Abstract
The utility of a satellite-based whitecap database for estimates of surface sea spray production and bubble-mediated gas transfer on a global scale is presented. Existing formulations of sea spray production and bubble-mediated CO2 transfer velocity involve whitecap fraction parametrization as a function of wind speed at 10 m reference height W(U10) based on photographic measurements of whitecaps. Microwave radiometric measurements of whitecaps from satellites provide whitecap fraction data over the world oceans for all seasons. Parametrizations W(U10) based on such radiometric data are thus applicable for a wide range of conditions and can account for influences secondary to the primary forcing factor, the wind speed. Radiometric satellite-based W(U10) relationship was used as input to: (i) the Coupled Ocean-Atmosphere Response Experiment Gas transfer (COAREG) algorithm to obtain CO2 transfer velocity and total CO2 flux; and (ii) the sea spray source function (SSSF) recommended by Andreas in 2002 to obtain fluxes of sea spray number and mass. The outputs of COAREG and SSSF obtained with satellite-based W(U10) are compared with respective outputs obtained with the nominal W(U10) relationship based on photographic data. Good comparisons of the gas and sea spray fluxes with direct measurements and previous estimates imply that the satellite- based whitecap database can be useful to obtain surface fluxes of particles and gases in regions and conditions difficult to access and sample in situ. Satellite and in situ estimates of surface sea spray production and bubble-mediated gas transfer thus complement each other: accurate in situ observations can constrain radiometric whitecap fraction and mass flux estimates, while satellite observations can provide global coverage of whitecap fraction and mass flux estimates.
Highlights
Direct measurements of near-surface ocean processes under high winds remain problematic due to the difficulty of operating surface vessels and in situ instrumentation under gale conditions
Whitecap fraction W—defined as the fraction of the sea surface covered with sea foam—is a forcing variable suitable for parameterizing surface fluxes associated with breaking waves and whitecaps such as surface production of sea spray aerosol and bubble-mediated gas exchange across the air-sea interface
Sensitivity of microwave frequency to foam thickness allows association of WA with W(TB) at 10 GHz and W with W(TB) at 37 GHz [5, 6]. Because this is a crude separation of active and residual whitecaps, we have developed a scaling factor R = WA/W, which, when applied to satellite-based data W(TB), can provide data for WA on a global scale [8]
Summary
Direct measurements of near-surface ocean processes under high winds remain problematic due to the difficulty of operating surface vessels and in situ instrumentation under gale conditions. Such measurements are necessary to better constrain parameterizations of near-surface ocean processes and improve predictions of weather and climate models. Whitecaps are the surface expression of various airsea processes enhanced by wave breaking. Whitecap fraction W—defined as the fraction of the sea surface covered with sea foam—is a forcing variable suitable for parameterizing surface fluxes associated with breaking waves and whitecaps such as surface production of sea spray aerosol and bubble-mediated gas exchange across the air-sea interface.
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