The EMAC (ECHAM-MESSy atmospheric model) uses ECHAM as the dynamical core to drive MESSy:

Modular Earth Submodel System (MESSy) is a software providing a framework for a standardized, bottom-up implementation of Earth System Models (or parts of those) with flexible complexity. "Bottom-up" means, the MESSy software provides an infrastructure with generalized interfaces for the standardized control and interconnection (=coupling) of "low-level ESM components" (dynamic cores, physical parameterizations, chemistry packages, diagnostics etc.), which are called submodels. MESSy comprises currently about 60 submodels (i.e., coded MESSy conform):

• infrastructure (= the framework) submodels
• diagnostic submodels
• atmospheric chemistry related submodels
• model physics related submodels

MESSy is a multi-institutional project.

The main design concept of MESSy is the strict separation of process description (=process and diagnostic submodels) from model infrastructure (e.g., memory management, input/output, flow control, ...).

Within MESSy, the operator splitting is formalized as the fundamental concept. Model codes are organized in 4 conceptual software layers, object oriented approaches are utilized as far as possible in the used programming language standard, and as far as feasible with respect to the computational performance.

The MESSy approach offers several benefits: A scalable model development is achieved and several different implementations of processes and diagnostics can coexist in the same model code. Nonetheless, the overall complexity remains controllable in a transparent and user friendly way. A high flexibility is achieved through the modularity, providing a research tool for a large community serving a wide variety of scientific needs.