Preliminary results on implementing a third-order turbulence closure in the Community Atmosphere Model

Description

This presentation describes the implementation and testing of a higher-order turbulence closure, an intermediately prognostic higher-order turbulence closure (IPHOC), into the Community Atmosphere Model (CAM) 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 skewed 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 for the coarse general circulation model (GCM) vertical grid-spacing.

The IPHOC replaces PBL, shallow convection, and cloud macrophysics parameterizations in CAM5. The coupling of CAM5 with IPHOC (CAM5-IPHOC) represents a more unified treatment of boundary-layer and shallow convective processes. Results from global simulations are presented and suggest that CAM5-IPHOC can provide a better treatment of boundary-layer clouds and processes, when compared to CAM5. However, the representation of the cloud processes in the horizontal scale of 100 km is challenging for the IPHOC, which is normally used to parameterize the PBL processes in the horizontal scale of 1 km. The global annual mean low cloud fraction and precipitation are compared among CAM5, CAM5-IPHOC, and a multi-scale modeling framework model with IPHOC. A few vertical cross-section plots along the GCSS/WGNE Pacific Cross-Section Intercomparison (GPCI) line and the south-east Pacific will also be shown in the poster.