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Peter G. Duynkerke and Phillip Hignett

Abstract

A model simulation is presented of the diurnal cycle of a marine stratocumulus-capped boundary layer. The model results are compared with observations obtained during the 1987 First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment marine stratocumulus project, made from San Nicolas Island, off the coast of California. Both the simulation and the observations show a marked diurnal variation in cloud properties, as a result of the decoupling of the boundary layer into a separate cloud and a subcloud layer. In the model simulation the decoupling is caused by the absorption of the solar radiation in the cloud layer.

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Robert W. Bergstrom, Philip B. Russell, and Phillip Hignett

Abstract

Measurements are presented of the wavelength dependence of the aerosol absorption coefficient taken during the Tropical Aerosol Radiative Forcing Observational Experiment (TARFOX) over the northern Atlantic. The data show an approximate λ −1 variation between 0.40 and 1.0 μm. The theoretical basis of the wavelength variation of the absorption of solar radiation by elemental carbon [or black carbon (BC)] is explored. For a wavelength independent refractive index the small particle absorption limit simplifies to a λ −1 variation in relatively good agreement with the data. This result implies that the refractive indices of BC were relatively constant in this wavelength region, in agreement with much of the data on refractive indices of BC. However, the result does not indicate the magnitude of the refractive indices.

The implications of the wavelength dependence of BC absorption for the spectral behavior of the aerosol single scattering albedo are discussed. It is shown that the single scattering albedo for a mixture of BC and nonabsorbing material decreases with wavelength in the solar spectrum (i.e., the percentage amount of absorption increases). This decease in the single scattering albedo with wavelength for black carbon mixtures is different from the increase in single scattering albedo for most mineral aerosols (dusts). This indicates that, if generally true, the spectral variation of the single scattering albedo can be used to distinguish aerosol types. It also highlights the importance of measurements of the spectral variation of the aerosol absorption coefficient and single scattering albedo.

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