Analyzing the Sensitivity of Cloud Properties to Parameters for Differing Cloud Types
Large uncertainties remain in simulating clouds in global climate models, partly due to multiple tunable parameters in cloud parameterizations. A multi-institutional team supported by the Department of Energy investigated the sensitivity of simulated shallow cumulus and stratocumulus clouds to different parameterizations. The investigation selected tunable parameters in a newly implemented cloud scheme in the single-column version of the Community Atmosphere Model version 5 (SCAM5), called Cloud Layers Unified by Binormals (CLUBB). The team found that most of the variance in simulated cloud fields can be explained by a small number of tunable parameters. They used a sophisticated statistical approach to explore the high-dimensional parameter space in CLUBB and analyzed the responses of simulated cloud fields to tunable parameters. They found that among 40 to 50 tunable parameters in CLUBB, only a handful are influential. The influential parameters are different for different types of clouds. Parameters related to water flux are found to be the most influential for stratocumulus. For shallow cumulus, the most influential parameters are those related to skewness of vertical velocity, reflecting the strong coupling between cloud properties and dynamics in this regime. The influential parameters in stratocumulus clouds are sensitive to the vertical resolution while little sensitivity is found for the shallow cumulus clouds. This study improves the understanding of the parameter dependence of this newly implemented scheme and reduces the number of tunable parameters for ongoing sensitivity and calibration study of global simulations.
Guo, Z., M. Wang, Y. Qian, V. Larson, P. Bogenschutz, G. Lin, S. Ghan, M. Ovchinnikov, C. Zhao, and T. Zhou. 2014. “A Sensitivity Analysis of Cloud Properties to CLUBB Parameters in the Single Column Community Atmosphere Model (SCAM5),” Journal of Advances in Modeling Earth Systems, DOI:10.1002/2014MS000315.