AOGCM: Atmosphere

The atmospheric component of the GEOS-5 AOGCM is Fortuna-2.5, the same that was used for the Modern-Era Retrospective Analysis for Research and Applications (MERRA), but with adjusted parametrization of moist processes and turbulence (Molod et al. 2012; Rienecker et al. 2008). The model has a finite volume dynamical core (Lin 2004) which is integrated with various physical packages through the ESMF.

The physics package includes parametrization of moist processes, radiation, turbulent mixing and surface fluxes. The moist component contains parametrization of convection using the Relaxed Arakawa-Schubert scheme (Moorthi and Suarez 1992), and the large scale precipitation and cloud cover model as described in Bacmeister et al. (2006). The radiation component includes parametrization for long wave (Chou 1990, 1992) and short wave radiation processes (Chou and Suarez 1994). The turbulence component consists of parametrization for vertical diffusivity, planetary boundary layer (PBL) and gravity wave drag (GWD). The free atmospheric turbulent diffusivities are based on the gradient Richardson number. The parametrization of PBL is based on Lock et al. (2000) scheme, acting together with scheme of Louis and Geleyn (1982). The Lock et al. (2000) scheme includes a representation of non-local mixing (driven by both surface fluxes and cloud-top processes) in unstable layers, either coupled to or decoupled from the surface, and an explicit entrainment parametrization. The original scheme was extended in GEOS-5 to include moist heating and entrainment in the unstable surface parcel calculations. GEOS-5 incorporates two gravity wave drag parameterizations, an orographic gravity wave drag formulation based on McFarlane (1987), and a formulation for non-orographic waves based on Garcia and Boville (1994). The surface exchange of heat, moisture and momentum between the atmosphere and land, ocean or sea ice surfaces are treated with a bulk exchange formulation based on Monin-Obukhov similarity theory.