Faculty candidate seminar - Adrian Callaghan

05/22/2019 - 12:45pm
Eckart 227
Event Description: 



DATE:          May 22nd, Wednesday, 12:45 p.m.  

LOCATION:     Eckart 227
SPEAKER:      Adrian Callaghan
            Imperial College London
Imaging Whitecap Foam to Study Upper Ocean and Air-Sea Interaction Processes
Wind-driven air-entraining breaking wave whitecaps play a crucial role in air-sea interaction and upper ocean physics: they dissipate wave energy, support air-to-sea momentum transfer, enhance ocean-atmosphere gas exchange and generate sea spray aerosols. Whitecaps are spatially and temporally intermittent in nature and are characterised by short bursts of intense turbulent motion and high air fractions. Consequently, making accurate in-situ field measurements of whitecap properties during active wave breaking is extremely challenging. However, because of the broadband scattering of light by the foam and bubbles within whitecaps, their occurrence and scale can be efficiently measured through digital image-based remote sensing of the ocean surface.
In this talk I will present results from a laboratory experiment designed to link sub-surface bubble plume evolution, surface foam evolution and breaking wave energy dissipation so that surface whitecap foam signatures can be used to infer total energy dissipation for individual breaking waves. The experimental data show that the volume of the sub-surface two-phase flow integrated in time during active wave breaking is almost linearly proportional to the total energy dissipated by breaking. The results are compared to three previous laboratory studies and found to be in good agreement over almost 3 orders of magnitude of energy dissipation. Crucially, the breaking wave two-phase flow “volume-time-integral” can be constrained using 2-dimensional surface whitecap foam signatures because the whitecap foam decay time can be used to infer bubble plume penetration depth. These laboratory results are used to formulate a model for the area of whitecap foam per unit sea surface area, or whitecap coverage (W). The W model is forced with the European Centre for Medium Range Forecasting (ECMWF) spectral wave model and results compared to a set of W measurements from the north-east Atlantic Ocean. The comparison between modelled and measured W yields important insights into the relative contribution of whitecaps in balancing wind energy input to the upper ocean, and will be discussed in the context of several field datasets of upper ocean turbulence and W.
Faculty Host:  Falk Feddersen (ffeddersen@ucsd.edu)
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