The Effect of Water Temperature on Air Entrainment, Bubble Plumes, and Surface Foam in a Laboratory Breaking-Wave Analogue
Here we present results of a laboratory study which investigated the effect of variable water temperature on air entrainment, bubble plume evolution, and surface whitecap foam are evolution by using a breaking wave analogue in the laboratory over a range of water temperatures (Tw = 5 °C to Tw = 30 °C) and different source waters. For filtered seawater, air entrainment was estimated to increase by 6 % between Tw = 6 °C and Tw = 30 °C, driven by increases in the measured surface roughness of the plunging water sheet. After air entrainment, the rate of loss of air through bubble degassing was more rapid at colder water temperatures within the first 0.5 s of plume evolution. After the first 0.5 s, the trend reversed and bubbles degassed more quickly in warmer water. Due to this time and temperature dependent fractionation of the submerged bubble distribution, large differences in the distribution of sub-surface air volume between the two water temperature extremes were observed to emerge during the bubble plume degassing phase. The largest observed temperature-dependent differences in sub-surface bubble densities occurred at radii greater than about 700 μm. Temperature-dependent trends observed in the sub-surface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the sub-surface bubble plume. The surface foam area integrated over the duration of the observational period increased with increasing water temperature by about 10% between the two temperature extremes examined. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined.