Shortwave Radiative Effect of Above Cloud Aerosols Over Global Oceans Derived From 6 Years of CALIOP and MODIS Observations

The shortwave direct radiative effect (DRE) of aerosols at TOA is strongly dependent on the underlying
surface. Over dark surfaces, the
scattering effect of aerosols is generally dominant, leading to negative
DRE (i.e., cooling) at TOA In contrast, when light-
absorbing aerosols reside above clouds, aerosol absorption is significantly amplified by cloud or surface reflection, which offsets or even exceeds the scattering effect of the aerosol leading to a less negative or even positive (i.e., warming) TOA DRE. Therefore, in order to understand the full complexity of aerosol radiative effects on climate, it is important to quantify the DRE under both clear-sky and cloudy-sky conditions. Based on 6 years of collocated CALIOP and MODIS (Moderate Resolution Imaging Spectroradiometer) observations, we investigated the shortwave direct radiative effect (DRE) of aerosols overlying low-level liquid phase clouds over global oceans. We noted that the occurrence frequency of ACA has strong geographical and seasonal variations (see figure below). Using 6 years of collocated CALIOP and MODIS data, and a novel method recently developed in {Zhang:2014ex}, we computed the DRE of ACA over global oceans. We found that ACA has a strong positive DRE over the south-east Atlantic region where absorbing smoke and polluted dusts are often observed overlying highly-reflective stratocumulus clouds. In contrast, the DRE is negative over the north-east Atlantic region where dusts are often found over broken cumulus clouds. We also carried out a comprehensive analysis of the impact of various uncertainty sources, including CALIOP and MODIS retrieval uncertainties, cloud diurnal variations and aerosol model assumption, on ACA DRE. The results from this study will be used in future research to constrain the ACA DRE in climate models References: Zhang, Z., Meyer, K., Platnick, S., Oreopoulos, L., Lee, D. and Yu, H.: A novel method for estimating shortwave direct radiative effect of above-cloud aerosols using CALIOP and MODIS data, Atmos. Meas. Tech., 7(6), 1777–1789, doi:10.5194/amt-7-1777-2014, 2014.