CLIMATE SENSITIVITY OF THE NCAR COMMUNITY CLIMATE MODEL (CCM2) TO HORIZONTAL RESOLUTION

The dependence on horizontal resolution of the climate simulated by th e National Center for Atmospheric Research Community Climate Model (CC M2) is explored. Simulations employing R15, T21, T31, T42, T63, and T1 06 horizontal spectral truncations are compared. Parameters associated with the diagnostic cloud scheme are modified for each resolution to provide similar global average cloud radiative forcing at each resolut ion. Overall, as with earlier studies, there are large differences bet ween the low resolution R15 and T21 simulations and the medium resolut ion T42 simulation. Many climate statistics show a monotonic signal wi th increasing resolution, with the largest variation occurring from lo w to medium resolution. Although the monotonic signal is often from th e low resolution simulations toward atmospheric analyses, in some case s it continues beyond the analyses at the highest resolution. Where co nvergence occurs, it is not always to the atmospheric analyses, and th e highest resolution simulations are not the best by all measures. Alt hough many climate statistics converge, the processes that maintain th e climate do not, especially when considered on a regional basis. The implication is that the finer scales are required to capture the nonli near processes that force the medium scales. Overall, it appears that, at a minimum, T42 resolution is required, but higher resolution would be better. Applications at T42 should take into consideration how mod el errors indicated by these resolution signals might affect any findi ngs.