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Modeling Framework: Flexible Modeling System (FMS)

Today’s climate models simulate highly complex systems. In response to increasingly complex models, the climate community has developed tools and methodologies to facilitate the modeling process and many common tasks (e.g., calendar management, grid generation, I/O). Such frameworks come with a number of advantages, including decreased model development time and increased compatibility of interfaces.

The Flexible Modeling System (FMS) is a software environment that supports the efficient development, construction, execution, and scientific interpretation of atmospheric, oceanic, and climate system models. This framework allows algorithms to be expressed on a variety of high-end computing architectures using common and easy-to-use expressions of the underlying platforms, spanning distributed and shared memory, as well as high-performance architectures. Scientific groups at GFDL can develop new physics and new algorithms concurrently, and coordinate periodically through this framework.

FMS Framework

Modeling frameworks for the construction of coupled models, made from independent model components, are now prevalent across this field. FMS was one of the first frameworks to be developed — since the advent of the Cray T3E in 1998 — and is still in use and under development today, using new architectures and new algorithms.

What is FMS

The Flexible Modeling System (FMS) is a software framework for supporting the efficient development, construction, execution, and scientific interpretation of atmospheric, oceanic, and climate system models. FMS consists of the following:

  1. A software infrastructure for constructing and running atmospheric, oceanic, and climate system models. This infrastructure includes software to handle parallelization, input and output, data exchange between various model grids, orchestration of the time stepping, makefiles, and simple sample run scripts. This infrastructure should largely insulate FMS users from machine-specific details.
  2. A standardization of the interfaces between various component models including software for standardizing, coordinating, and improving diagnostic calculations of FMS-based models, and input data preparation for such models. Common preprocessing and post-processing software are included to the extent that the needed functionality cannot be adequately provided by available third-party software.
  3. Contributed component models that are subjected to a rigorous software quality review and improvement process. The development and initial testing of these component models is largely a scientific question, and would not fall under FMS. The quality review and improvement process includes consideration of (A) compliance with FMS interface and documentation standards to ensure portability and inter-operability, (B) understandability (clarity and consistency of documentation, comments, interfaces, and code), and (C) general computational efficiency without algorithmic changes.
  4. A standardized technique for version control and dissemination of the software and documentation.

FMS does not include the determination of model configurations, parameter settings, or the choice amongst various options. These decisions require scientific research. Similarly, the development of new component models is a scientific concern that is outside of the direct purview of FMS. Nonetheless, infrastructural changes to enable such developments are within the scope of FMS. The collaborative software review process of contributed models is therefore an essential facet of FMS.

Dependencies and installation

The following external libraries are required when building libFMS

  • NetCDF C and Fortran (77/90) headers and libraries
  • Fortran 2003 standard compiler
  • Fortran compiler that supports Cray Pointer
  • MPI C and Fortran headers and libraries (optional)
  • Libyaml header and libraries (optional)
  • Linux or Unix style system

Please see the Build and Installation page for more information on building with each build system.

Compiler Support

For most production environments and large scale regression testing, FMS is currently compiled with the Intel classic compiler (ifort) but will be transitioning to the llvm-based ifx intel compiler when it is available for production.

Below shows the status of our compiler support for various compilers and versions. Testing was done on CentOS 8, with additional testing using a larger cray SLES system. MPICH is used as the MPI library except for the intel compilers, which use intel's mpi library. Compilers used in our Github continuous integration testing are in bold.

Compiler Version Builds Successfully Unit Testing
intel classic(ifort) 2021.6.0 yes passes
GNU (gfortran) 9.3.0 yes passes
intel oneapi (ifx) 2021.6.0 yes passes
GNU (gfortran) 11.2.0 yes passes
HP/Cray (cce) 9.1.1 yes not passing
Nvidia/PGI(nvfortran) 22.9 no not passing
AMD (aocc) 3.2.0 no(compiles,fails to link) not passing


Source code documentation for the FMS code base is available at The documentation is generated by doxygen and updated upon releases, and a copy of the site can be obtained through the gh-pages branch or generated manually with ./configure --enable-docs && make -C docs. For more information on documentating the code with doxygen please see the documentation style guide.


The United States Department of Commerce (DOC) GitHub project code is provided on an 'as is' basis and the user assumes responsibility for its use. DOC has relinquished control of the information and no longer has responsibility to protect the integrity, confidentiality, or availability of the information. Any claims against the Department of Commerce stemming from the use of its GitHub project will be governed by all applicable Federal law. Any reference to specific commercial products, processes, or services by service mark, trademark, manufacturer, or otherwise, does not constitute or imply their endorsement, recommendation or favoring by the Department of Commerce. The Department of Commerce seal and logo, or the seal and logo of a DOC bureau, shall not be used in any manner to imply endorsement of any commercial product or activity by DOC or the United States Government.

This project code is made available through GitHub but is managed by NOAA-GFDL at