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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-450
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
19 May 2017
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Unveiling aerosol-cloud interactions Part 1: Cloud contamination in satellite products enhances the aerosol indirect forcing estimate
Matthew W. Christensen1,2, David Neubauer3, Caroline Poulsen1, Gareth Thomas1, Greg McGarragh2, Adam C. Povey4, Simon Proud2, and Roy G. Grainger4 1RAL Space, STFC Rutherford Appleton Laboratory, Harwell, OX11 0QX, United Kingdom
2Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, OX1 3PU, United Kingdom
3Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, 8092, Switzerland
4National Centre for Earth Observation, University of Oxford, Oxford, OX1 3PU, United Kingdom
Abstract. Increased concentrations of aerosol can enhance the albedo of warm lowlevel cloud. Accurately quantifying this relationship from space is challenging due in part to contamination of aerosol statistics near clouds. Aerosol retrievals near clouds can be influenced by stray cloud particles in areas assumed to be cloud-free, particle swelling by humidification, shadows and enhanced scattering into the aerosol field from (3D radiative transfer) clouds. To screen for this contamination, we have developed a new 5 Cloud-Aerosol Pairing Algorithm (CAPA) to link cloud observations to the nearest aerosol retrieval within the satellite image. The distance between each aerosol retrieval and nearest cloud is also computed in CAPA.

Results from two independent satellite imagers, the Advanced Along Track Scanning Radiometer (AATSR) and MODerate Resolution Imaging Spectroradiometer (MODIS) show a marked reduction in the strength of the intrinsic aerosol indirect forcing when selecting aerosol pairs that are located farther away from the clouds (−0.28 ± 0.26 W/m2) compared to those 10 including pairs that are within 15 km of the nearest cloud (−0.49 ± 0.18 W/m2). The larger aerosol optical depths in closer proximity to cloud artificially enhance the relationship between aerosol loading, cloud albedo, and cloud fraction. These results suggest that previous satellite-based radiative forcing estimates represented in key climate reports may be exaggerated due to including retrieval artefacts in the aerosol located near clouds.


Citation: Christensen, M. W., Neubauer, D., Poulsen, C., Thomas, G., McGarragh, G., Povey, A. C., Proud, S., and Grainger, R. G.: Unveiling aerosol-cloud interactions Part 1: Cloud contamination in satellite products enhances the aerosol indirect forcing estimate, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-450, in review, 2017.
Matthew W. Christensen et al.
Matthew W. Christensen et al.

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Short summary
A new Cloud-Aerosol Pairing Algorithm (CAPA) is developed to quantify the impact of near-cloud aerosol retrievals on satellite-based aerosol-cloud statistical relationships. We find that previous satellite-based radiative forcing estimates of aerosol-cloud interactions represented in key climate reports are likely exaggerated by up to 50% due to including retrieval artefacts in the aerosols located near clouds. It is demonstrated that this retrieval artefact can be corrected in current products.
A new Cloud-Aerosol Pairing Algorithm (CAPA) is developed to quantify the impact of near-cloud...
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