
Key features of RRTMG_LW are:

Absorption coefficient data for the kdistributions are obtained directly from the
linebyline radiative transfer model, LBLRTM,
which has been extensively validated against observations, principally at the
ARM SGP site. Data are consistent with those used in RRTM_LW_v3.0.1, which is
described here.

Fluxes and heating rates can be calculated over sixteen contiguous bands in the
longwave (103250 cm^{1}, or 3.081000 microns). The individual band
ranges (in wavenumbers, cm^{1}) are: 10350, 350500, 500630,
630700, 700820, 820980, 9801080, 10801180, 11801390, 13901480, 14801800,
18002080, 20802250, 22502380, 23802600, and 26003250.

When results are integrated over the full longwave spectrum, the 26003250
cm^{1} band includes a small adjustment to add the contribution over
the spectral interval from 3250 cm^{1} to infinity.

Modeled molecular absorbers are water vapor, carbon dioxide, ozone,
nitrous oxide, methane, oxygen, nitrogen and several halocarbons (CFC11,
CFC12, CFC22, and CCL4)

Uses reduced set of gintervals (140) for integration over absorption in
each band relative to full set of gintervals used in RRTM_LW (256)

Includes McICA (MonteCarlo Independant Column Approximation) capability
to represent subgrid cloud variability with random, maximumrandom,
maximum, exponential, and exponentialrandom options for cloud overlap;
the exponential and exponentialrandom methods allow specification of the
required decorrelation length as a constant or as a variable that varies
as a function of latitude and day of the year; References: Barker et al.
(2003); Pincus et al., JGR, (2003); Oreopoulos et al., ACP, (2012)

Performs radiative transfer for a single (diffusivity) angle (angle = 53 deg;
secant angle = 1.66) and improves accuracy in profiles with high water by
varying the angle in some bands as a function of total column water

Coding has been reformatted to use many FORTRAN90 features

Model able to run either as a column model or as a callable subroutine

Fluxes calculated by RRTMG_LW agree with those computed by LBLRTM within 1.0
W/m2 at all levels, and the computed cooling rates generally agree to within 0.1
K/day in the troposphere and 0.3 K/day in the stratosphere

Water clouds:

The optical properties of water clouds are calculated for each spectral
band from the Hu and Stamnes parameterization. The optical depth,
singlescattering albedo, and asymmetry parameter are parameterized as
a function of cloud equivalent radius and liquid water path.
Reference: Hu, Y. X., and K. Stamnes, An accurate parameterization
of the radiative properties of water clouds suitable for use in climate
models. J. Climate, Vol. 6, 728742, 1993.

Ice clouds:

The optical properties of ice clouds are calculated for each spectral band
from the Fu et al. ice particle parameterization.
Reference: Fu, Yang, and Sun, J. Climate,
Vol 11, 1998, pp. 22232237, 1998.

Aerosols:

Aerosol absorption in the longwave can be included by providing the bulk
aerosol optical depth at the midpoint of each spectral band.

Absorption coefficients and other initialization data can be optionally input
through a netCDF data file. This feature was developed and provided by
Patrick Hofmann and Robert Pincus of NOAA.

An optional feature is available to calculate the change in upward flux by
layer as a function of surface temperature. This can be used to approximate
adjustments in upward flux caused only by a change in surface temperature
in a GCM at time intervals between full radiation calls. This is derived
using the precalculated derivative of the Planck function with respect to
temperature.
