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RRTM_LW is a broadband rapid radiative transfer model that utilizes the correlated-k approach to calculate longwave fluxes and heating rates efficiently and accurately.

Key features of RRTM_LW are:

• K-distributions are obtained directly from a line-by-line radiative transfer code, [LBLRTM] (https://www.github.com/AER-RC/LBLRTM), which has been extensively validated against observations, principally at the ARM climate research facilities
• Water vapor continuum absorption coefficients are consistent with those in [MT_CKD 2.5.](https://www.github.com/AER-RC/MT_CKD)
• Fluxes and cooling rates can be calculated over sixteen bands contiguous bands in the longwave (10-3250 cm-1)
• Scattering capability is available through the radiative transfer solver DISORT
• Modeled molecular absorbers are: water vapor, carbon dioxide, ozone, nitrous oxide, methane, oxygen, nitrogen and halocarbons.
• Fluxes calculated by RRTM agree with those computed by LBLRTM within 1.5 W/m2 at all levels, and the computed cooling rates 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, single-scattering 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, 728-742, 1993.
• Ice clouds:
  • The optical properties of ice clouds are calculated for each spectral band from the ice particle parameterization from Fu, Yang, and Sun (J. Climate, Vol 11, 1998, pp. 2223 - 2237) or from the ice particle parameterization available from the STREAMER model v3.0.