Spherical Harmonic Discrete Ordinate Method (SHDOM)
for Atmospheric Radiative Transfer
Model Capabilities
This program computes unpolarized monochromatic or spectral band
radiative transfer in a one, two, or three-dimensional medium for either
collimated solar and/or thermal emission sources of radiation. The
properties of the medium can be specified completely generally, i.e. the
extinction, single scattering albedo, Legendre coefficients of the
scattering phase function, and temperature for the particular wavelength
or spectral band may be specified at each input grid point. Radiances
at any angle, hemispheric fluxes, net fluxes, mean radiances, and net
flux convergence (related to heating rates) may be output anywhere in
the domain. For highly peaked phase functions the delta-M method may be
chosen, in which case the radiance is computed with an untruncated phase
function single scattering correction. A correlated k-distribution
approach is used for the integration over a spectral band. There may be
uniform or spatially variable Lambertian reflection and emission from
the ground surface. Several types of bidirectional reflection
distribution functions (BRDF) for the surface are implemented, and more
may be added easily. The horizontal boundaries may be either periodic
or open.
Method in Brief
The SHDOM uses an iterative process to compute the source function
(including the scattering integral) on a grid of points in space. The
angular part of the source function is represented with a spherical
harmonic expansion. Solving for the source function instead of the
radiance field saves memory, because there are often parts of a medium
where the source function is zero or angularly very smooth (hence few
spherical harmonic terms). The other reason for using spherical
harmonics is that the scattering integral is more efficiently computed
than in discrete ordinates. A discrete ordinate representation is used
in the solution process because the streaming of radiation is more
physically (and correctly) computed in this way. An adaptive grid that
chooses where to put grid points is useful in atmospheric radiative
transfer because the source function is usually rapidly varying in
some regions and slowly varying in others.
The iterative method is equivalent to a successive order approach.
For each iteration 1) the source function is transformed to discrete
ordinates at every grid point, 2) the integral form of the radiative
transfer equation is used to compute the discrete ordinate radiance at
every grid point, 3) the radiance is transformed back to spherical
harmonics, and 4) the new source function is computed from the radiance
in spherical harmonics. As with all order of scattering methods the
number of iterations increases with the single scattering albedo and
optical depth. A sequence acceleration method is used to speed up
convergence, which is typically achieved in under 50 iterations. During
the solution process, the grid cells with the integral of the source
function difference above a certain limit are split in half, generating
new grid points. The number of spherical harmonic terms kept at each
grid point also changes as the iterations proceed (i.e. adaptive
spherical harmonic truncation).
When to Use SHDOM
SHDOM is the most capable explicit representation (or deterministic) 3D
atmospheric radiative transfer model developed so far. However, it
cannot handle all modeling situations, and Monte Carlo methods are often
superior. SHDOM is appropriate for atmospheric media in which the
radiative transfer can be resolved; i.e. the optical depth across the
adaptive grid cells is small compared to unity. If many of the input
grid cells in a 3D medium have optical depth greater than 1, then SHDOM
is likely to prove computationally infeasible. This typically implies
that the SHDOM grid spacing needs to be 100 meters or smaller for
simulations of 3D clouds. Monte Carlo methods are also faster and more
accurate than SHDOM for simulations in which there are relatively few
radiative quantities output. For example, SHDOM is a poor choice when
horizontal domain average quantities are desired. SHDOM is a good
choice when it resolves the radiative transfer and many radiative
quantities are desired, e.g. the radiance field across the domain top or
the 3D distribution of heating.
What's New
A new SHDOM distribution was made available on June 20, 2003. The new
features are
- A new optical property file generation system for arbitrary
mixtures of particles, including ice crystal scattering for the
shortwave. The system consist of make_mie_table.f90 and
make_ice_table.f90 for creating single scattering tables for spherical
particles and eight shapes of ice crystals, and propgen.f90 for
producing property files with mixtures of particles at each grid point.
Propgen produces more accurate optical property files than cloudprp
because it adaptively creates new tabulated phase functions to keep the
phase function error for particle mixtures within user specified
tolerances.
- A modified version of the 6S ocean surface reflectance BRDF model
in SHDOM.
- A visualization output mode for SHDOM that simulates camera or
cross-track scanning images having accurate radiance values with the
correct geometric perspective. An example script creates camera images
for an animation of a 3D stratocumulus cloud field.
- New longwave and shortwave broadband k-distribution programs based
on the Rapid Radiative Transfer Model (RRTM) from AER, Inc.
Several scientists have used SHDOM for 1D modeling, so a
plane-parallel version, called SHDOMPP, has been developed. SHDOMPP is
optimized for plane-parallel radiative transfer, making it faster and
more accurate. More information and a distribution file is available .
Documentation
The online documentation
contains details on how to run the code.
A conference paper (PDF)
from the ARM science team meeting (2003) describes and gives examples
for the new optical property generation system and the SHDOM
visualization output.
A journal article on the SHDOM algorithm is available as a
PDF file (though the figures are poor).
The reference is
Evans, K. F., 1998: The spherical harmonic discrete ordinate method
for three-dimensional atmospheric radiative transfer.
J. Atmos. Sci., 55, 429-446.
Distribution
The model is being freely distributed via this Web page. The
distribution README lists the files in the
distribution. A list of the changes
in the update releases is also available.
SHDOM is distributed as a gzipped Unix tar file. To be a registered
user for update notifications fill in the following form before
downloading the distribution:
If you have trouble downloading the distribution from the Web, you can
try the anonymous ftp site (ftp://nit.colorado.edu/pub/shdom/).
More Stuff
Results of tests and examples illustrating how to best use SHDOM are
below.
This material is based upon work supported by the National Science
Foundation under Grant ATM-9421733. Any opinions, findings, and
conclusions or recommendations expressed in this material are those of
the author(s) and do not necessarily reflect the views of the National
Science Foundation. Updates to SHDOM since 1997 have been funded by
the Atmospheric Radiation Measurement program of the Department of
Energy under grant DE-A1005-90ER61069.
Last modified: January 23, 2006
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