boundary layers. Related results should feed into an improved model of phase-averaged momentum equations, to be used in future general circulation models. For the latter, a novel approach will be tested using thickness-weighted momentum equations in vertically Lagrangian coordinates.

The main products of T4 are to identify dominant energy input mechanisms from wind to waves using a wind-input reconstruction method and to quantify the dissipative roles of microscale and air-entraining breaking waves for the air-sea energy budget. Establishing a validated numerical wave tank to study momentum transfer across the free surface for different wind-wave scenarios in this phase of the CRC, we will characterize wind/wave conditions for their influences on surface stresses and energy transfer and provide an energetically consistent treatment of surface wave effects in general circulation ocean models.

Two-Phase Flow Simulations of Surface Waves in Forced Conditions

Due to mostly very high Reynolds numbers, it is hardly possible to perform Direct Numerical Simulations (DNS).

Malte Loft, PhD T4

Hello everyone, my name is Malte Loft and I work on the ”T4 Surface Wave-Driven Energy Fluxes at the Air-Sea Interface” subproject as a PhD student at the Hamburg University of Technology (TUHH).

I studied dual mechanical engineering at the Hamburg University of Applied Sciences and specialised in fluid mechanics at the University of Rostock as part of a Master’s degree. In September 2021, I started my PhD to investigate the energy fluxes at the air-sea interface using high-resolution CFD simulations (WP2).

Our goal is to resolve the small-scale processes that dominate the energy exchange as well as to identify the individual mechanisms as a function of the wind wave conditions, e.g. the wave age or wave slope of the current sea state. Due to mostly very high Reynolds numbers, it is hardly possible to perform Direct Numerical Simulations (DNS). Therefore, a hybrid turbulence model (Detatched Eddy Simulation, DES) is used for our simulations. First, a numerical wind-wave tank is developed to reproduce relatively simple laboratory conditions and to validate the numerical model with experimental results (WP1). In the animation shown, a non-linear surface wave can be seen propagating from left to right, involving strong wind forcing. Air separation events and highly turbulent structures are clearly visible. Due to our fully coupled model, we are able to extract the pressure fields and surface stresses at any point in space and can also include the influence of surface tension effects in our investigations. Furthermore, we produce large amounts of data during our simulations in order to determine phase-averaged quantities using triple decomposition. In other words, fields of pressure or velocity that correlate with the respective sea state, detached from turbulent fluctuations. With all this data, we hope to gain deep insights into the physical processes that determine the mechanical energy flow at the air-sea interface.

In the future, we will extend the application of our model to more complex scenarios, e.g. to highly non-linear sea states of the Baltic Sea, including further phenomena such as wave breaking. Another goal is to formulate the findings into improved parameterisations, in particular to improve the boundary conditions of current ocean models (WP3).

Here you can see a short video.

  • Holand, K., Kalisch, H., Buckley, M. et al. (2023). Identification of wave breaking from nearshore wave-by-wave records. Phys. Fluids 35(9), 092105, doi: https://doi.org/10.1063/5.0165053

  • Loft, M., Kühl, N., Buckley, M.P., Carpenter, J.R., Hinze, M., Veron, F. & Rung, T. (2023). Two-Phase Flow Simulations of Surface Waves in Wind-Forced Conditions. Phys. Fluids 35(7): 072108, doi: https://doi.org/10.1063/5.0156963

  • Carpenter, J.R., Buckley, M.P., & Veron, F. (2022). Evidence of the critical layer mechanism in growing wind waves. J. Fluid Mech. 94(26), doi: https://doi.org/10.1017/jfm.2022.714.

  • Merckelbach, L.M. and Carpenter, J.R. (2021): Ocean Glider Flight in the Presence of Surface Waves. J. Atmos. Ocean Tech., 38(7), 1265-1275, doi: 10.1175/JTECH-D-20-0206.1.

  • Bjørnestad, M., Buckley, M., Kalisch, H., Streßer, M., Horstmann, J., Frøysa, H.G., Ige, O.E., Cysewski, M. & Carrasco-Alvarez, R. (2021): Lagrangian measurements of orbital velocities in the surf zone. Geophys. Res. Lett. 48(21), e2021GL095722, doi: https://doi.org/10.1029/2021GL095722

  • Buckley, M., Veron, F. & Yousef, K. (2020). Surface viscous stress over wind-driven waves with intermittent airflow separation, J. Fluid Mech. 905(31), doi: https://doi.org/10.1017/jfm.2020.760