In addition to reflecting and absorbing incoming solar radiation, (known as the direct radiative effect), aerosols also have an important impact on cloud properties and their associated radiative effects. These are known as aerosol indirect effects (AIE). The most common forms of AIE include aerosols acting as cloud condensation nuclei, reducing cloud droplet size, increasing cloud albedo, which reflects more solar radiation back into space. This is known as the first indirect effect. Reduced cloud droplet size can delay the onset of precipitation, increasing cloud lifetime, which is known as the second indirect effect. This research analyzes indirect effects for a 6-year period between March 2000 and December 2005 for several regions around the globe to study the impact of differing aerosol concentrations, species, and atmospheric conditions on observed indirect effects. One of the most important factors in whether or not aerosols and clouds are located at the same level within at atmospheric column. This research shows that AIE differs significantly from region to region, and that some of these differences can be explained by the relative vertical distributions of aerosols and clouds.

An example of this difference can be observed by comparing the Arabian Sea and the Bay of Bengal. The first AIE is maximized in the Arabian Sea during the summer months, corresponding to an increase in elevated dust aerosols and near surface sea salt aerosols. Examples of the relative aerosol and cloud height layers during this time indicate that clouds are located at similar heights, between 2 and 3 km ASL. These estimates are based off examples using MISR stereo height data and CALIPSO backscatter profiles. Since aerosols and clouds are co-located at the same atmospheric level, the opportunity for the indirect effect to occur is maximized. In the Bay of Bengal, similar atmospheric conditions are present, but no corresponding summer-time increase in the indirect effect was observed. In this region, the concentration of elevated dust aerosols is much less. Also, a greater proportion of the clouds are high cirrus type with lie well above any aerosols present. Thus, aerosol indirect effects are less likely to occur. These examples highlight the importance of vertical profiles when analyzing possible aerosol-cloud interactions and how total column measurements only may produce misleading results.