Improved Low-cloud Simulation from the Community Atmosphere Model with a Third-order Turbulence Closure

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Description

This study describes the implementation and testing of a higher-order turbulence closure, an intermediately-prognostic higher-order turbulence closure (IPHOC), into the Community Atmosphere Model version 5 (CAM5). The third-order turbulence closure introduces a joint double-Gaussian distribution of liquid water potential temperature, total water mixing ratio, and vertical velocity to represent any turbulence circulations. The distribution is inferred from the first-, second-, and third-order moments of the variables given above and is used to diagnose cloud fraction and grid-mean liquid water mixing ratio, as well as the buoyancy term and fourth-order terms in the equations describing the evolution of the second- and third-order moments. In addition, a diagnostic planetary boundary layer (PBL) height approach has been incorporated in IPHOC in order to resolve the strong inversion above PBL. The IPHOC replaces PBL, shallow convection, and cloud macrophysics parameterizations in CAM5. The coupling of CAM5 with IPHOC (CAM5-IP) represents a more unified treatment of boundary layer and shallow convective processes. Results from global climate simulations are presented and suggest that CAM5-IP can provide a better treatment of boundary layer clouds and turbulence processes, when compared to CAM5. The low cloud amounts near the west coast of the subtropical continents are well produced in CAM5-IP and the global and annual mean low cloud fraction is within 5% of satellite observations. A more reasonable cloud regime transition from low to deep convective clouds is produced in CAM5-IP than in CAM5. The vertical structures in cloud fraction and condensate are improved. Although the potential for realistic simulation of cloud processes is great with the IPHOC approach, there are many challenges. Firstly, the short time step needs to increase for the scheme to be extensively used. Secondly, the global and annual mean liquid water path is still underestimated. Using IPHOC to replace the deep convection scheme may be an approach to solve this problem because the deep convective scheme does not produce enough liquid water in CAM5. Finally, the correlation of the surface precipitation between CAM5-IP and GPCP observation decreases. This may be related to the coupling between IPHOC and other physical processes, such as microphysics and deep convective processes.