RACORO: Case Study Generation for Continental Boundary Layer Clouds

Monday, May 12, 2014 - 07:00
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As part of the FAst-physics System TEstbed and Research (FASTER) project, case studies have been constructed to assess and improve model representation of continental boundary-layer clouds (stratus, stratocumulus, cumulus) using aircraft data from the RACORO Campaign [Routine Atmospheric Radiation Measurement (ARM) Aerial Facility (AAF) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations], and ARM ground-based remote-sensing observations. Three 60-hour case study periods are selected that capture the temporal evolution of cumulus, stratiform and drizzling boundary layer cloud systems. Cases are included that are more mixed or transitional in nature compared to purely canonical cases. Collectively, the case studies include: daytime stratus, stratocumulus, and cumulus formation and breakup under varying aerosol conditions, strong and weak nocturnal jets, and nighttime stratus formation and deepening within a residual layer. The aircraft data include aerosol number size distributions in a wide range of aerosol loadings fit to lognormal distributions for concise representation in models with two-moment microphysics. Aerosol hygroscopicity (kappa) is determined from observations as a function of size and mapped to the lognormal modes for use in models. The low kappa values observed (0.05-0.30) suggest a large aerosol organic fraction. The case studies are being developed to run side-by-side using large eddy simulation (LES) models (see Endo et al. poster) and single-column models (see Lin et al. poster). Large-scale forcing conditions are obtained: from the ARM continuous forcing for the standard domain and a reduced domain, derived from ECMWF for a standard domain and a reduced domain, and derived from a Multi-Scale Data Assimilation (MS-DA) System. The goal is to provide a diverse set of observationally constrained cases of cloudy boundary layer evolution for use by ESM, ASR and the general modeling communities to improve our understanding of continental cloudy boundary layers, aerosol influences upon them, and their representation in cloud-scale and global-scale models. These continental case studies offer an important complement to previous boundary layer case studies, which mainly focus on marine boundary layer clouds. Overall, the periods sample a wide range of cloudy boundary-layer conditions but they use a modular specification, which makes it easy for modelers to run them.

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