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What is EMAC?

EMAC (ECHAM/MESSy Atmospheric Chemistry) is a numerical chemistry-climate model. It couples the ECHAM5 atmospheric general circulation model, which computes winds, temperature, clouds and radiation on a 3-D global grid, with the Modular Earth Submodel System (MESSy), which supplies chemistry, emissions, aerosol and other processes as interchangeable submodels. It is used to study the composition of the atmosphere from the surface to the middle atmosphere, and how it responds to emissions and climate change.

How it works

The atmosphere is divided into a grid of boxes in longitude, latitude and height. In every time step, the model:

1. Computes the physics of the atmosphere (winds, temperature, humidity, clouds, radiation).
2. Transports every chemical species with the winds, including vertical transport by convection and mixing in the boundary layer.
3. Adds emissions from the surface (anthropogenic, biomass burning, vegetation), and lightning NO_x.
4. Runs the chemistry in each grid box: the same kind of calculation as in a box model, but done for every box.
5. Removes material by dry deposition and wet scavenging.
6. Writes output such as 3-D fields of ozone, OH, VOCs and aerosol.

Technical details (click to open)

Resolution and vertical range

EMAC is run at a chosen horizontal resolution (set by the spectral truncation, for example T42, roughly 2.8° × 2.8°) and a chosen number of vertical levels, from the surface up to the middle atmosphere in some configurations. Finer resolution captures more detail but needs far more computing time, so it is chosen according to the question.

Free-running vs. "nudged" simulations

• Free-running simulations compute their own weather. They are suited to long-term climate questions and statistics.
• Nudged (specified-dynamics) simulations push the model's winds and temperature towards meteorological reanalysis data, so that the simulated weather follows real events. This allows direct comparison with measurements from a given day, flight or campaign.

Chemistry

Chemistry is handled by MECCA, the same module used in the CAABA box model, so the mechanism can be shared between the two. Detailed organic chemistry can be added with the Mainz Organic Mechanism, described in Pozzer et al. (2022), who evaluated EMAC with it.

Computing

EMAC is a large Fortran code run in parallel on supercomputers. A typical set-up involves choosing a configuration, preparing input files (emissions, boundary conditions) and submitting a batch job. Output is stored as netCDF files and analysed with tools such as Python, Ferret or NCL.

EMAC and the box model

	CAABA/MECCA box model	EMAC
Scope	One air parcel	The whole globe, 3-D
Transport	None	Winds, convection, mixing
Chemistry	MECCA	MECCA (same mechanism)
Cost	Seconds to minutes on a laptop	Hours to days on a supercomputer
Best for	Understanding the chemistry in detail	Regional and global distributions, budgets, scenarios

The two complement each other: a box model reveals which reactions matter in an observed air mass, and a global model like EMAC shows how widely that matters once transport and emissions are included.

Key references

• Jöckel, P. et al. (2006). The atmospheric chemistry general circulation model ECHAM5/MESSy1. Atmos. Chem. Phys., 6, 5067–5104.
• Jöckel, P. et al. (2010). Development cycle 2 of the Modular Earth Submodel System (MESSy2). Geosci. Model Dev., 3, 717–752.
• Roeckner, E. et al. (2006). Sensitivity of simulated climate to horizontal and vertical resolution in the ECHAM5 atmosphere model. J. Climate, 19, 3771–3791.
• Pozzer, A. et al. (2022). Simulation of organics in the atmosphere: evaluation of EMACv2.54 with the Mainz Organic Mechanism. Geosci. Model Dev., 15, 2673–2710. doi:10.5194/gmd-15-2673-2022

This is a general technical overview of the model, not a description of specific simulations. For details, consult the references above and the MESSy documentation.