Diagnostic studies of ECHAM GCMs

Abstract

The two latest generations of MPI ECHAM AGCMs, ECHAM3 and ECHAM4, have been performed the test run in relatively high resolution (T106) with the prescribed AMI boundary conditions. There are major changes made in ECAHM4 T106 from ECHAM3 T106 in the radiation scheme, the treatment of radiation absorption by water vapor and cloud water and the calculation methods of advection of water vapor and cloud water, etc. It is shown that the simulation of annual mean heat balance at ocean surface has been greatly improved in ECHAM4 T106 with respect to the GEBA observations. While the annual mean state is important to the global climate system's heat and water balance, the distribution of global heat and water divergence determines the energy and water mass transport in the atmosphere and between the ocean and atmosphere. The diagnostic studies of ECHAM AGCMs' implied oceanic meridional heat transport and their comparison with available observational data, and the break-down of model annual surface heat balance terms highlight the importance of model's treatment in radiation absorption of atmospheric water vapor, the cloud radiation forcing calculation and latent heat contribution from hydrological balance requirement. The ECHAM GCMs' response with the ocean-atmosphere boundary condition is also investigated with MIT's ECHAM4 T42 datasets, which are obtained with the two different boundary conditions. The ECHAM4 model is capable in simulating the annual mean implied oceanic meridional heat transport with fare accuracy in T42 resolution, while the interannual variation is clearly shown.