Design and Integration of Submodels in the Modular Earth Submodel System (MESSy): A Case Study with JSBACH

Abstract. The Modular Earth Submodel System (MESSy) provides a flexible framework for coupling and managing specific physical, chemical, or biological processes in the earth system within a comprehensive Earth system model. The central component of these framework is the so called ’basemodel’, which typically represents core dynamical processes such as energy and mass conservation. It describes the resolved state and tendencies to the dynamical state variables, e.g. temperature, wind and humidity. However, additional unresolved processes, such as chemical reactions, aerosol dynamics, or biological interactions, often require more specialized description, which are represented by dedicated ’submodels’. A submodel system like MESSy enables the seamless integration of these additional components, allowing researchers to extend and refine the functionality of a basemodel according to their scientific objectives. Thanks to its modular structure, MESSy allows users to activate or deactivate individual submodels as required, depending on which processes are being studied. This design improves computational efficiency by focusing resources on relevant mechanisms, and supports controlled comparisons between different process representations, such as varying cloud, radiation, chemistry or land-surface models, under identical experimental conditions. This modularity is technically achieved by adhering to MESSy’s coding standards and architecture, which are implemented in Fortran. The framework distinguishes between a central core layer and multiple submodel layers that communicate via well-defined interfaces. Once compiled, simulations can be reconfigured dynamically via namelists, offering a high degree of flexibility for model experimentation and eliminating the need for recompilation. As an example, the land surface and vegetation model JSBACH (Jena Scheme for Biosphere–Atmosphere Coupling in Hamburg) was coupled to the ECHAM/MESSy Atmospheric Chemistry General Circulation Model (EMAC) to improve surface hydrology representation add a dedicated vegetation scheme and several additional couplings between land and atmosphere. JSBACH substantially enhances the model’s versatility, offering many additional output variables with minimal impact on computational performance. Its development illustrates both the scientific and technical challenges of translating physical processes into modular Fortran code while following the MESSy’s coding standards. The resulting system demonstrates how a modular submodel architecture can enhance adaptability, reproducibility, and computational efficiency in earth system modeling.