Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 1 2010 10.5194/acpd-10-1559-2010 http://www.atmos-chem-phys-discuss.net/10/1559/2010/ http://www.atmos-chem-phys-discuss.net/10/1559/2010/acpd-10-1559-2010.html http://www.atmos-chem-phys-discuss.net/10/1559/2010/acpd-10-1559-2010.pdf 1559 1593 2010-01-20 Cluster analysis of midlatitude oceanic cloud regimes – Part 1: Mean cloud and meteorological properties N. D. Gordon n.gordon@leeds.ac.uk J. R. Norris Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA now at: School of Earth and Environment, University of Leeds, Leeds, UK Clouds play an important role in the climate system by reducing the amount of shortwave radiation reaching the surface and the amount of longwave radiation escaping to space. Although dependent on type and location, clouds produce more cooling than warming in the global average. Accurate simulation of clouds in computer models remains elusive, however, pointing to a lack of understanding of the connection between large-scale dynamics and cloud properties. This study uses a <i>k</i>-means clustering algorithm to group 21-years of satellite cloud data over midlatitude oceans into seven clusters and demonstrates that the cloud clusters are associated with distinct large-scale dynamical conditions. Three clusters correspond to low-level cloud regimes with different cloud fraction and cumuliform or stratiform characteristics, but all occur under large-scale descent and a relatively dry free troposphere. The "small cumulus" regime is most prevalent equatorward of 40&deg; in all seasons; the "large cumulus" regime is associated with a relatively cold troposphere and primarily occurs during winter; and the "stratocumulus/stratus" regime occurs under a temperature inversion and relatively warm free troposphere and predominates during summer. 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