Microscale wave breaking and air‐water gas transfer

Abstract

Abstract. Laboratory results showing that the air-water gas transfer velocity k is correlated with mean square wave slope have been cited as evidence that a wave-related mechanism regulates k at low to moderate wind speeds [JShne et al., 1987;Bock et al., 1999]. Csanady [1990] has modeled the effect of microscale wave breaking on air-water gas transfer with the result that k is proportional to the fractional surface area covered by surface renewal generated during the breaking process. In this report we investigate the role of microscale wave breaking in gas transfer by determining the correlation between k and AB, the fractional area coverage of microscale breaking waves. Simultaneous, colocated infrared (IR) and wave slope imagery is used to verify that AB detected using IR techniques corresponds to the fraction of surface area covered by surface renewal in the wakes of microscale breaking waves. Using measurements of k and AB made at the University of Washington wind-wave tank at wind speeds from 4.6 to 10.7 m s -1, we show that k is linearly correlated with AB, regardless of the presence of surfactants. This result is consistent with Csanady's [1990] model and implies that microscale wave breaking is likely a fundamental physical mechanism contributing to gas transfer.

The FuGas 2.3 Framework for Atmosphere–Ocean Coupling: Comparing Algorithms for the Estimation of Solubilities and Gas Fluxes

“…This fits the general theory that under low-moderate wind speeds, the k w is set by surface renewal and micro-scale wave breaking (i.e., by k wind ), increasing with steeper younger waves [28,51,65], and only under higher wind speeds is k w set by bubbles from breaking waves (i.e., k bubble ) [41][42][43][44][45][46][47][48][49][50]. The preponderance of surface renewal and micro-scale wave breaking at the coastal ocean turn it more susceptible to additional factors driving k w .…”

“…The preponderance of surface renewal and micro-scale wave breaking at the coastal ocean turn it more susceptible to additional factors driving k w . As an example, under the low-moderate wind speeds often verified at the coastal ocean, the hydration reaction enhances k w [19] and surfactants attenuate surface renewal [28,58,59,65,66]. [20] to compilation by Sarmiento and Gruber [2].…”

“…In addition to their single influence, these factors also interact. For instance, while surfactants increase surface tension, although decreasing the water surface renewal and micro-scale wave breaking [28,65], there is no report of them affecting whitecap from breaking waves. Surfactants are expected to be more relevant on the coastal ocean due to their supply from land and their effect being more pronounced under low-moderate winds [60,66].…”

Influence of rain on air‐sea gas exchange: Lessons from a model ocean

“…An IR imager is ideally suited to measure the skin temperature because the optical depth of the infrared radiation detected, roughly O(10 À5 m) [McAlister, 1964;McAlister and McLeish, 1970], is much less than the thickness of the TBL. Recently developed IR imaging techniques have quantified signatures of thermal variability that result from renewal processes such as large-scale wave breaking [Jessup et al, 1997], microbreaking [Zappa, 1999;Zappa et al, 2001Zappa et al, , submitted manuscript, 2004, nearsurface shear, and free-convective patchiness [Zappa et al, 1998]. Similarly, rain will generate turbulence directly at the air-water interface and will contribute to, if not dominate, the disruption of the TBL.…”

“…This change in the wave spectra is linked to reduced gas exchange [Bock et al, 1999]. Microscale wave breaking has been suggested as a dominant mechanism for gas exchange at low to moderate wind speeds over the ocean [Zappa et al, 2001] (see also C. J. Zappa et al, Microbreaking and the enhancement of air-water gas transfer velocities, submitted to Journal of Geophysical Research, 2004) (hereinafter referred to as Zappa et al, submitted manuscript, 2004) and may explain the observed correlation between k and surface roughness.…”