Parameterization of sea-salt optical properties and physics of the associated radiative forcing J. Li1, X. Ma1, K. von Salzen1, and S. Dobbie2 1Canadian Centre For Climate Modelling and Analysis, Science and Technology Branch, Environment Canada, University of Victoria, Victoria, British Columbia, Canada 2Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
Abstract. The optical properties of sea-salt aerosol have been parameterized at shortwave
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, to our knowledge, this is the first
published sea-salt parameterization to provide a parameterization for both
shortwave 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, including the dependency on sea-salt column loading, effective
variance, solar angle, and surface albedo. From these sensitivity tests, it is
sea-salt forcings for both shortwave 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. Forcing sensitivity studies
show that the influence of effective variance for sea-salt is minor;
therefore, only one value of effective variance is used in the parameterization.
The dependence of sea-salt forcing with solar zenith
angle illustrates an interesting result that sea-salt can generate a positive
top-of-the-atmosphere result (i.e. warming) when the solar zenith angle is
relatively small (i.e. <30°). Finally, it is found that the surface
albedo significantly affects the shortwave radiative forcing, with the forcing
diminishing to zero as the surface albedo tends to unity.
Citation: Li, J., Ma, X., von Salzen, K., and Dobbie, S.: Parameterization of sea-salt optical properties and physics of the associated radiative forcing, Atmos. Chem. Phys., 8, 4787-4798, doi:10.5194/acp-8-4787-2008, 2008.