Getting Started with GCHP

GCHP Home

Overview

GEOS-Chem High Performance (GCHP) represents the next generation of GEOS-Chem. Capabilities that distinguish GCHP from standard GEOS-Chem (GEOS-Chem "classic" or GCC) include:

• Flexible-resolution simulations, from ~4° x 5° (C24) to ~0.25° x 0.3125° (C360) without the need for code edits or recompilation
• Use of the cubed-sphere (CS) grid, which eliminates the need for polar filtering and brings GEOS-Chem into line with the NASA GEOS AGCM
• MPI distributed-memory parallelization which enables running a single job across multiple machines

This page will guide you through the wiki pages to get started with the latest release of GCHP, including how to:

1. Set up your environment
2. Download source code
3. Generate a run directory
4. Perform a simulation
5. Post-process output data

Whether you plan on doing research in the near-term with GCHP or not, we encourage you to set up the model, play with it, and join the GCHP community by signing up for the GCHP Working Group mailing list. GCHP is very much a team effort and we invite you to contribute by testing it out and telling us how it goes by contacting the GEOS-Chem Support Team (geos-chem-support [at] as.harvard.edu). We also welcome you to join our GCHP Slack Workspace for easy contact with other GCHP users. Contact the Lizzie Lundgren (elundgren [at] seas.harvard.edu) to join.

GCHP Quick Start Guide

Before beginning, please review the GCHP Hardware and Software Requirements to confirm you have sufficient resources available. Then proceed through the following chapters.

1. Downloading Source Code
2. Obtaining a Run Directory
3. Setting Up the GCHP Environment
4. Compiling
5. Basic Example Run
6. Run Configuration Files
7. Advanced Run Examples
8. Output Data
9. Developing GCHP

Frequently Asked Questions

I do not have a high performance compute cluster. Can I run GCHP?

Yes, you can run GCHP on as little as a single node as long as there are at least 6 cores available. We are working to make serial runs on a single core possible in the future.

Is GCHP only for high resolution runs?

No! You can run GCHP at the cubed-sphere grid equivalent of 4x5 (c24) and 2x2.5 (c48). The GCHP standard simulation run directory is set up for c48 by default starting in GCHP v11-02e but can easily be changed to whatever resolution you would like to run at by updating the runConfig.sh bash script in the run directory. Unlike GEOS-Chem classic, a single GCHP run directory can be used for any run resolution.

What are the different cubed-sphere resolutions and how do they compare to lat/lon grids?

Below we summarize the standard resolution specifications and the recommended timesteps (based on the timesteps used in GEOS-Chem "classic").

Standard resolution specification	Approximate CS equivalent(s)	Suggested timestep (s)
4° x 5°	C24	1200
2° x 2.5°	C48, C45	900
1° x 1.25°	C96, C90	600
0.5° x 0.625°1	C192, C180	300
0.25° x 0.3125°2	C384, C360	300
0.125° x 0.15625°	C7203	180

¹ Native resolution of MERRA-2 product from GMAO
² Native resolution of GEOS-FP product from GMAO
³ Native cubed-sphere resolution of GEOS-5

I changed my model resolution and my run completed successfully. Are there any scientific considerations I should be aware of before drawing conclusions from my results?

Yes, there are several resolution-dependent settings or inputs that will result in degraded output quality if the model resolution increases from the default c24. These include lightning scale factors, dust parameterization, regridded cloud variables, and use of the Voronoi timezones. The GCHP Development Team is working to resolve these dependencies. Until then, please contact the GEOS-Chem Support Team for more information.

I don't have met fields at the lat/lon equivalent of the cubed-sphere resolution I want to run at. What should I use?

As a general rule of thumb you may run GCHP at cubed-sphere resolutions up to a factor of two larger than your input met resolution equivalent. For example, you could use 2x2.5 input met for both c24 and c48 runs. GCHP will generally run successfully with even higher ratios, such as 2x2.5 input met for c180. However, in these cases you should use caution in your interpretation of results. In general it is always best to use the finest met resolution available and 0.25x0.3125 is the default input met resolution starting in v11-02d. See the [Shared Data Directories wiki page] for instructions on how to access fine resolution meteorology files.

Should I change my simulation timestep if I run at very high resolution?

You may increase your timestep to improve speed but this is not required. A 30 minute dynamics timestep and 60 minute chemistry timestep may be adequate for your purposes. We recommend that you never reduce your timestep from the default of 10 minutes for dynamics and 20 minutes for chemistry/emissions.

How can I get a restart file for a different cubed-sphere resolution?

All GCHP run directories come with symbolic links to five resolutions of restart files stored in ExtData/SPC_RESTARTS: c24, c48, c96, c180, and c360. You can generate restart files for other resolutions by regridding an existing restart file using the Fortran tool csregridtool for created by Sebastian Eastham. Python regridding tools are also avaialbe and will be included in the gcpy package currently in development. Contact GCST member Lizzie Lundgren for more information.

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GCHP Home

--Lizzie Lundgren (talk) 16:20, 7 March 2018 (UTC)