MECO(n) stands for MESSy-fied ECHAM and COSMO models nested n times. It features an on-line coupling of the global model EMAC with the regional model COSMO/MESSy. On-line coupling means, that the necessary boundary data for the regional model are exchanged during run-time between the EMAC and COSMO/MESSy instances, i.e. the global and regional model instances run in parallel. The exchange of the data is realized via single-sided, non-blocking MPI communication.
MECO(n) features an arbitrary number of different model instances. A global EMAC instance can drive various COSMO/MESSy instances at different geographical locations at the same time, and the COSMO/MESSy instances themselves can drive various finer resolved COSMO/MESSy instances in different regions, which themselves could drive further COSMO/MESSy nests.
More information on MECO(n) and an evaluation are provided by:
Kerkweg, A. and Jöckel, P.: The 1-way on-line coupled atmospheric chemistry model system MECO(n) – Part 1: Description of the limited-area atmospheric chemistry model COSMO/MESSy, Geosci. Model Dev., 5, 87–110, https://doi.org/10.5194/gmd-5-87-2012, 2012.
Hofmann, C., Kerkweg, A., Wernli, H., and Jöckel, P.: The 1-way on-line coupled atmospheric chemistry model system MECO(n) – Part 3: Meteorological evaluation of the on-line coupled system, Geosci. Model Dev., 5, 129–147, https://doi.org/10.5194/gmd-5-129-2012, 2012.
Mertens, M., Kerkweg, A., Jöckel, P., Tost, H., and Hofmann, C.: The 1-way on-line coupled model system MECO(n) – Part 4: Chemical evaluation (based on MESSy v2.52), Geosci. Model Dev., 9, 3545–3567, https://doi.org/10.5194/gmd-9-3545-2016, 2016.
Kerkweg, A., Hofmann, C., Jöckel, P., Mertens, M., and Pante, G.: The on-line coupled atmospheric chemistry model system MECO(n) – Part 5: Expanding the Multi-Model-Driver (MMD v2.0) for 2-way data exchange including data interpolation via GRID (v1.0), Geosci. Model Dev., 11, 1059–1076, https://doi.org/10.5194/gmd-11-1059-2018, 2018.

MECO(n) global-regional model system with refinements over Europe. Image: DLR, CC BY-NC-ND 3.0