The Apparent Bluing of Aerosols Near Clouds
Download a printable PDF
Marshak, A., NASA Goddard Space Flight Center
Radiation Measurements
Radiative Processes
Marshak, A., G. Wen, J.A. Coakley, L.A. Remer, N.G. Loeb, and R.F. Cahalan, 2008. A simple model of the cloud adjacency effect and the apparent bluing of aerosols near clouds. J. Geophys. Res., 113, D14S17, doi: 10.1029/2007JD009196.
Numerous studies ba
There are two ways that clouds affect the retrievals of aerosols: (i) the existence of small amounts of sub-pixel sized clouds in pixels identified as being cloud-free and (ii) an enhancement in the illumination of the cloud-free column through the reflection of sunlight by nearby clouds. When the pixels are relatively large, only the first type (cloud contamination) is considered. The second type (cloud adjacency effect) is more pronounced when satellite pixels are relatively small. Both cloud contamination and the cloud adjacency effect may increase substantially reflected radiation and thus lead to significant overestimates of the AOT. These two types of cloud effects, however, have different impacts on the retrieved AOT: sub-pixel clouds increase AOT by increasing the apparent contribution due to large particles (aerosol “coarse” mode), cloud adjacency mostly increases the apparent contribution due to small particles (aerosol “fine” mode). We have justified and quantified the second factor by using a simple stochastic cloud model to obtain the radiative flux reflected by broken clouds and comparing this flux with that obtained with the molecules in the atmosphere causing extinction, but no scattering. A simple model was described for estimating the cloud-induced enhanced reflectances of cloud-free columns in the vicinity of clouds. The enhancement was assumed to be due entirely to Rayleigh scattering. For the shorter wavelengths where molecular scattering is relatively large, attributing the enhancement to the illumination of the Rayleigh scattering atmosphere by sunlight reflected from nearby clouds proved reasonable for scenes with dark surfaces, broken, low-level cumulus clouds, and an aerosol la
The results of numerical simulations of the enhancement are in relatively good agreement with the simple model, although the model underestimates somewhat the enhancement for the particular scenes studied, cumulus cloud fields retrieved from collocated moderate-resolution imaging spectroradiometer (MODIS) and Atmosphere-Surface Turbulent Exchange Research (ASTER) images over a biomass burning region in Brazil.
The one-la The assumption that the enhancement of the cloud-free column is due to molecular scattering leads naturally to a larger increase of AOT for shorter wavelengths, or to a “bluing” of aerosols near clouds. As a result, in contrast to cloud contamination by sub-pixel clouds, the cloud adjacency effect will increase the apparent aerosol “fine” mode fraction rather than the “coarse” mode fraction. We showed that the enhanced illumination of cloud-free columns is a key part of characterizing aerosol properties in the vicinity of clouds. In satellite ba