Impact of cumulus cloud spacing on Landsat atmospheric

A Landsat 7 Enhanced Thematic Mapper Plus (ETM+) image acquired over the Southern Great Plains site of Department of Energy's Atmospheric Radiation Measurement Program during the Atmospheric Radiation Measurement Enhanced Shortwave Experiment II is used to study the effect of clouds on reflected radiation in clear gaps in a cumulus cloud field. A technique using the spectral information of background and clouds is applied to identify clouds. The path radiance technique is used to extract the apparent path radiance in a clear region of the cumulus cloud field. The result shows that the apparent path radiance is enhanced by nearby clouds in both band 1 (blue) and band 3 (red) of ETM+. More importantly, the magnitude of the enhancement depends on the mean cloud-free distance in the clear patches. For cloud-free distances <0.5 km the enhancement of apparent path radiance is more than 0.025 and 0.015 (reflectance units) in band 1 and band 3, respectively, which corresponds to an enhancement of apparent aerosol optical thickness of ∼0.25 and ∼0.15. Neglecting of the three-dimensional cloud effect would lead to underestimates of surface reflectance of ∼0.025 and ∼0.015 in the blue and red band, respectively, if the true aerosol optical thickness is 0.2 and the surface reflectance is 0.05. The enhancement decreases exponentially with mean cloud-free distance, reaching asymptotic values of 0.09 for band 1 and 0.027 for band 3 at a mean cloud-free distance about 2 km. The asymptotic values are slightly larger than the mean path radiances retrieved from a completely clear region: 0.086 and 0.024 for the blue and red bands, respectively.