ExoRT

June 14, 2026 · View on GitHub

A two-stream radiative transfer code for 3D climate models and offline 1-D calculations.

📦 GitHub Repository
✉️ Eric T. Wolf — eric.wolf@colorado.edu

Overview

ExoRT is a flexible two-stream radiative transfer code designed for use with 3D climate models. It includes builds for a 1-D offline version and for direct interfacing with CESM1.2.1-ExoCAM and now with CESM3-planets.

Citation

If you use ExoRT, please cite:

Wolf, E. T., Kopparapu, R., Haqq-Misra, J., & Fauchez, T. J. (2022). ExoCAM: A 3D Climate Model for Exoplanet Atmospheres. The Planetary Science Journal, 3, 7. https://doi.org/10.3847/PSJ/ac3f3d

Integration with 3D models

NCAR CESM1.2.1 Integration

  • see the ExoCAM Github Repository for details and instructions: ExoCAM
  • Recommended version for CESM1.2.1-ExoCAM (as of September 2020): 3dmodels/src.cam.n68equiv
  • Files in 3dmodels/src.cam.n68equiv/ are kept identical to their counterparts in source/src.n68equiv/ and source/src.main/. The only intentional exception is sys_rootdir.F90, which encodes a machine-specific data path. When updating gas species in source/, always sync the corresponding files to 3dmodels/ in the same commit.

NCAR CESM3-Planets Integration

  • ExoRT n68equiv is now natively integrated into the CESM3-planets framework as an external component.
  • NEW for CESM3-planets (as of April 2026): 3dmodels/src.cam7.n68equiv
  • GitHub Repository: NCAR/CESM3-planets
  • Project Wiki: CESM3-planets Wiki

Directory Structure

ExoRT/
├── source/
│   ├── src.main/        # Drivers for offline calculation and shared radiation routines
│   ├── src.misc/        # Miscellaneous files and stubs from CESM origin (needed for offline runs)
│   └── src.n*/          # Radiative transfer versions (n## = number of spectral intervals + descriptor)
├── data/
│   ├── cia/             # Collision-induced absorption data
│   ├── cloud/           # Cloud optical properties (Mie)
│   ├── continuum/       # CO2, H2O continuum coefficients (from MTCKD / LBLRTM)
│   ├── kdist/           # Correlated k-distributions
│   └── solar/           # Stellar spectra
├── iofiles/             # Input/output files for the 1-D model
├── build/               # Build directory for the 1-D model
├── run/                 # Run directory for the 1-D model
├── 3dmodels/            # File sets to be linked with CESM
└── tools/               # Pre/post-processing scripts (Python and IDL)

Radiative Transfer Versions

Recommended for all terrestrial planet cases as of September 2020.

The H₂O/CO₂/CH₄/C₂H₆ correlated-k data correspond to the configuration published in Wolf et al. (2022). NH₃ and CO (HITRAN 2024) were added later as additional absorbers on top of that baseline and do not affect runs that omit those species.

  • Correlated-k coefficients from HELIOS-K (Grimm et al. 2015)
  • H₂O: HITRAN 2016, Voigt lineshape, 25 cm⁻¹ cutoff, plinth removed; self/foreign continuum from MT_CKDv3.3 fit to Gauss points
  • CO₂: HITRAN 2016, Perrin & Hartmann (1989) sub-Lorentzian lineshape, 500 cm⁻¹ cutoff, with CO₂–CO₂ CIA
  • CH₄: HITRAN 2016, Voigt lineshape, 25 cm⁻¹ cutoff
  • C₂H₆: HITRAN 2016, Voigt lineshape, 25 cm⁻¹ cutoff
  • NH₃: HITRAN 2024, Voigt lineshape, 25 cm⁻¹ cutoff (added 2026-04-27; 3-D interface 2026-04-29)
  • CO: HITRAN 2024, Voigt lineshape, 25 cm⁻¹ cutoff (added 2026-04-27; 3-D interface 2026-04-29)
  • CIA: N₂–N₂, N₂–H₂, H₂–H₂ from HITRAN; CO₂–H₂ and CO₂–CH₄ from Turbet et al. (2020)
  • 68 spectral intervals, 8 Gauss points
  • Gas overlap via equivalent extinction absorption method (Amundsen et al. 2016): major gas treated with full 8-point correlated-k; minor species added as grey absorbers, selected on the fly
  • Pressure range: 10 bar – 0.01 mb
  • Temperature range: 100 K – 500 K
  • Reference: Wolf et al., PSJ 3:7 (2022)

src.n84equiv

  • Same as n68equiv, with additional bins shortward of 0.24 µm
  • Use for F stars (6500 K < T < ~10,000 K)

src.n28archean

Designed for Archean climate simulations.

  • Species: H₂O, CO₂, CH₄, N₂, H₂; HITRAN 2004, 28 spectral bins
  • H₂O and CO₂ continuum from MT_CKD2.5 (see Halevy et al. 2009)
  • Mixed gas k-distributions via LBLRTM (Mlawer et al. 1997; Shi et al. 2009)
  • CO₂ up to multi-bar; reasonable agreement at 2-bar dry CO₂
  • CH₄ up to 0.01 bar ⚠️ Older line list misses CH₄ near-IR absorption
  • H₂O ⚠️ Overestimates near-IR absorption around M-dwarfs due to coarse bands
  • Up to 100 bar total pressure; N₂–N₂, N₂–H₂, H₂–H₂ CIA
  • Reference: Wolf & Toon, Astrobiology 13(7), 1–18 (2013)

src.n42h2o

  • Species: H₂O, N₂, H₂; 42 spectral bins; HITRAN 2012
  • H₂O single-gas k-distributions via HELIOS-K (Grimm et al. 2015)
  • Up to 10 bar total pressure; N₂–N₂, N₂–H₂, H₂–H₂ CIA
  • Reference: Kopparapu et al., ApJ 845:5 (2017)

src.n68h2o

  • Same as n42h2o, extended to 68 spectral bins

Building the Model

cd ../ExoRT/build

# Build specific version(s)
make n42h2o
make n68equiv

macOS users: the default compiler is ifort, which Intel discontinued and never ported to Apple Silicon (arm64). On any modern (M-series) Mac you must build with gfortran by adding USER_FC=gfortran to every make command, e.g. make USER_FC=gfortran n68equiv. See macOS (Apple Silicon) below.

The executable is copied to ../ExoRT/run/.

Linux / Discover (default)

The default compiler is ifort. Ensure nf-config is on your PATH (loaded automatically via the NetCDF module on Discover):

module load netcdf
make n68equiv

macOS (Apple Silicon)

ifort is not available on Apple Silicon — Intel's compiler was never ported to arm64 and was discontinued in 2023. Use gfortran instead:

make USER_FC=gfortran n68equiv

Dependencies: NetCDF-Fortran via Anaconda is recommended:

conda install -c conda-forge netcdf-fortran

Verify nf-config is on your PATH before building:

which nf-config

Runtime environment: The NetCDF libraries must be findable at runtime.

csh/tcsh:

setenv DYLD_LIBRARY_PATH /opt/anaconda3/lib

bash:

export DYLD_LIBRARY_PATH=/opt/anaconda3/lib

Add the appropriate line to your ~/.cshrc or ~/.bashrc.

Note on compiler warnings: The gfortran build produces warnings from CESM-heritage infrastructure files (infnan.F90, wrap_nf.F90, shr_sys_mod.F90) that ifort accepts silently. These are suppressed with -w in the Makefile and are benign for the current codebase, but are candidates for cleanup during refactoring of src.misc.

Running the Model

  1. Place your input file RTprofile_in.nc in the ../ExoRT/run/ directory.
    Template inputs are in ../ExoRT/iofiles/.

  2. Set the solar spectrum and number of vertical levels in:

    source/src.main/exoplanet_mod.F90

  3. Run the executable:

    cd ../ExoRT/run
./n42h2o.exe

Output is written to RTprofile_out.nc.

Input File: RTprofile_in.nc

Note: pverp = pver + 1 (interface levels = midpoint levels + 1)

Gas species variables are optional: if a variable is absent from the input file, ExoRT sets that species to zero and prints a clean diagnostic rather than an error. Only TS, PS, the pressure/temperature/height arrays, the albedos, coszrs, mw, and cp are required.

VariableDimensionRequiredDescription
ts(1)yesSurface temperature (K)
ps(1)yesSurface pressure (Pa)
tmid(pver)yesTemperature at layer midpoints (K)
tint(pverp)yesTemperature at layer interfaces (K)
pdel(pver)yesPressure thickness of each layer (Pa)
pint(pverp)yesPressure at interface levels (Pa)
zint(pverp)yesHeight at interfaces (m)
asdir(1)yesSW albedo, direct
asdif(1)yesSW albedo, diffuse
aldir(1)yesNear-IR albedo, direct
aldif(1)yesNear-IR albedo, diffuse
coszrs(1)yesCosine of solar zenith angle
mw(1)yesMolecular weight of dry air (g/mol)
cp(1)yesSpecific heat of dry air (J/kg/K)
h2ommr(pver)optionalH₂O specific humidity, kg(wv)/kg(air)
co2mmr(pver)optionalCO₂ mass mixing ratio (dry)
ch4mmr(pver)optionalCH₄ mass mixing ratio (dry)
c2h6mmr(pver)optionalC₂H₆ mass mixing ratio (dry)
nh3mmr(pver)optionalNH₃ mass mixing ratio (dry)
commr(pver)optionalCO mass mixing ratio (dry)
o2mmr(pver)optionalO₂ mass mixing ratio (dry)
o3mmr(pver)optionalO₃ mass mixing ratio (dry)
h2mmr(pver)optionalH₂ mass mixing ratio (dry)
n2mmr(pver)optionalN₂ mass mixing ratio (dry)

Use tools/makeColumn.py to generate input files. Species with VMR set to 0.0 are automatically omitted from the output file.

Output File: RTprofile_out.nc

Contains all input variables plus the following radiative flux and heating rate fields.

VariableDimensionDescription
LWUP(pverp)Longwave upwelling flux (W m⁻²)
LWDN(pverp)Longwave downwelling flux (W m⁻²)
SWUP(pverp)Shortwave upwelling flux (W m⁻²)
SWDN(pverp)Shortwave downwelling flux (W m⁻²)
LWUP_SPECTRAL(ntot_wavlnrng, pverp)Spectral LW upwelling flux (W m⁻² per interval)
LWDN_SPECTRAL(ntot_wavlnrng, pverp)Spectral LW downwelling flux (W m⁻² per interval)
SWUP_SPECTRAL(ntot_wavlnrng, pverp)Spectral SW upwelling flux (W m⁻² per interval)
SWDN_SPECTRAL(ntot_wavlnrng, pverp)Spectral SW downwelling flux (W m⁻² per interval)
LWHR(pver)Longwave heating rate (K/Earth day)
SWHR(pver)Shortwave heating rate (K/Earth day)

Data Directory Contents

DirectoryContents
data/aerosol/Aerosol optical constants and CARMA production rates
data/cia/Collision-induced absorption data
data/continuum/CO₂ and H₂O continuum coefficients from MTCKD (LBLRTM)
data/kdist/Correlated k-distributions
data/cloud/Cloud optical properties (Mie scattering)
data/solar/Stellar spectra

Tools

Python scripts in tools/ handle the primary 1-D pre/post-processing workflow. Legacy IDL scripts (.pro) remain for reference.

Building input profiles

tools/makeColumn.py — generates RTprofile_in.nc from a P-T profile and user-specified gas mixing ratios. Edit the USER SETTINGS block at the top or pass CLI flags:

cd tools/
python makeColumn.py --defaults                        # show current settings
python makeColumn.py --output RTprofile_in.nc          # use USER SETTINGS
python makeColumn.py --profile TS273K \
    --co2vmr 400e-6 --nh3vmr 1e-6 \
    --output RTprofile_in_nh3.nc
python makeColumn.py --zero-h2o --output RTprofile_in_dry.nc

Gas species with VMR = 0.0 are omitted from the file; ExoRT reads absent species as zero. Available P-T profiles (from profile_data.py):

TagDescription
TS273KMoist-adiabat Earth, Ts = 273 K, 300 levels
US1976US Standard Atmosphere 1976, 49 levels
smart_2bar_t2502-bar dry CO₂ Mars atmosphere, Ts ≈ 250 K, 69 levels

Plotting spectral output

tools/plotspectra_1D.py — plots SW and LW spectra from RTprofile_out.nc. The spectral grid is auto-detected from the file. Supports side-by-side comparison of two runs.

cd tools/
python plotspectra_1D.py                               # interactive display
python plotspectra_1D.py --save                        # save spectra_sw.png / spectra_lw.png
python plotspectra_1D.py --f2 ../run/RTprofile_out_ref.nc --label2 "no NH3"

SW panel: wavelength axis (µm), W m⁻² µm⁻¹. LW panel: wavenumber axis (cm⁻¹), W m⁻² cm. Also prints a band-by-band flux table to stdout.

References

ExoRT applications

Correlated-k / line-by-line

Gas overlap

  • Amundsen, D.S. et al. (2016). Accurate treatment of transmission, emission, and scattering in parameterised radiation schemes for a general circulation model. A&A, 564, A59. https://doi.org/10.1051/0004-6361/201323169

Line shapes / CO₂ broadening

  • Perrin, M.Y. & Hartmann, J.M. (1989). Temperature-dependent measurements and modeling of absorption by CO₂–N₂ mixtures in the far line-wings of the 4.3 µm CO₂ band. JQSRT, 42(4), 311–317. https://doi.org/10.1016/0022-4073(89)90077-0

Continuum absorption

  • Halevy, I., Pierrehumbert, R.T. & Schrag, D.P. (2009). Radiative implications of CO₂ cloud formation in a Neoproterozoic snowball Earth. JGR Atmospheres, 114, D18112. https://doi.org/10.1029/2009JD011915

CIA data

  • Turbet, M. et al. (2020). CO₂ condensation is a serious limit to the deglaciation of Earth-like planets. Earth and Planetary Science Letters, 531, 115759. https://doi.org/10.1016/j.epsl.2019.115959
  • Gordon, I.E. et al. (HITRAN collaboration) — CIA data retrieved from the HITRAN database.