Edward C. Monahan

A Model of Marine Aerosol Generation Via Whitecaps and Wave Disruption

We have, over the past several years, as one element in the development of a time-dependent model of the aerosol population of the marine atmospheric boundary layer, attempted to define, in terms of aerosol droplet radius (r) and 10m-elevation wind speed (U), a model of open-ocean sea-surface aerosol generation. This source function is represented by the expression dF(r, U)/dr, which states the rate of production of marine aerosol droplets, per unit area of the sea surface, per increment of droplet radius. In the initial modeling efforts only the indirect aerosol production mechanisms associated with the bursting of whitecap bubbles (see Figure 1) were considered. The model for sea surface aerosol generation by the indirect mechanisms, first introduced in our Canberra SSAG-1 (Monahan, et al, 1979) and Manchester SSAG-2 (Monahan, 1980) papers, is given by Equation 1, where W is the

Whitecaps and the passive remote sensing of the ocean surface

{\rm{d}}{{\rm{F}}_0}/{\rm{dr = W}}{\tau ^{ - 1}}{\rm{dE/dr}}

Acoustically relevant bubble assemblages and their dependence on meteorological parameters

instantaneous fraction of the sea surface covered by whitecaps, τ is the time constant characterizing the exponential whitecap decay (measured in seconds), and dE/dr is the differential whitecap aerosol productivity, i.e. the number of droplets per increment droplet radius produced during the decay of a unit area of whitecap (expressed in m−2 μm−1 ). The necessary expression for W(U) was obtained from shipboard photographic observations of white- caps (Monahan, 1971; Toba and Chaen, 1973), while values for τ and dE/dr were derived from measurements made using the University College, Galway, whitecap simulation tank.

THE ROLE OF OCEANIC WHITECAPS IN AIR-SEA GAS EXCHANGE

Abstract The optimal power-law expression for the dependence of oceanic whitecap coverage fraction W on 10 m elevation wind speed U as determined by ordinary least squares fitting applied to the combined whitecap data sets of Monahan (1971) and Toba and Chaen (1973), is W = 2.95 × 10−6 U3.52. The equivalent expression, obtained by the application of the technique of robust biweight fitting, is W = 3.84 × 10−6 U3.41. These expressions fit the combined data set better than any of the previously published equations.

The Ocean as a Source for Atmospheric Particles

Abstract Whitecap coverage (W), which influences the apparent microwave brightness temperature and short-wave albedo of the sea surface, is not only a strongly non-linear function of the l0m-elevation wind speed (U), but also varies with changes in the stability of the lower atmosphere (i.e. with alterations in the water-air temperature difference AT), and with changes in the surface-sea water temperature (Tw). Thus wind retrieval algorithms to be applied to open ocean data from whitecap-detecting satellite instruments should ideally be of the form, U(W, δT, Tw, d), where d is a measure of the effective wind duration. The wind speed associated with the onset of whitecapping, while also varying with AT and Tw, is typically 3 to 3-5ms-1, not the often quoted 7ms-1

Occurrence and Evolution of Acoustically Relevant Sub-Surface Bubble Plumes and their Associated, Remotely Monitorable, Surface Whitecaps

The part that sea spray plays in the air-sea transfer of heat and moisture has been a controversial question for the last two decades. With general circulation models (GCMs) suggesting that perturbations in the Earths surface heat budget of only a few W m−2 can initiate major climatic variations, it is crucial that we identify and quantify all the terms in that heat budget. Thus, here we review recent work on how sea spray contributes to the sea surface heat and moisture budgets. In the presence of spray, the near-surface atmosphere is characterized by a droplet evaporation layer (DEL) with a height that scales with the significant-wave amplitude. The majority of spray transfer processes occur within this layer. As a result, the DEL is cooler and more moist than the atmospheric surface layer would be under identical conditions but without the spray. Also, because the spray in the DEL provides elevated sources and sinks for heat and moisture, the vertical heat fluxes are no longer constant with height. We use Eulerian and Lagrangian models and a simple analytical model to study the processes important in spray droplet dispersion and evaporation within this DEL. These models all point to the conclusion that, in high winds (above about 15 m/s), sea spray begins to contribute significantly to the air-sea fluxes of heat and moisture. For example, we estimate that, in a 20-m/s wind, with an air temperature of 20°C, a sea surface temperature of 22°C, and a relative humidity of 80%, the latent and sensible heat fluxes resulting from the spray alone will have magnitudes of order 150 and 15 W/m2, respectively, in the DEL. Finally, we speculate on what fraction of these fluxes rise out of the DEL and, thus, become available to the entire marine boundary layer.

Fresh Water Whitecaps

A detailed physical model of the life history of a typical bubble plume, from its formation by a breaking wave to its dissipation into the background bubble population, is given, and the relationship between the early, acoustically relevant stages in bubble-plume development and the associated, remotely detectable whitecap is described. The manner in which the fraction of the sea surface covered by stage A spilling crests and by stage B mature whitecaps depends upon wind speed and upon wind stress or friction velocity is investigated. Formal expressions are given whereby near-surface bubble concentrations can be estimated from observations of fractional whitecap coverage or from measurements of the 10-m elevation wind speed. >

The Role of Whitecap Bubbles in Air–Sea Heat and Moisture Exchange

The currently available oceanic piston velocities, based on radon profiles, and the Galway climatological atlas of world ocean whitecap coverage, have been combined to demonstrate that a statistically significant correlation exists between piston velocity and whitecap coverage. This result is in accord with a simple model of gas transfer in which the sole effective path for sea-to-air gas transfer is via isolated turbulent whitecap vents. The effective piston velocity was found to be equal to 2.3 + 1.25 × 10−3U3 m/day, where U is the 1Om-elevation wind speed in ms−1.