Subcloud controls on shallow-cumulus dilution

Description

A combination of bulk-entrainment retrievals and large-eddy simulations (LES) is used to investigate the impacts of subcloud processes on dilution in nonprecipitating active (i.e. buoyant) shallow cumuli. The entrainment retrievals are conducted over a 5-yr period (2015-2019) at the SGP and ENA observatories, using two different methods. The first method, applied to individual cumuli, uses an entraining parcel model to estimate entrainment based on observed cloud-top height. The second method, applied to one-hour cumulus ensembles, uses a similarity theory of shallow-cumulus transports to infer entrainment based on CAPE, cloud-base mass flux, and cloud-layer depth. Analogous to the observational analysis, the simulations are based on two LES model inter-comparison studies of a continental (ARM-SGP; Brown et al. 2002) and a maritime (BOMEX; Siebesma et al. 2003) case. The retrievals and simulations agree that the cloud dilution depends inversely on both subcloud wind speed (in maritime environments) and cloud-base mass flux, the latter being closely related to cloud width. Other subcloud parameters thought to influence bulk entrainment, such as subcloud-layer depth (i.e. cloud-base height) and sensible heat flux show more mixed signals, suggesting that these parameters are less influential and/or induce competing effects that complicate the sensitivity. Perhaps surprisingly, the two strongest subcloud sensitivities can both be explained by an inverse relationship between cloud-layer turbulence intensity and cloud dilution. The simulations suggest that, although stronger turbulence induces stronger cloud-environmental mixing, this enhanced mixing does not necessarily lead to enhanced dilution within the buoyant cloud cores. Rather, it leads to a wider buffer zone, or “shell”, surrounding the cloud that helps to protect the buoyant cloud cores from dry environmental air.