The atmospheric general circulation model ECHAM 5. PART I: Model description

A detailed description of the fifth-generation ECHAM model is presented. Compared to the previous version, ECHAM4, a number of substantial changes have been introduced in both the numerics and physics of the model. These include a flux-form semi-Lagrangian transport scheme for positive definite variables like water components and chemical tracers, a new longwave radiation scheme, separate prognostic equations for cloud liquid water and cloud ice, a new cloud microphysical scheme and a prognostic-statistical cloud cover parameterization. The number of spectral intervals is increased in both the longwave and shortwave part of the spectrum. Changes have also been made in the representation of land surface processes, including an implicit coupling between the surface and the atmosphere, and in the representation of orographic drag forces. Also, a new dataset of land surface parameters has been compiled for the new model. On the other hand, horizontal and vertical diffusion, cumulus convection and also the spectral dynamics remain essentially unchanged