Climatological Sensitivities of Shallow-Cumulus Bulk Entrainment in Continental and Oceanic Locations

Cumulus entrainment is a complex process that has long challenged conceptual understanding and atmospheric prediction. To investigate this process observationally, two retrievals are used to generate multiyear climatologies of shallow-cumulus bulk entrainment (epsilon) at two Atmospheric Radiation Measurement cloud observatories, one in the U.S. Southern Great Plains (SGP) and the other in the Azores archipelago in the eastern North Atlantic (ENA). The statistical distributions of epsilon thus obtained, as well as certain environmental and cloud-related sensitivities of epsilon, are consistent with previous findings from large-eddy simulations. The retrieved epsilon robustly increases with cloud-layer relative humidity and decreases in wider clouds and cloud ensembles with larger cloud-base mass fluxes. While epsilon also correlates negatively with measures of cloud-layer vigor (e.g., maximum in-cloud vertical velocity and cloud depth), the extent to which these metrics actually regulate epsilon (or vice versa) is unclear. Novel sensitivities of epsilon include a robust decrease of epsilon with increasing subcloud wind speed in oceanic flows, as well as a decrease of epsilon with increasing cloud-base mass flux in individual cumuli. A strong land-ocean contrast in epsilon is also found, with median values of 0.5-0.6 km(-1) at the continental SGP site and 1.0-1.1 km(-1) at the oceanic ENA site. This trend is associated with drier and deeper cloud layers, along with larger cloud-base mass fluxes, at SGP, all of which favor reduced epsilon. The flow dependence of retrieved epsilon implies that its various sensitivities should be accounted for in cumulus parameterization schemes.