An LES study of continental boundary layer cumulus clouds during the RACORO field campaign at the ARM SGP site

Abstract:

A case study of continental boundary layer cumulus clouds is performed by multiple large-eddy simulation (LES) models in the FAst-physics System TEstbed and Research (FASTER) project, based on the boundary-layer-cloud observations during the RACORO Campaign [Routine Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations] at the ARM Climate Research Facility’s Southern Great Plains (SGP) site. Two LES models, GISS DHARMA and BNL WRF-FASTER, are driven by continuous large-scale and surface forcings, and constrained by multi-modal and temporally-varying aerosol number size distribution profiles derived from aircraft observations. Simulated transitions of cloud macrophysical properties and thermodynamic structure are examined by ground-based remote sensors and profiling measurements by the aircraft observations. The two LES models both capture the significant transitions of the evolving cumulus-topped boundary layers in the three daytime periods. The aircraft observations are then used to evaluate cloud microphysical properties under the varying aerosol and cloud conditions, providing physical insights into the parameterizations necessary at LES scale. Linear variations of droplet number concentration with liquid water content found in each horizontal leg are reproduced in the simulations. The LES data are further used as references to investigate the performance of shallow cumulus scheme in CAM in simulating the RACORO clouds.