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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.68 n.2 supl.TIIProc Concepción  2004

http://dx.doi.org/10.4067/S0717-65382004000300016 

Gayana 68(2): 402-404, 2004

TECHNIQUE ESTIMATING SEA SPRAY PRODUCTION RATE FOR GLOBAL CLIMATE MODELS FROM SATELLITE-DERIVED MEASURES OF OCEANIC WHITECAP COVERAGE
 
 

Edward C. Monahan

Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, CT 06340-6048, USA edward.monahan@uconn.edu


ABSTRACT

By combining laboratory-determined values for the number of sea spray droplets, per increment of droplet radius, that are produced during the decay of an oceanic whitecap of initial unit surface area, and values determined from both laboratory experiments and field observations for the e-folding time associated with the decay of individual oceanic whitecaps, with satellite-derived measures of the fraction of the sea surface covered at any instant by spilling wave crests (Stage A whitecaps), the number of sea spray droplets, per droplet radius increment, that are produced per unit time, per unit area of the ocean surface, can be estimated. This sea spray aerosol source function can be incorporated in any global climate model where the role of spray droplets in influencing maritime cloudiness, in affecting the over-ocean radiative balance, and in contributing to the sea-air moisture flux, need be explicitly parameterized.


INTRODUCTION

Knowledge of the rate of production of sea spray droplets on the surface of the world ocean is crucial to any comprehensive modeling of the earth's climate. These droplets, once injected into the marine atmospheric boundary layer, will adjust in response to the local relative humidity and ambient air temperature. In so doing, they will contribute an ocean-atmosphere latent heat flux and a moisture flux, and often a sensible heat flux (see, e.g., Andreas et al., 1995). Many of the saline droplets, or sea-salt particles, that are left after the initial adjustment of the sea spray droplets to the surrounding atmosphere, will be mixed upward in the marine atmosphere by turbulent or convective processes to where they can serve as marine cloud condensation nuclei (Woodcock et al., 1963; Cipriano et al., 1987). Thus sea spray droplets not only are important to the moisture and heat budgets of the marine troposphere, but they also are a factor in determining the extent and character of the cloud cover present over the world's ocean. And the extent and character of the cloud cover over the 70% of the world's surface covered by ocean influences the effective visible albedo of the earth.
Given the acknowledged importance of sea spray aerosols in determining the climate of planet Earth, a method has long been sought whereby the instantaneous rate of injection into the atmosphere of sea spray droplets could be assessed. Clearly it is very difficult to directly measure the rate of production of sea spray droplets at one location in a field setting, and impossible to directly and simultaneously measure the rate of droplet production at anything approaching a sufficient number of locations spread over the entire world ocean to adequately characterize the important global droplet-related heat, moisture, and condensation nucleus flux for modelling or predictive purposes. Here is a need that can be met by the application of remote sensing techniques, as the parents of most of the relevant sea spray droplets are the bubbles which make up the whitecaps that form when waves break at sea (Blanchard, 1963), and the extent to which the ocean surface is covered by whitecaps can be determined via the passive microwave remote sensing of the apparent brightness temperature of the sea surface (Ross et al., 1970; Nordberg, 1971; Stogryn, 1972, Webster, et al., 1976; Monahan and O'Muircheartaigh, 1986).

METHODS

The present author and his collaborators undertook to explicitly relate the rate of sea spray droplet production to oceanic whitecap coverage in a series of experiments begun in the late 1970s. Their basic marine aerosol generation formula, first published in Monahan et al. (1983) and subsequently refined, is given by Equation 1, where ¶F0/¶r80, term 4, represents the number of sea spray droplets, per unit droplet radius increment, produced per unit time, per unit area of sea surface. The subscript 80 on this term, and elsewhere, refers to droplet radii that have been adjusted to an ambient relative humidity of 80%. Likewise, ¶Elab/¶r80,labeled term 1, represents the number of sea spray droplets, per unit droplet radius increment, that are produced during the decay of a whitecap of initial unit surface area. Term 1 has been evaluated via a series of laboratory experiments, which were refined over the years (e.g., Monahan et al., 1986;Woolf et al., 1987; Woolf et al., 1988; Bowyer et al., 1990; Stramska et al., 1990). Term 2, Ñ, represents the e-folding time associated with the decay of individual oceanic whitecaps, and has been determined from both laboratory experiments and field observations. The remaining term (term 3), WA, stands for the fraction of the sea surface covered by spilling wave crests ,i.e., by Stage A whitecaps.

(1)

In the first papers in which this expression appeared, term 3 was simply given as W, the fraction of the sea surface covered by whitecaps. As this research team's understanding of the evolution with time of individual whitecaps developed (e.g., Monahan, 1989; Monahan, 2001), this expression has been refined. Equation 1 is explicitly couched in terms of WA, the fraction of the sea surface covered by Stage A whitecaps, but it can equally well be restated in terms of WB, the fraction of the sea surface covered by decaying foam patches, i.e., by Stage B whitecaps. In such a reformulation, the magnitude of ¶Elab/¶r80 is adjusted to reflect the characteristic ratio of WA/WB. The distinction between WA and WB discussed in, e.g., Monahan (1989), is consistent with the distinction between breaking wave crests and foam made by Bondur and Sharkov (1982), and by Bortkovskii (1987).

For additional details on how whitecaps influence the microwave emissivity of the sea surface, and 

F0/ r80 = Elab [S,T,etc.]/ r80 x WA/t

(4) (1) (3)/(2)
hence the apparent microwave brightness temperature of that surface, the reader is referred to Monahan (2002), and the papers cited therein.

Discussion

In the absence of readily available daily, or more frequent, global maps of sea surface Stage A whitecap coverage, the author, his colleagues, and other workers, have used variants of Equation 1, in which one or another relationship describing how whitecap coverage varies with 10-meter elevation wind speed, or with wind speed and atmospheric stability, has been substituted for the WA term in this equation. Such expressions are to be found, for example, in Monahan (1971), Monahan and O'Muircheartaigh (1980), Monahan and O'Muircheartaigh (1986), or Monahan and Lu (1990). The substitution of such WA(U), or WB(U), expressions, where U is the wind speed, into Equation 1 yields expressions such as Equation 2.

(2)

Here WA is shown as a simple power law, consistent with the vast majority of the WA and WB expressions, described, e.g., in Monahan and O'Muircheartaigh (1980). When an experimentally derived value for ô is likewise entered into such an expression, one obtains something akin to Equation 3 (see, e.g., Monahan et al., 1986).

(3)

The use of expressions such as Equation 3 has unfortunately caused a number of authors to focus their attention on the limitations presumed to result from the parameterization of whitecap coverage in terms of one, U, or two, e.g., U and DT, independent variables, and to further identify these limitations with any general application of Equation 1. It should be stressed that the preferred application of Equation 1 is one where WA is determined directly from remote sensing data, and not inferred from measurements of meteorological variables. WA is by far the independent variable of choice.

ACKNOWLEDGMENTS

The work summarized above was made possible by 

F0/ r80 = Elab[S,T,etc.]/ r80 x C1Ul/t
¶ F0/¶r80 = ¶ Elab[S,T,etc.]/¶r80 x 1.088 x 10-6U3.41
 

the long-term support of the research of the author and his collaborators by the U.S. Office of Naval Research.

REFERENCES

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