An Updated CLUBB PDF Closure Scheme to Improve Low Cloud Simulation in CAM6.

Cloud Layers Unified By Binormals (CLUBB) adopts the joint probability density function (PDF) to close higher‐order turbulence and diagnose the cloud fraction. CLUBB assumes the equal PDF width of vertical velocity (w $w$) as the default closure scheme, called Analytic Double Gaussian 1 (ADG1). In this study, a new unequal width PDF closure scheme is adopted in CLUBB, in which the PDF width of w $w$ is assumed to be associated with skewness and kurtosis of vertical velocity. Offline calculation shows that the updated closure scheme improves the tail in the joint PDF and produces the saturation area from the joint PDF of the total water and liquid water potential temperature closer to that derived from large‐eddy simulations than ADG1. The new PDF scheme implemented in Community Atmosphere Model Version 6 is shown to increase low cloud fraction by 20%–30% in most stratocumulus‐to‐cumulus transition regions. The updated closure scheme is found to improve the low cloud simulation by simulating more symmetric turbulence in stratocumulus and stronger turbulent transport in shallow cumulus. Furthermore, the updated closure scheme enhances the cloud fraction near the cloud base by interacting with the microphysical scheme. Our work indicates the importance of continuous improvement in the PDF closures for higher‐order turbulence schemes. Plain Language Summary: Low clouds play an important role in global energy and water cycles, and their simulations in global climate models remain challenging. Here we improved the turbulence parameterization scheme in a climate model. The new scheme updates the width assumption of vertical velocity, temperature and moisture distribution, which improves the representation of extreme skewed turbulence and makes the turbulence parameterization closer to physical reality. The updated scheme is shown to improve the distribution of moisture and temperature in better agreement with high‐resolution simulation results, and increase low cloud fraction by 20%–30% in most low cloud regions compare with observations. Key Points: An updated unequal width probability density function (PDF) closure scheme is implemented in Community Atmosphere Model Version 6The new PDF closure scheme produces the joint PDF of vertical velocity, total water and liquid water potential temperature closer to large‐eddy simulation resultsThe new PDF closure scheme improves low cloud fraction simulations by 20%–30% in most stratocumulus‐to‐cumulus transition regions [ABSTRACT FROM AUTHOR]