The REMO-API: Steps towards a flexible regional earth system model

The regional Earth System Model REMO.

The regional climate model (RCM) REMO was originally developed at the Max-Planck-Institute for Meteorology, nowadays, it is further developed and maintained by the Climate Service Center Germany (GERICS) in Hamburg, Germany. Like most other RCMs, REMO has been developed starting from an existing numerical weather prediction (NWP) model: the Europa-Modell (EM) of the German Weather Service DWD. Additionally, the physical parameterization package of the general circulation model ECHAM4 has been implemented. In numerous studies, the combination of the EM dynamical core plus the ECHAM4 physical parameterization schemes proved its ability to realistically reproduce regional climatic features and is therefore used as the standard setup in recent applications.

REMO is configured to run on almost all regional setups of the World Climate Research Programme Coordinated Regional Downscaling Experiment (WCRP-CORDEX) and, therefore, requires different configurations depending on the region of interest. Consequently, the model has been continuously developed by its user community resulting in a number of different coupled and configured REMO versions. The atmospheric component of REMO is coupled to three different hydrology models and three ocean/sea-ice models. A chemistry module for tropospheric chemistry is also available as well as a number of subroutine models such as a glacier model or the lake model FLAKE.

However, due to its long history of legacy codes, the REMO model is not easy to maintain and the different coupled model versions are usually based on different REMO versions and are difficult to reintegrate into the main branch. At the same time, the model is continuously developed towards a high-resolution model (from 50km originally to ~3km today). A non-hydrostatic dynamical core has been introduced into the latest version called REMO-NH, which requires an update of the original ECHAM4 parameterizations, e.g., new turbulence and micro-physic schemes.

It is highly desirable to reintegrate coupled REMO legacy developments into the new REMO-NH model to evolve the non-hydrostatic model version into a flexible regional Earth system model which can be coupled to other models of the HGF-ESM project. Therefore, we have started to restructure the model components into separate software units which can be combined and managed on a meta level using a suite of python tools. Each unit represents a certain model component and implements an interface that is defined at the top of the model tree hierarchy. A carefully and strict definition of interfaces allows different parallel implementations of a specific component to be maintained separately. In addition, it opens up the possibility for external couplings to be included much easier and more flexible than before since the model data can be managed in a centralized way including meta data information for easy access and exchange with external and internal model components.

A clear separation of model components and strict interface design also allows for much easier community developments to be reintegrated and promotes a lively interaction of different expertise to come together in an elegant development process. The work on this is ongoing at GERICS in the context of the HGF-ESM project.