PNNL

Abstract

The role of large-scale forcing (LSF) in the development of shallow convection at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) atmospheric observatory is investigated for the 11 June 2016 case using large-eddy simulations (LES) provided by the U.S. Department of Energy’s LES ARM Symbiotic Simulation and Observation (LASSO) activity. The two contrasting LSFs generate different types of convective clouds, i.e., nonprecipitating shallow convective clouds and precipitating cumulus congestus, mainly due to temperature advection in the free troposphere, which modulates the strength of the capping inversion and therefore the buoyancy of the air parcels rising from the atmospheric boundary layer (ABL). The inversion, together with the large-scale updrafts, controls the penetration of the ABL thermals into the free troposphere, leading to cumulus congestus in the case of a weaker inversion. As the congestus reaches the middle troposphere, a secondary cloud peak that is decoupled from the positively buoyant thermals is generated near the cloud top as the cloudy air is detrained to the surrounding cold environment. In contrast, clouds remain shallow in the case of a strong inversion. Differences between the two simulations, which result from the uncertainty in LSF, are amplified over time, as mixed-phase clouds are formed near the top of the congestus in the weaker inversion case. This high dependency of LES results to LSF stresses the importance of accurate LSF by large-scale and mesoscale models to weather simulations at LES resolutions.