Convergence of atmospheric simulations with increasing horizontal resolution and fixed forcing scales

A series of experiments with the NCAR Community Climate Model, Version 2 (CCM2) is examined in which the grid and scale of the physical parameterizations are held fixed while the horizontal resolution of the dynamical core is increased. The convergence characteristics of these dual-resolution simulations are compared to those of simulations with the standard model in which the resolutions of both the dynamical core and physical parameterizations are increased together, as traditionally done in atmospheric model convergence studies. With the standard model the upward branch of the local Hadley circulation increases in strength with increasing horizontal resolution and does not converge by T170 truncation. As the dynamical resolution is increased, but the parameterization resolution held fixed, the dual-resolution model simulations converge to a state close to that produced by the standard model at the fixed parameterization resolution. The mid-latitude transient aspects do not converge with increasing resolution when the scale of the physics is held fixed. The nonlinear interactions in the dynamics create finer scales, with or without the finer scale forcing. However, the lower resolution T42 scales appear to converge. These convergence characteristics are shared by both the dual-resolution model and the standard model when horizontal resolution is increased.