The numerical global atmosphere-chemistry model EMAC (ECHAM/MESSy Atmospheric Chemistry) is a modular global climate and chemistry simulation system with sub-models for the calculation of processes in the troposphere and the middle atmosphere, their interaction with the ocean and land surfaces, and of anthropogenic influences. The dynamic model ECHAM5 uses the spectral transformation method and was developed at the Max-Planck Institute for Meteorology and the University of Hamburg. It is based on the weather forecast model of the European Centre for Medium-Range Weather Forecast. ECHAM5 is the fifth generation of this model. It can be operated in different horizontal and vertical resolutions.
The Modular Earth Submodel System MESSy currently comprises more than 40 submodels to describe processes and to perform diagnoses; among them the ocean model MPIOM, the Lagrangian transport scheme ATTILA, the chemistry model MECCA and many others. Because of its modularity EMAC can be operated in several modes: as general circulation model (GCM) or as a quasi chemistry transport model (QCTM) without feedbacks from the chemistry to the dynamics. All modes optionally include an ocean model. Furthermore, it is possible to nudge the model with meteorological analyses or re-analyses data to allow for a direct comparison with observations.
ECHAM calculates the development of weather systems (temperatures, winds, clouds etc.) and optionally the chemical composition of the atmosphere around the globe with a horizontal resolution of 100 to 500 km and with time steps between 5 and 40 minutes (depending on the spatial resolution).