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Rick Pernak edited this page Mar 20, 2020 · 6 revisions

RRTMG_LW is a radiative transfer model that utilizes the correlated-k approach to calculate longwave fluxes and heating rates efficiently and accurately for application to GCMs.

Clear sky comparison of the latest version of RRTMG_LW relative to LBLRTM.

Key features of RRTMG_LW are:

  • Absorption coefficient data for the k-distributions are obtained directly from the line-by-line 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 on its RTWeb site, and differences between RRTMG and RRTM have their own Wiki page.
  • Fluxes and heating rates can be calculated over sixteen contiguous bands in the longwave (10-3250 cm-1, or 3.08-1000 microns). The individual band ranges (in wavenumbers, cm-1) are:
  1. 10-350
  2. 350-500
  3. 500-630
  4. 630-700
  5. 700-820
  6. 820-980
  7. 980-1080
  8. 1080-1180
  9. 1180-1390
  10. 1390-1480
  11. 1480-1800
  12. 1800-2080
  13. 2080-2250
  14. 2250-2380
  15. 2380-2600
  16. 2600-3250.
  • When results are integrated over the full longwave spectrum, the 2600-3250 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 (CFC-11, CFC-12, CFC-22, and CCL4)
  • Uses reduced set of g-intervals (140) for integration over absorption in each band relative to full set of g-intervals used in RRTM_LW (256)
  • Includes McICA (Monte-Carlo Independant Column Approximation) capability to represent sub-grid cloud variability with random, maximum-random and maximum options for cloud overlap; References: Barker et al. (2003), Pincus et al., JGR, (2003)
  • 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 m-2 at all levels, and the computed cooling rates generally agree to within 0.1 K day-1 in the troposphere and 0.3 K day-1 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 Fu et al. ice particle parameterization.
    • Reference: Fu, Yang, and Sun, J. Climate, Vol 11, 1998, pp. 2223-2237, 1998.
  • Aerosols:
    • Aerosol absorption in the longwave can be included by providing the bulk aerosol optical depth at the mid-point 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 pre-calculated derivative of the Planck function with respect to temperature.
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