Parametric Behaviors of CLUBB in Simulation of Low Clouds in the Community Atmosphere Model (CAM5)

Abstract:

A high-order turbulence cloud parameterization, called Cloud Layers Unified by Binormals (CLUBB), has been recently implemented into the Community Atmosphere Model Version 5 (CAM5) to provide a unified treatment for turbulence, stratiform cloud macrophysics, and shallow convection. In this study, we investigate the sensitivity of simulated low clouds (i.e. shallow cumulus and stratocumulus) to selected tunable parameters of CLUBB in the both single column and global version of CAM5. A quasi-Monte Carlo (QMC) sampling approach is adopted to effectively explore the high-dimensional parameter space and a generalized linear model is adopted to study the responses of simulated cloud fields to tunable parameters. Based on the assembled tests, our results show that most of the variance in simulated cloud fields can be explained by a small number of tunable parameters. For the single column version, the parameters related to Newtonian and buoyancy-damping terms of total water flux are found to be the most influential parameters for stratocumulus, while the parameters related to skewness of vertical velocity are found to be the most influential ones for shallow cumulus. The influential parameters in global simulations are generally consistent with those identified in the single column simulations, although significant regional deviations can be found. These regional deviations are related to particular cloud regimes and cloud microphysics scheme. The sets of parameters that perform favorably against observations are further identified and analyzed. This study improves understanding of the CLUBB behaviors associated with parameter uncertainties and provides valuable insights for other high-order cloud schemes.