Strategies used to empirically derive the aerosol indirect radiative forcing from space-borne observations invariably seek regional scale correlations between retrieved aerosol and cloud properties to determine the response of clouds to changes in aerosol burdens. Through such correlations several studies have shown that droplet radii decrease and cloud visible optical depths and cloud cover increase as aerosol optical depths increase. These trends are consistent with those expected for the aerosol indirect radiative forcing. Such trends, however, may be due in part to physical processes that have no relationship to the effects of aerosols on clouds and in part to biases in the cloud properties derived using threshold methods. For example, the enhancement in aerosol burden in cloudy regions could be due to 1) increases in the cloud contamination of aerosol retrievals performed in the vicinity of clouds, 2) enhancements in the apparent reflectivities of cloud-free columns that arise from the scattering of sunlight that was reflected by nearby clouds, and 3) the growth of aerosol particles in the humid cloud environment or by cloud processing which increases the mass of the residual particles when droplets evaporate. Here CALIPSO and A-Train observations, primarily MODIS radiance, aerosol, and cloud products, will be used to investigate differences in the MODIS and CALIPSO aerosol properties in the vicinity of the clouds. Differences in detection strategies and sensitivities of the passive (MODIS) and active (CALIOP) sensors, should provide insight into the different roles played by cloud contamination, enhanced illumination, and the response of particles to the cloud environment. In addition, a partly cloudy imager pixel retrieval will be used to study the response of clouds to changes in aerosol loading. The partly cloudy imager pixel retrievals reduce the potential for biases that are possible in the MODIS MOD06 cloud product which produce trends in cloud optical depths and droplet effective radii that coupled with the rise in aerosol optical depth with increasing cloud cover could, on their own, be mistaken for the indirect effect of aerosols.
