RACORO Continental Boundary Layer Cloud Investigations: Large-Eddy Simulations of Cumulus Clouds and Evaluation with In-Situ and Ground-Based Observations

Abstract:

Observation-based modeling case studies have been developed to study continental boundary layer clouds, aerosol influences upon them, and their representation in cloud- and global-scale models. Three 60-h cases are based on observations obtained during the RACORO field campaign at the ARM Climate Research Facility’s Southern Great Plains site. This study examines large-eddy simulation (LES) of one of the three cases that is dominated by boundary layer cumulus clouds. Two LES models are driven by continuous large-scale and surface forcings and are constrained by multimodal and temporally varying aerosol number size distribution profiles derived from aircraft observations. We compare simulated cloud macrophysical and microphysical properties with ground-based remote sensing and aircraft observations. The LES simulations capture the observed transitions of the evolving cumulus-topped boundary layers during the three daytime periods and generally reproduce variations of droplet number concentration with liquid water content (LWC), corresponding to the gradient between the cloud centers and cloud edges at given heights. While observed LWC values fall within the range of simulated values, observed droplet number concentrations are commonly higher than simulated, although differences remain on par with estimated experimental error in the aircraft measurements. Sensitivity studies examine the influences of bin microphysics versus bulk microphysics, aerosol advection, supersaturation treatment, and aerosol hygroscopicity. Simulated macrophysical cloud properties are found to be insensitive to microphysics treatment in this non-precipitating case, but microphysical properties are especially sensitive to bulk microphysics supersaturation treatment and aerosol hygroscopicity.