Dominant Cloud Controlling Factors for Low-Level Cloud Fraction: Subtropical Versus Extratropical Oceans

To improve cloud feedback understanding and simulation, observations have been used to quantify the rate of change of cloud radiative properties as a function of specific environmental metrics (or cloud controlling factors; CCFs). The study focuses on low-level cloud dominated regions during 2006-2010. For each ocean gridpoint, Spearman's rank correlation coefficients of daily mean observed cloud fraction versus (a) 10-m wind, (b) sensible heat flux (SHF), (c) sea surface temperature, (d) estimated inversion strength (EIS), (e) 850 hPa vertical velocity, and (f) the M parameter ( increment & theta;skin800hPa ${\increment}{\theta }_{\text{skin}}<^>{800\text{hPa}}$) are sorted to identify the dominant CCFs in both extratropics (30 & DEG;-60 & DEG;N/S) and subtropics (30 & DEG;S-30 & DEG;N). A novel map for visualizing dominant CCFs for low-level cloud fraction reveals that: increment & theta;skin800hPa ${\increment}{\theta }_{\text{skin}}<^>{800\text{hPa}}$ dominates in the subtropical stratocumulus regions while 10-m winds dominate in shallow cumulus regions but in the extratropics, a different inversion structure diagnostic (EIS) dominates, while SHF dominates in western boundary current areas. Clouds in the lower levels of the atmosphere cover up to 80% of the oceans in some specific areas within the & PLUSMN;30 & DEG; latitude band (i.e., the subtropics regions) but also cover a large portion of the mid-latitude oceans (i.e., the extratropics). In the subtropics, cloud cover has been found to depend principally on sea surface temperature and the stability or thermal structure of the lower levels of the troposphere. Using satellite observations and reanalysis data, we test whether these relationships are found everywhere, and importantly, we find that these dependencies are not uniform across the global oceans. Where cloud cover is large, stability is the strongest driver of cloud cover, but the atmospheric vertical structure of this stability most strongly correlated with the clouds differs between the subtropics and extratropics. Where cloud cover is relatively small, near surface wind speed has the strongest relationship with cloud cover in the subtropics, while at higher latitudes and more specifically in the western boundary current regions, air-sea energy exchanges (i.e., sensible heat flux) dominate. These regionally varying relationships are depicted with a novel map of dominant cloud controlling factors. This map can help with evaluation and development of climate models. A novel map of dominant controlling factors for low-level oceanic cloud fraction reveals regionally varying patternsSubtropical and extratropical low clouds depend on stability, but the dominant stability metric varies by region, challenging modelsSurface wind speed dominates in cumulus-dominated regions, sensible heat fluxes dominate in western boundary current regions in winter