Abstract
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 found that 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.
Highlights
In the last several decades, interest in sea-salt aerosol has increased, partly because it is essential for explaining a significant portion of the differences in clear-sky top-of-theatmosphere irradiance between observations and modeling results over the oceans (Haywood et al, 1997; Li et al, 2006)
We investigated the sensitivity of the simulated forcing to variations in effective radius, effective variance, surface albedo and solar zenith angle
It is found for both the mid-latitude summer (MLS) and sub-Arctic winter (SAW) the strong forward diffraction peak will contribute to the upthat the shortwave forcing is positive for zenith angles less scatter and reflection
Summary
In the last several decades, interest in sea-salt aerosol has increased, partly because it is essential for explaining a significant portion of the differences in clear-sky top-of-theatmosphere irradiance between observations and modeling results over the oceans (Haywood et al, 1997; Li et al, 2006). By the end of this century, IPCC scenarios are predicting an increase in sea-salt burden based on future global wind patterns and strengths. This might entail an estimated additional −0.8 W/m2 direct seasalt forcing and −1.16 W/m2 indirect forcing (Penner et al, 2001). A key aspect of the work has been to provide an accurate representation of the response of sea-salt optical properties and radiative forcings to changes in relative humidity. It is proposed here to directly consider the size distribution of the wet sea-salt aerosol particles for optical properties. In contrast to other studies based on global climate models, this approach allows a more rigorous evaluation of fundamental aspects of sea-salt radiative forcing
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