Simulation of the present-day climate with the ECHAM model: Impact of model physics and resolution

A detailed description of the third-generation atmospheric general circulation model ECHAM is presen- ted. The climatology of ECHAM3, as simulated at low and high resolution (T21 and T42, respectively), is compared with ECMWF analyses (1981-1990) and also with the climatology of two earlier low reso- lution versions ECHAM 1 and ECHAM2. At low resolution, the impact of the improved model physics is evident primarily in the simulated time- mean state which is more successfully reproduced by the more recent model versions, particularly by ECHAM3. The impact of increased horizontal resolution can be identified not on] v by a generally impro- ved time-mean circulation, particularly in the Southern Hemisphere, but mosbsignificantly by the incre- ased level of high-frequency variability. In the low-frequency range, however, the impact of increased horizontal resolution is modest, and all models fail to reproduce the observed level of low-frequency intraseasonal variabilit