Investigation of aerosol optical depth retrieval errors caused by aerosol vertical profile assumptions using CALIPSO data

Estimation of aerosol direct and indirect effects requires accurate knowledge of aerosol properties both away from and in the vicinity of clouds. Currently, aerosol optical depth at global scale is primarily obtained via satellite retrievals. However, numerous assumptions are embedded in satellite aerosol retrieval algorithms, including the vertical density profile of the aerosols. The mixing ratio of aerosol determines the volume mean phase function at a given height. If the assumption of aerosol layer height in the retrieval is different from the actual aerosol height, then the retrieved aerosol optical depth will be in error.

New aerosol profile measurements taken around the globe by CALIPSO allow us to investigate the importance of aerosol profile assumptions on satellite aerosol optical depth retrievals. Here we present new aerosol profile data from CALIPSO measurements, sorted to characterize the variability of aerosol profiles from different regions of the world. Using the typical profiles from CALIPSO, we estimate the uncertainty in retrieved aerosol optical depths due to differences between the true aerosol vertical profile and the assumed profile. In addition, we examine whether the uncertainty depends on the presence of clouds near the aerosol layers. To obtain these estimates, full three-dimensional radiative transfer computations are performed for the various profiles and aerosol optical depth retrievals are simulated using look-up tables computed for the range of observed aerosol vertical profiles. Retrieval errors obtained as a function of assumed aerosol profile, solar zenith angle, and view zenith angle are summarized.