nanodisort

Python bindings for CDISORT, a C implementation of the DISORT (Discrete Ordinates Radiative Transfer) solver.

Overview

DISORT is a widely-used radiative transfer solver for plane-parallel atmospheres. It uses the discrete ordinates method to solve the radiative transfer equation with support for:

• Multiple scattering
• Absorption and emission
• Thermal emission (Planck function)
• Various boundary conditions
• Pseudo-spherical geometry for direct beam

The CDISORT library offers improved performance over the original Fortran version through dynamic memory allocation and full double precision arithmetic.

Installation

pip install nanodisort

Prerequisites:

• Supported platforms: Linux, macOS
• Python 3.10 and later
• NumPy 1.20 or later
• nanobind-backend (installed automatically)

Quickstart

Single Rayleigh-scattering layer illuminated by a solar beam at 60° zenith:

import numpy as np
import nanodisort as nd
from nanodisort.utils import phase_functions as pf

# Configure dimensions and allocate
ds = nd.DisortState()
ds.nstr = 16     # number of streams
ds.nlyr = 1      # number of layers
ds.nmom = 16     # phase function moments (>= nstr)
ds.ntau = 2      # output optical depth levels
ds.numu = 6      # output polar angles
ds.nphi = 1      # output azimuthal angles
ds.nphase = 100  # phase-angle grid points for the Buras-Emde correction
ds.allocate()

# Flags
ds.usrtau = True                     # user-specified output levels
ds.usrang = True                     # user-specified viewing angles
ds.lamber = True                     # Lambertian lower boundary
ds.quiet = True                      # suppress C-level output
ds.intensity_correction = True       # enable intensity correction
ds.old_intensity_correction = False  # use the Buras-Emde algorithm (default)

# Optical properties
ds.dtauc = np.array([0.1])          # extinction optical thickness
ds.ssalb = np.array([1.0])          # single-scattering albedo (pure scattering)
ds.pmom = pf.rayleigh(ds.nmom).reshape(-1, 1)  # Rayleigh phase function moments

# Buras-Emde correction: tabulate the phase function on a scattering-angle grid.
# mu_phase holds the scattering-angle cosines, shape (nphase,); phase holds the
# phase function values per layer, shape (nlyr, nphase). Here we use the analytic
# Rayleigh phase function P(mu) = 0.75 * (1 + mu**2).
mu_phase = np.linspace(-1.0, 1.0, ds.nphase)
ds.mu_phase = mu_phase
ds.phase = (0.75 * (1.0 + mu_phase**2)).reshape(1, -1)

# Output levels and viewing angles
ds.utau = np.array([0.0, 0.1])                    # TOA and surface
ds.umu = np.array([-1.0, -0.5, -0.1, 0.1, 0.5, 1.0])  # cosines of polar angles
ds.phi = np.array([0.0])                           # azimuthal angle [degrees]

# Boundary conditions
ds.fbeam = np.pi  # solar irradiance [W/m²]
ds.umu0 = 0.5     # cosine of solar zenith angle (60°)
ds.phi0 = 0.0     # solar azimuth [degrees]
ds.albedo = 0.0   # black surface
ds.fisot = 0.0    # no diffuse illumination from above

# Solve
ds.solve()

# Fluxes at each output level: shape (ntau,)
print(ds.rfldir)  # direct-beam downward irradiance
print(ds.rfldn)   # diffuse downward irradiance
print(ds.flup)    # diffuse upward irradiance

# Radiance field: shape (numu, ntau, nphi)
print(ds.uu.shape)

License

This project is licensed under the GNU General Public License v3.0 or later (GPL-3.0-or-later), consistent with the CDISORT library it wraps.

Acknowledgements

nanodisort authors are grateful to the developers of the many implementations of DISORT, and, in particular, to

• Timothy E. Dowling (original CDISORT C translation);
• Robert Buras (phase function extensions);
• Arve Kylling (pseudo-spherical approximation, testing);
• the original DISORT Fortran authors (Knuth Stamnes, Si-Chee Tsay, Warren Wiscombe and Kolf Jayaweera).

Citation

If you use nanodisort in your research, please cite the CDISORT paper and the nanodisort repository:

@software{Leroy_nanodisort,
    author = {Leroy, Vincent and Emde, Claudia},
    license = {GPL-3.0-or-later},
    title = {{nanodisort}},
    url = {https://github.com/eradiate/nanodisort},
    version = {0.2.0}
}