Climatology of cloud entrainment in marine and continental shallow cumulus and its sensitivity to environmental factors

Abstract:

Shallow cumulus form in several regions around the globe and as a result influence the global radiative budget. Shallow cumulus lifetime is believed to be largely influenced by the entrainment process; a process that occurs at scales much smaller than the resolution of GCMs. Parameterizing the entrainment process for GCMs requires understanding the factors impacting its variability. Current parameterizations assume that cloud entrainment is a function of either environmental thermodynamics or cloud dynamics and none explicitly account for both. The current study aims to further document how entrainment rate is impacted by environmental thermodynamics and cloud dynamics and report on its climatological variability in non-precipitating shallow cumulus clouds as observed at two sites: the continental Southern Great Planes site (2011-2019) and the marine Eastern North Atlantic site (2015-2019).Considering that bulk entrainment rate retrievals may carry large uncertainty, we apply two methods: the individual cloud method of Jensen & Del Genio (2006), which is based on CAPE arguments and the cloud ensemble method of Drueke et al. (2019), which is based on a TKE similarity theory. For both methods, cloud macrophysical and dynamical characteristics are estimated using vertically pointing radar and Doppler lidar observations and environmental characteristics are estimated using AERI, RWP and ECOR observations.

Consistent with other shorter-term observational studies, we find that more humid, higher CAPE and shallower cloud conditions are robustly associated with stronger cloud dilution. A weaker relationship between cloud width and cloud base mass flux is also observed where wider clouds with larger cloud-base mass fluxes are associated with decreased dilution. Consistent with the large-eddy simulations of Drueke et al (2019), relationships are found to vary with continentality, and marine clouds are often found to have higher entrainment rate than continental clouds for the same fixed sets of environmental conditions. The long entrainment rate record compiled in this study can be leveraged to enhance physical understanding of shallow cumulus convection, develop new, more comprehensive parameterizations and/or to evaluate the representation of shallow cumuli in large-eddy simulations.