Articles | Volume 17, issue 21
Atmos. Chem. Phys., 17, 13151–13164, 2017
Atmos. Chem. Phys., 17, 13151–13164, 2017

Research article 07 Nov 2017

Research article | 07 Nov 2017

Unveiling aerosol–cloud interactions – Part 1: Cloud contamination in satellite products enhances the aerosol indirect forcing estimate

Matthew W. Christensen1,2, David Neubauer3, Caroline A. Poulsen1, Gareth E. Thomas1, Gregory R. McGarragh2, Adam C. Povey4, Simon R. Proud2, and Roy G. Grainger4 Matthew W. Christensen et al.
  • 1RAL Space, STFC Rutherford Appleton Laboratory, Harwell, OX11 0QX, UK
  • 2Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, OX1 3PU, UK
  • 3Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, 8092, Switzerland
  • 4National Centre for Earth Observation, University of Oxford, Oxford, OX1 3PU, UK

Abstract. Increased concentrations of aerosol can enhance the albedo of warm low-level 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 (3-D radiative transfer) clouds. To screen for this contamination we have developed a new 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 radiative forcing when selecting aerosol pairs that are located farther away from the clouds (−0.28±0.26 W m−2) compared to those including pairs that are within 15 km of the nearest cloud (−0.49±0.18 W m−2). 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 the inclusion of retrieval artefacts in the aerosol located near clouds.

Short summary
The 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.
Final-revised paper