Cluster analysis of midlatitude oceanic cloud regimes – Part 1: Mean cloud and meteorological properties

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 k-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° 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. Three clusters correspond to vertically extensive cloud regimes with tops in the middle or upper troposphere. They differ according to the strength of large-scale ascent and enhancement of tropospheric temperature and humidity: "deep altostratus" has the smallest forcing, "weak frontal" is in the middle, and "strong frontal" has the largest forcing. The frontal cloud regimes occur most frequently in storm track regions. The final cluster, "cirrus" is associated with a lower troposphere that is dry and an upper troposphere that is moist and experiencing weak ascent and horizontal moist advection. This information builds a foundation for producing an observational estimate of the midlatitude ocean cloud response to warming that is independent of confounding meteorological influences.