Photographic measurements of bubble populations from oceanic whitecaps

This thesis describes a photographic technique for measuring the size spectra of bubbles from breaking wind waves. Bubble spectra have been measured at depths of 0.5 m to 2.0 m and windspeeds of 2 ms'1 to 14 ms"1 in various sea states and meteorological conditions. Sampled bubble populations were found to be highly variable in time. Large fluctuations from the mean were caused by clouds of bubbles from breaking waves near the camera. The bubble density was found to be a strong function of windspeed. Few bubbles were found in windspeeds below 6 ms'l. For windspeeds between 6 ms—1 and 14 ms—1 the air volume entrained by bubbles was found to be proportional to approximately the fifth power of windspeed. over the depth range of(L5 to Zilm. The state of development of the wave height spectrum also appeared to influence the bubble density in one case where the waves were well below their fully developed height due to a limited wind fetch. Comparisons with previous and less extensive data from other investigators revealed a similar behaviour of the bubble population. Bubble spectra measured by photographic techniques from the present work and previous investigations all show peaks in the bubble size spectrum from 50 um to 70 um radius. These are unlikely to be physical peaks but are caused by difficulties in resolving bubbles from 20 um to 50 um radius. Evidence from laboratory measurements and a numerical model, V suggest that the bubble spectrum has no peak and that the concentration of bubbles continues to increase with decreasing radius down to 30 um