Analysis of co-located MODIS and CALIPSO observations near clouds

[摘要] This paper aims at helping synergistic studies in combining data fromdifferent satellites for gaining new insights into two critical yet poorlyunderstood aspects of anthropogenic climate change, aerosol-cloudinteractions and aerosol radiative effects. In particular, the paperexamines the way cloud information from the MODIS (MODerate resolutionImaging Spectroradiometer) imager can refine our perceptions based on CALIOP(Cloud-Aerosol Lidar with Orthogonal Polarization) lidar measurements aboutthe systematic aerosol changes that occur near clouds.

The statistical analysis of a yearlong dataset of co-located global maritimeobservations from the Aqua and CALIPSO (Cloud-Aerosol Lidar and InfraredPathfinder Satellite Observation) satellites reveals that MODIS'smultispectral imaging ability can greatly help the interpretation of CALIOPobservations. The results show that imagers on Aqua and CALIPSO yield verysimilar pictures, and that the discrepancies – due mainly to wind drift anddifferences in view angle – do not significantly hinder aerosol measurementsnear clouds. By detecting clouds outside the CALIOP track, MODIS revealsthat clouds are usually closer to clear areas than CALIOP data alone wouldsuggest. The paper finds statistical relationships between the distances toclouds in MODIS and CALIOP data, and proposes a rescaling approach tostatistically account for the impact of clouds outside the CALIOP track evenwhen MODIS cannot reliably detect low clouds, for example at night or oversea ice. Finally, the results show that the typical distance to cloudsdepends on both cloud coverage and cloud type, and accordingly varies withlocation and season. In maritime areas perceived cloud free, the globalmedian distance to clouds below 3 km altitude is in the 4–5 km range.

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