Observed variation in the decay time of oceanic whitecap foam

Abstract

[1] Whitecap foam decay times for 552 individual breaking waves determined from digital images of the sea surface are reported. The images had sub-centimeter pixel resolution and were acquired at frame rates between 3 and 6 frames per second at the Martha's Vineyard Coastal Observatory over a 10-day period in 2008, subdivided into 4 observation periods. Whitecap foam decay times for individual events varied between 0.2 s to 10.4 s across the entire data set. A systematic positive correlation between whitecap foam decay time and maximum whitecap foam patch area was found for each observation period. For a given whitecap size within each observation period, the decay times varied between a factor of 2 and 5, with the largest variation occurring during unsteady environmental forcing conditions. Within observation periods, bin-averaged decay times varied by up to a factor of 4 across the range of foam patch areas. Between observation periods, the effective whitecap foam decay time, which we define as the area-weighted mean decay time, varied by a factor of 3.4 between 1.4 s and 4.8 s. We found a weak correlation between decay times and individual event-averaged breaking wave speeds. The variation in the active breaking area across all 4 observation periods was small, indicating relatively uniform surface whitecap area generating potential. We speculate that the variation in the foam decay times may be due to (i) the effect of surfactants on bubble and foam stability, and (ii) differences between bubble plume characteristics caused by a variation in breaking wave type.

A review of sea-spray aerosol source functions using a large global set of sea salt aerosol concentration measurements

“…The water temperature influences the water surface tension, density and viscosity, all of which may all affect SSA production both through bubble and wave breaking formation (e.g., Thorpe et al, 1992;Callaghan et al, 2012). For particles larger than about D p = 0.1 µm, Mårtens-son et al (2003) found a strong decrease in SSA production in cold water compared to that in warmer water, while for smaller particles the opposite was found.…”

“…Especially when water depth gets smaller than the distance between wave crests, waves grow steeper and are more inclined to break and generate white water (Massel, 2007). There are many more subtle properties within wave field-wind interaction, such as changes in wind speed and direction (Callaghan et al, 2012), and old waves (swell) out-distancing/out-lasting the wind. One way to account for some of the ocean surface properties is to use the surface stress rather than the model Table 2).…”

Effect of soluble surfactant on bubble persistence and bubble‐produced aerosol particles

“…Since we consider a range of source fluxes derived from measurements above different types of whitecaps (laboratory and ambient), we simply assume that w = 1 for each case. For the decay time constant t, we use the whitecap area-weighted mean decay time of 4.2 s found in recent foam decay observations [Callaghan et al, 2012], in which t ranged from 0.2 to 10.4 s for 552 individual whitecaps observed at a single coastal observation site.…”

Primary marine aerosol emissions from the Mediterranean Sea during pre-bloom and oligotrophic conditions: correlations to seawater chlorophyll <i>a</i> from a mesocosm study

“…Inversely, concentrations of dimethylsulfoniopropionate (DMSP), the precursor to DMS, increased by up to 50 %. In the remote ocean, DMS was predicted by modeling studies to be one of the main precursors for CCN in the marine boundary layer, and studies have shown that regional DMS emission changes could affect CCN sensitivity (Cameron-Smith et al, 2011;Woodhouse et al, 2013).…”

“…Aerosol emissions are also dependent on the chemical composition of the seawater due to the presence of a rich and varied mixture of organic material. These organics can affect the waters' ability to form whitecaps (Callaghan et al, 2012) and change bubble lifetime (Garrett, 1967). Large-scale marine aerosol source functions used in models have started to include seawater composition Spracklen et al, 2008;Vignati et al, 2010) by focusing on parameterizations of the correlation between surface water chlorophyll a (chla) concentrations and aerosol organic fractions Rinaldi et al, 2013).…”