The Evolution and Anatomy of the GEOS Atmospheric Model

Matthew Thompson

The Goddard Earth Observing System Atmospheric General Circulation Model (GEOS AGCM) has been designed in a modular fashion using what are known as “grid components.” This componentized design, which is based on the Earth System Modeling Framework (ESMF), facilitates the import and addition of new modules (grid components) that allow the model to compute impacts of different physical, chemical and biological processes. This modular approach eases the management and maintenance of the GEOS model code base: various applications of the GEOS model may require either simple or more complex representations of different processes. For instance, the GEOS FP system used for weather analysis and prediction uses a very simple representation of atmospheric chemistry (linearized production rates and loss frequencies), while the GEOS Chemistry-Climate Model (GEOS CCM) invokes a detailed chemistry mechanism in order to investigate interactions between ozone loss and the atmospheric circulation. Note that the ocean/ice components of GEOS are not captured in this visualization.

Credit: This illustration and the accompanying animation were made using the open-source visualization software package, Gource.

This image shows the “anatomy” of the GEOS model base code, valid in April 2017. Clusters of grid components provide the functional capabilities. To the right-hand side of the image, the available dynamical cores are included in a number of grid components — three versions of the “Finite-Volume” (FV) dynamical core (Lin, 2004) along with the classic “Aries” dynamical core (Suarez et al., 1995) are options for the GEOS model. Grid components for computing physical processes are shown in the central region of the illustration, at which point the branches to the upper, lower and left-most regions illustrate the multiple options available to the user. Modules for atmospheric chemistry computations provide some of the more interesting structures, with options available for “linearized” chemistry (PChem), and the implementations into the CCM of stratospheric ozone chemistry (Stratchem: Pawson et al., 2008) and NASA’s “Global Modeling Initiative” (GMI COMBO) troposphere-stratosphere chemistry package (Oman et al., 2013), and most recently the addition the GEOS-Chem chemistry module (CITATION) into the GEOS infrastructure. Likewise, multiple options exist for computing aerosol distributions: the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART; Colarco et al., 2010), Modular Aerosol Module (MAM), and Carma models are included in their own grid components.

The color-coded key (upper left) highlights the various types of files used in the model. The majority are Fortran (colored red), the language which represents the bulk of the model. Fortran has long been the dominant programming language of choice in General Circulation Models and this continues as models such as GEOS evolve to include many facets of Earth System Science simulation. The next largest is the resource-control (RC) files (colored yellow). These files are used by the model to provide information on how the model should run, what files that should be used for input, and other data that can change from run to run. The largest user of RC files at the left-center is the bright yellow branch corresponding to the GOCART aerosol model, which makes extensive use of RC files to control various scenarios corresponding to different sets of emissions. The rest of the colors represent the other sundry files used in a large model, such as Makefiles to control the build process, Perl and Python scripts for various pre- and post-processing tasks, etc.

The animation below shows the growth of the GEOS atmospheric model source code, compiled by accessing the time history of the model code using Gource. The striking aspects of the animation include the early configuration as a “traditional” atmospheric general circulation model (AGCM) with modules for “dynamics,” “atmospheric physics,” and “land surface” followed by the sporadic “explosive” expansions of the modeling capabilities, especially as the packages of aerosol and chemistry grid components are added. These major advances arise from a combination of increased computational capacity, that allows for an increasingly complex array of processes and their interactions to be represented inside the GEOS model, and the data-driven approach to modeling adopted in the GMAO, that motivates expansion of the model to include representations of many different processes that may be outside the required scope of other models. Captions in the lower portion of the animation indicate the introduction of these major additions as new branches spring to life. More subtle changes in the GEOS model structure are also revealed in the animations, as additional code layers are introduced to enhance the modularity of the code in order to enhance functionality.

A larger version of this animation is available for download. Links: MPEG-4 format, WEBM format, and OGG format


Colarco, P., A. da Silva, M. Chin, and T. Diehl, 2010: Online simulations of global aerosol distributions in the NASA GEOS-4 model and comparisons to satellite and ground‐based aerosol optical depth, J. Geophys. Res., 115, D14207, doi:10.1029/2009JD012820.

Lin, S., 2004: A “Vertically Lagrangian” Finite-Volume Dynamical Core for Global Models. Mon. Wea. Rev., 132, 2293–2307, doi: 10.1175/1520-0493(2004)132<2293:AVLFDC>2.0.CO;2.

Long, M. S., R. Yantosca, J. E. Nielsen, C. A. Keller, A. da Silva, M. P. Sulprizio, S. Pawson, and D. J. Jacob, 2015: Development of a grid-independent GEOS-Chem chemical transport model (v9-02) as an atmospheric chemistry module for Earth system models. Geosci. Model Dev., 8, 3, 595-602. doi:10.5194/gmd-8-595-2015.

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Pawson, S., R. S. Stolarski, A. R. Douglass, P. A. Newman, J. E. Nielsen, S. M. Frith, and M. L. Gupta, 2008: Goddard Earth Observing System Chemistry-Climate Model Simulations of Stratospheric Ozone-Temperature Coupling Between 1950 and 2005. J. Geophys. Res., 113, D12103. doi:10.1029/2007JD009511.

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Suarez, M.J. and L.L. Takacs, 1995: Technical report series on global modeling and data assimilation. Volume 5: Documentation of the AIRES/GEOS dynamical core, version 2, NASA-TM-104606-VOL-5, REPT-95B00069-VOL-5, NAS 1.15:104606-VOL-5, 51pp.


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