A newly identified cloud oscillation in atmospheric model simulations
Klein, Stephen - Lawrence Livermore National Laboratory
Area of research:
Journal Reference:Zheng X, S Klein, H Ma, P Caldwell, V Larson, A Gettelman, and P Bogenschutz. 2017. "A cloudy planetary boundary layer oscillation arising from the coupling of turbulence with precipitation in climate simulations." Journal of Advances in Modeling Earth Systems, 9(4), doi:10.1002/2017MS000993.
With a variety of modeling approaches, Lawrence Livermore National Laboratory scientists in the Atmospheric, Earth, and Energy Division uncovered properties and causes of a newly identified cloud oscillation in atmospheric model simulations, and reported on the physically consistent changes to model formulations that can effectively control the oscillation.
This study uncovers model deficiencies in the simplified coupling of turbulence with precipitation and in the treatment of the precipitation evaporation. This study may draw the attention of the modeling and observational communities to the need for consistency between the coupled sub-grid processes in atmospheric models.
Current atmospheric model development for the representation of moist convection and turbulence is trending toward a more unified treatment, which makes interactions between parameterizations tighter and more explicit. In this study, a new cloudy planetary-boundary-layer (PBL) oscillation related to interaction between parameterizations is identified in the Community Atmosphere Model (CAM). This oscillation occurs most often in the marine cumulus cloud regime. The oscillation occurs only if the modeled PBL is strongly decoupled and precipitation evaporates below the cloud. Global simulations prove that turning off the evaporation-variance coupling and improving the precipitation fraction parameterization can effectively reduce the cloudy PBL oscillation in marine cumulus clouds. This study uncovers model deficiencies in the simplified coupling of turbulence with precipitation and in the treatment of the precipitation evaporation, and highlights the need for consistency between the coupled sub-grid processes in atmospheric model development.