Abstract
Abstract. Aerosols are often advected above cloud decks, and the amount of aerosols over cloud has been assumed to be similar to that at the same heights in nearby clear sky. In this assumption, cloud and aerosol above cloud-top height are considered randomly located with respect to each other. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed here to investigate this assumption on global scales. The CALIPSO data reveal that the aerosol optical depth (AOD) above low cloud tends to be smaller than in nearby clear sky during the daytime, and the opposite is true during the nighttime. In particular, over oceanic regions with wide-spread low cloud, such as the tropical southeastern Atlantic Ocean and northeastern Pacific Ocean, the daytime AOD above low cloud is often 40 % smaller than in surrounding clear skies.
Highlights
IntroductionThe magnitude of the global aerosol direct radiative forcing (due to absorption and scattering of incoming sunlight) has been estimated to range from −0.85 to 0.15 W m−2 in the 5th Intergovernmental Panel on Climate Change report (Myhre et al, 2013a)
The magnitude of the global aerosol direct radiative forcing has been estimated to range from −0.85 to 0.15 W m−2 in the 5th Intergovernmental Panel on Climate Change report (Myhre et al, 2013a)
We study the aerosol optical depth (AOD) above low clouds since aerosols are typically advected above low clouds, and the low-cloudtop height provides a more uniform reference altitude than a combination of all clouds
Summary
The magnitude of the global aerosol direct radiative forcing (due to absorption and scattering of incoming sunlight) has been estimated to range from −0.85 to 0.15 W m−2 in the 5th Intergovernmental Panel on Climate Change report (Myhre et al, 2013a). This uncertainty range is based partly on aerosol simulations (Myhre et al, 2013b) and on semiempirical studies (e.g., Myhre, 2009; Su et al, 2013) where aerosol observations were used to constrain the aerosol simulations. This means that cloud amount and albedo become important factors in determining the forcing of absorbing aerosols. Chand et al (2009) found that, for biomass burning smoke (with a single-scattering albedo, SSA, of 0.85) over the southeastern Atlantic stratocumulus clouds, the net radiative forcing changes the sign from negative to positive when the cloud fraction exceeds 40 %
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