Parameterization of sea-salt optical properties and physics of the associated radiative forcing

J. Li (1) , X. Ma (1) , K. von Salzen (1) , S. Dobbie (2)

1 CCCma - Canadian Centre for Climate Modelling and Analysis
2 Institute for Atmospheric Science [Leeds]

The optical properties of sea-salt aerosol have been parameterized at solar and longwave wavelengths. The optical properties were parameterized in a simple functional form in terms of the ambient relative humidity based on Mie optical property calculations. The proposed parameterization is tested relative to Mie calculations and is found to be accurate to within a few percent. In the parameterization, the effects of the size distribution on the optical properties are accounted for in terms of effective radius of the sea-salt size distribution. This parameterization differs from previous works by being formulated directly with the wet sea-salt size distribution (and compared to AERONET results) and, to our knowledge, this is the first published sea-salt parameterization to provide a parameterization for both solar and longwave wavelengths. We have used this parameterization in a set of idealized 1-D radiative transfer calculations to investigate the sensitivity of various attributes of sea-salt forcing, such as dependence with sea-salt column loading, effective variance, solar angle, and surface albedo. From these sensitivity tests, it is found that sea-salt forcings for both solar and longwave spectra are linearly related to the sea-salt loading for realistic values of loadings. The radiative forcing results illustrate that the shortwave forcing is an order of magnitude greater than the longwave forcing results and opposite in sign, for various loadings. Studies of the sensitivity of forcing results to variations in effective variance show there to be minimal variation; therefore, only one value of effective variance is used in the parameterization. The dependence of sea-salt forcing with solar angle illustrates an interesting result that sea-salt can generate a positive top-of-the-atmosphere result (i.ewarming) when the solar zenith angle is relatively small 30° Finally, it is found that the surface albedo significantly affects the solar radiative forcing, with the forcing diminishing to zero as the surface albedo tends to unity. We anticipate this new sea-salt optical property parameterization will be useful for GCM models due to its simplicity, computational efficiency, and that its sensitivities have been explored and summarized in this work.

Domaines: Océan, Atmosphère

J. Li, X. Ma, K. von Salzen, S. Dobbie. Parameterization of sea-salt optical properties and physics of the associated radiative forcing. Atmospheric Chemistry and Physics Discussions, 2008, 8 (2), pp.5813-5845. ⟨hal-00303419⟩