Aerosol Effects on Satellite Retrievals of Cloud Optical Properties in the South Atlantic Ocean

Satellite retrievals of cloud optical depth and effective radius are shown to be underestimated in the South Atlantic Ocean, where biomass-burning aerosols typically reside above low-level, liquid water stratocumulus clouds. A radiative transfer model is used to calculate top of the atmosphere (TOA) reflectances for a case where an aerosol layer is elevated above a liquid cloud in the South Atlantic Ocean. Both absorbing and scattering aerosols are modeled at varying aerosol optical depths (AOD) at 0.86 μm and 2.10 μm. Optimal estimation methods are used to convert reflectance pairs into a retrieved cloud optical depth and effective radius, similar to the Moderate Resolution Imaging Spectroradiometer (MODIS, aboard Aqua satellite) algorithm. Absorbing aerosols lead to an underestimation of the retrieved cloud optical depth by up to 60 – 80%, while scattering aerosols can lead to an over- or underestimate of the optical depth by up to 10%, depending on the cloud thickness. Effective radius retrievals can be over- and underestimated if absorbing aerosols are present, depending on the AOD. Scattering aerosols result in an overestimation of effective radius by up to 0 – 10%. A-Train satellite observations from August 2006 – December 2010 in the South Atlantic Ocean also show that the MODIS biases in cloud optical depth and effective radius are likely affecting retrieved properties and hence retrieved cloud droplet number concentrations when the aerosol layer is vertically separated from the cloud.