Extending 3D near-cloud corrections from shorter to longer wavelengths

Satellite observations have shown a positive correlation between cloud amount and aerosol optical thickness (AOT); it can be explained by the humidification of aerosols near clouds, and/or by cloud contamination by sub-pixel size clouds and the cloud adjacency effect. The last effect may substantially increase reflected radiation in cloud-free columns, leading to overestimates in the retrieved AOT. For clear-sky areas near boundary layer clouds the main contribution to the enhancement of clear sky reflectance at shorter wavelengths comes from the radiation scattered into clear areas by clouds and then scattered to the sensor by air molecules. Because of the wavelength dependence of air molecule scattering, this process leads to a larger reflectance increase at shorter wavelengths, and can be corrected using a simple two-layer model. However, correcting only for molecular scattering skews spectral properties of the retrieved AOT.

This presentation discusses a new approach that extends 3D near-cloud corrections from shorter to longer wavelengths. It assumes that (a) the 3D correction for the shortest wavelength is known, and (b) the linear relationship between radiances at shorter and longer wavelengths has the same slope as the relationship between the 3D errors at the same wavelengths. The new approach has been tested with 3D radiances simulated for 26 cumulus fields from Large-Eddy Simulations, supplemented with 40 aerosol profiles.