Examining Differences between Observed and Large-Eddy Simulations of Shallow Cumulus Cloud-Base Vertical Velocity

Wednesday, December 12, 2018 - 08:00
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Model parameterizations of continental boundary layer clouds remain challenged when it comes to representing the observed properties of these clouds in part because small-scale turbulence and convection are not properly represented. To perform model evaluation and adjustments, observational constraints are needed on critical parameters such as cloud-base vertical velocity and its relationship to cloud cover. In this study conducted at the ARM Facility Southern Great Plains (SGP) observatory in Oklahoma, Doppler lidar observations are used as a benchmark for first-light ensemble large-eddy simulations (LES) conducted by the LES ARM Symbiotic Simulation and Observation (LASSO) project (https://www.arm.gov/capabilities/modeling/lasso). The observational component of this work extends on the long-term single-site work of Lamer and Kollias [2015] by incorporating observation from nearby extended facilities providing critical information on regional variability. Results indicate that simulations significantly underestimate the frequency of occurrence of downdrafts at cloud base. These results are robust to changes in LES model used, forcing source, and horizontal resolution. An investigation of factors that may contribute to these differences is described.

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