RRTMG_LW Description

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 here.
• 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: 10-350, 350-500, 500-630, 630-700, 700-820, 820-980, 980-1080, 1080-1180, 1180-1390, 1390-1480, 1480-1800, 1800-2080, 2080-2250, 2250-2380, 2380-2600, and 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, maximum, exponential, and exponential-random options for cloud overlap; the exponential and exponential-random 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,  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.

Key differences between RRTMG_LW and RRTM_LW are:

• RRTMG_LW 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).
• RRTMG_LW includes McICA (Monte-Carlo Independant Column Approximation) capability to represent sub-grid cloud variability with random, maximum-random, maximum, exponential, and exponential-random options for cloud overlap; References: Barker et al. (2003); Pincus et al., JGR, (2003); Oreopoulos et al., ACP, (2012); RRTM_LW does not have McICA, but it does include representations for random and maximum-random cloud overlap.
• RRTMG_LW performs radiative transfer only for a single (diffusivity) angle (angle = 53 deg; secant angle = 1.66) and varies this angle to improve accuracy in profiles with high water; RRTM_LW can use multiple angles for radiative transfer
• RRTMG_LW coding has been reformatted to use many FORTRAN90 features.
• RRTMG_LW includes aerosol absorption capability.
• RRTMG_LW can be used as a callable subroutine and adapted for use within global or regional models.
• RRTMG_LW can optionally read the required input data either from a netCDF file or from the original RRTM_LW source data statements.
• RRTMG_LW can provide the change in upward flux with respect to surface temperature, dF/dT, by layer for total sky and clear sky.