Direct comparison of LES of cumulus convection with ARM-SGP observations

 
Poster PDF

Authors

Thijs Heus — Cleveland State University
Roel Neggers — University of Cologne
Philipp Griewank — University Cologne
Neil Lareau — University of Nevada Reno

Category

Warm low clouds, including aerosol interactions

Description

In this work we do a direct comparison between cumulus observations over the ARM-SGP with numerical results from LES, using a simulator style approach. Recently, an algorithm has been developed to retrieve cloud size from Total Sky Imaging, which allows us to continuously monitor the cloud size distribution of shallow cumulus convection. In this study we compare the retrieved distribution with LASSO-based Large Eddy Simulations. To do so, we have developed a TSI Simulator using the Blender ray tracing engine. Using the simulator, we perform the following studies: 1) Validating the TSI projection algorithm via cross referencing with the cloud field from the LES, with a specific emphasis on the error introduced by cloud sides and background clouds for larger zenith angles; 2) Comparison of LES and TSI observations of cloud size distributions using the simulator. We find that the cloud size distribution matches well between TSI and LES, but that the TSI shows a bias in the timing of early morning cloudiness, that can be replicated with LES-TSI. A further way to evaluate the LES fields is to compare them to long term composites of upward pointing 1D ground measurements of cumulus clouds chord lengths. To make the model-observation comparison as direct as possible we imitate a ground observation in the LES field. This is done either by sampling a single column of the model domain at full temporal resolution, or by taking individual 3D snapshots and running the observation along the ground. On one hand we can use this comparison to evaluate and explore the limitations of the LASSO simulations, on the other hand the much higher sample size available from the LES fields allows us to extend the analysis beyond what is currently possible from observations.