Aerosol Properties Near Clouds from MODIS and CALIPSO

Aerosols contribute the second-largest forcing to climate change, but with the largest uncertainty among all forcing agents. It is well known that aerosols influence the climate directly by scattering and absorbing incoming solar radiation and indirectly by serving as cloud condensation nuclei to modify the microphysics, radiative properties, and lifetime of clouds. On the contrary, clouds can also influence aerosols. Specifically, cloud droplets provide the aqueous environment for precursor gas phase substances to form new particulate matter, and for existing particles to grow. Thus, aerosol properties near clouds in “cloud-clear transition zone” or “twilight zone” are fundamentally different from those in clear air. Since more than 60% of Earth is covered by clouds and about half of all clear-sky areas are within 4 km from low clouds, ignoring the change of aerosol properties near clouds will bias aerosol radiative forcing estimates in both shortwave and longwave spectral regions and better quantifying aerosol properties near clouds is essential for reducing uncertainties of aerosol forcing on global climate.

Aerosol properties are routinely retrieved from space by passive and actively remote sensing methods. It is well known that clouds also affect the passive aerosol remote sensing by enhancing clear-sky reflected radiation through from clouds scattering into clear areas. This is called 3D radiative effect. We had developed a 2-Layer model to correct the 3D radiative effect over ocean. In this study, we expand the validation of the model to areas over land. Here, we perform the corrections for MODIS spectral reflectance over Amazon region. For better understanding near-cloud aerosol properties, we will compare MODIS retrieved aerosol properties, both before and after the correction, with co-located CALIPSO aerosol data which are not affected by the 3D radiative effect. We will further assess the radiative impact of aerosol near clouds before and after the 3D correction for MODIS and compare the results with that for aerosols from CALIPSO data.