Application of CERES observations to correction of MODIS aerosol optical thickness in the vicinity of cloud: a case study over the SGP site of the ARM program

This paper presents an example of applying CERES observations to the correction of MODIS aerosol optical thickness values in the vicinity of clouds. Collocated MODIS, CERES, and ASTER images are acquired near the SGP (Southern Great Plain) site of the ARM (Atmospheric Radiation Measurement) Program. The high-resolution ASTER image (~60 km x 60 km) shows detailed information of cloud structure and surface reflectance. In the image, there is an isolated optically thick cloud with horizontal extension ~ 20 km x 30 km reaching to an altitude ~ 5 km. We find that MODIS retrieved aerosol optical thickness increases towards the cloud edges with maximum of aerosol optical thickness about 3-4 larger than that in the clear region 20 km away from the cloud. CERES simultaneous observed upward shortwave flux also increases towards cloud.

For the first order approximation, we apply CERES broadband observations to estimate MODIS narrow spectral band fluxes in computing the cloud-induced reflectance enhancement. A substantially large aerosol correction up to ~0.08 is made at wavelength 0.47 microns, while a less but still significant correction up to ~0.02 is made at wavelength 0.66 microns. We further compare CERES-based estimates of cloud-induced enhancement and corresponding aerosol optical thickness corrections to those based on rigorous 3D radiative computations. We find the results are similar. This suggests that CERES observations from the same spacecraft can be applied to correct errors in MODIS aerosol optical thickness retrieval due to cloud radiative adjacency effects.