The atmospheric general circulation model ECHAM-4: Model description and simulation of present-day climate

TLDR

Compared to the previous ECHAM3, the fourth‑generation ECHAM‑4 introduces substantial numerical and physical changes. The paper presents a detailed description of the ECHAM‑4 model. ECHAM‑4 incorporates a semi‑Lagrangian transport scheme, a revised ECMWF radiation scheme, turbulence‑based vertical diffusion, a convection closure based on instability, updated diffusion, cloud, and land‑surface parameterizations, a new land‑surface dataset, and its climatology from two extended AMIP T42L19 runs is compared with ECMWF analyses. Biases from ECHAM3 persist, with a too‑cold polar upper troposphere/lower stratosphere, large zonal wind errors above 200 hPa, and still low low‑frequency variability, though reduced by about 50 % relative to ECHAM3.

Abstract

A detailed description of the fourth-generation ECHAM model is presented. Compared to the previous version, ECHAM3, a number of substantial changes have been introduced in both the numerics and physics of the model. These include a semi-Lagrangian transport scheme for water vapour, cloud water and trace substances, a new radiation scheme (ECMWF) with modifications concerning the water vapour continuum, cloud optical properties and greenhouse gases, a new formulation of the vertical diffusion coefficients as functions of turbulent kinetic energy, and a new closure for deep convection based on convective instability instead of moisture convergence. Minor changes concern the parameterizations of horizontal diffusion, stratiform clouds and land surface processes. Also, a new dataset of land surface parameters have been compiled for the new model. The climatology of the model, derived from two extended AMIP simulations at T42L19 resolution, is documented and compared with ECMWF operational analyses. Some of the biases noted for the previous model version remain virtually unchanged. For example, the polar upper troposphere and lower stratosphere is much too cold, and the zonal wind errors become very large above the 200 hPa level. Furthermore, the low-frequency variability is still too small but the errors are reduced by about 50% compared to ECHAM3.