The atmosphere is very similar to the GFDL Atmospheric Model 2 (AM2), as described by the GFDL Global Atmospheric Model Development Team (GFDL GAMDT, 2004), but using the finite volume grid of Lin (2004), as implemented in CM2.1 (Delworth et al., 2004). The atmosphere used here employs the M30 grid, with a latitudinal resolution of 3 degrees and a longitudinal resolution of 3.75 degrees. It has 24 vertical levels, the same number as used in CM2.1. This results in 44% of the number of grid cells in the CM2.1 atmosphere, a relatively modest decrease in resolution. However, the atmospheric time steps are also increased accordingly, from 0.5 to 1.5 hours for the tracer time step, from 6 to 9 minutes for the baroclinic timestep, and from 2 to 3 hours for the radiative time step. The coarsened temporal discretization leads to an additional overall decrease in computational cost.
The change in discretization required a small readjustment of two cloud parameters in order to maintain a net radiation balance close to zero under modern climate, which had negligible impact on atmospheric dynamics. In addition, the mountain gravity drag parameter G* (GFDL GAMDT 2004) was decreased from 1.0 to 0.5 in order to compensate for the coarser horizontal resolution. The final alteration made was the replacement of the sea salt aerosol climatology of Haywood et al. (1999), used in the radiation calculation, with a more recent estimate from Ginoux et al. (2006). As in CM2.1, the land component is the Land Dynamics model of Milly and Shmakin (2002), which includes a river routing scheme but no terrestrial ecosystem (see GFDL GAMDT, 2004).