Joint Seminar: Exploiting Optical Remote Sensing of Foam Evolution to Estimate Rates of Energy Dissipation and Air Entrainment By Oceanic Whitecaps

Abstract

Ocean-atmosphere exchange processes are enhanced by air-entraining breaking waves known as whitecaps. In particular, exchange processes such as aerosol-production flux and bubble-mediated gas exchange are directly related to the magnitude of air entrainment by whitecaps. In addition, the scale and frequency of occurrence of whitecaps is a direct manifestation of wave field energy dissipation rates. Quantifying the link between the occurrence of air entrainment rates within whitecaps and wave field energy dissipation can offer new ways in which bubble-mediated exchange processes are parameterised and represented in coupled ocean-atmosphere models. However, direct measurements of the rates of energy dissipation and air entrainment by oceanic breaking waves are incredibly difficult to make due to the requirements of making time and space-resolved measurements within the upper tens of centimetres of the air-sea interface inside high air fraction turbulent flows. Alternative methods to estimate and link these quantities are therefore needed.

In this seminar I will discuss a series of papers in which colleagues and I have developed and applied a novel framework to estimate energy dissipation and air entrainment by individual whitecaps through a new interpretation of remotely-sensed whitecap foam evolution. I will introduce key results from controlled laboratory experiments that quantify the relationship between whitecap foam evolution and breaking wave energy dissipation and show important similarities in foam evolution between laboratory and oceanic breaking waves. I will then use a new energy dissipation-based model for oceanic whitecap coverage to demonstrate how the ECMWF spectral wave model, ecWAM, can be used to reproduce field observations of whitecap coverage. Estimates of the rate of air entrainment per unit sea surface using whitecap coverage measurements will also be presented and compared to other models, and the first estimates of the breaking strength parameter for individual oceanic whitecaps will be shown.  To conclude, I will discuss ongoing work with colleagues aimed at (i) generating large datasets of the statistics of individual breaking waves and (ii) developing energy dissipation-based parameterisations of air-sea gas exchange and aerosol production flux.

Short Bio:

Adrian Callaghan’s interests revolve around breaking surface gravity waves, air entrainment and bubble-mediated air-sea fluxes. He graduated with a PhD in Physical Oceanography from the University of Galway, Ireland, in 2009. Thereafter he spent time at the Scripps Institution of Oceanography in California, before moving to the Department of Civil and Environmental Engineering in Imperial College London as a Royal Society eric Shooter Fellow. He is now an Associate Professor at Imperial where he teaches undergraduate and graduate classes on fluid mechanics, wave mechanics and air-sea interaction.