The atmospheric general circulation model ECHAM5 Part II: Sensitivity of simulated climate to horizontal and vertical resolution

The most recent version of the Max Planck Institute for Meteorology atmospheric general circulation model, ECHAM5, is used to study the impact of changes in horizontal and vertical resolution on seasonal mean climate. In a series of AMIP-style experiments with resolutions ranging between T21L19 and T159L31, the systematic errors and convergence properties are assessed for two vertical resolutions. At low vertical resolution (L19) there is no evidence for convergence to a more realistic climate state for resolutions higher than T42. At higher vertical resolution (L31), on the other hand, the root-mean-squared-errors decrease monotonously with increasing horizontal resolution. Furthermore, except for T42, the L31 versions are superior to their L19 counterparts, and the improvements become more evident at increasingly higher horizontal resolutions. This applies, in particular, to the zonal mean climate state and also to stationary wave patterns in boreal winter. As in previous studies, increasing horizontal resolution leads to a warming of the troposphere, most prominently at mid-latitudes and, hence, to a poleward shift and intensification of the mid-latitude westerlies. Increasing the vertical resolution has the opposite effect, almost independent of horizontal resolution: Whereas the atmosphere is colder at low and middle latitudes, it is warmer at high latitudes and close to the surface. In addition, increased vertical resolution results in a pronounced warming in the polar upper troposphere and lower stratosphere where the cold bias is reduced by up to 50% compared to L19 simulations. Consistent with these temperature changes is a decrease and equatorward shift of the mid-latitude westerlies. The substantial benefits in refining both horizontal and vertical resolution are in accord with scaling arguments deduced from quasi-geostrophic theory implying that horizontal and vertical resolution ought to be chosen consistently