RRTMG_SW Description

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

Clear sky comparison of the latest version of RRTMG_SW (using 2 streams) relative
to RRTM_SW (using 16 streams with DISORT).

Key features of RRTMG_SW 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_SW_v2.5, which is described here.
• Fluxes and heating rates can be calculated over fourteen contiguous bands in the shortwave (820-50000 cm^-1)
• Modeled sources of extinction are water vapor, carbon dioxide, ozone, methane, oxygen, nitrogen, aerosols, and Rayleigh scattering
• A two-stream algorithm is used to perform scattering calculations; Reference: Oreopoulos and Barker (1999).
• Uses reduced set of g-intervals (112) for integration over extinction in each band relative to full set of g-intervals used in RRTM_SW (224).
• 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).
• Coding has been reformatted to use many FORTRAN90 features.
• Model able to run either as a column model or as a callable subroutine.
• Clear sky agreement with RRTM_SW is generally within 3 W/m² for flux and for heating rate within 0.1 K/d in the troposphere and 0.3 K/d 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 parameterization, which assumes the ice crystals are hexagonal and randomly-oriented in space.  The optical depth, single-scattering albedo, and asymmetry parameter are parameterized as a function of the generalized effective size of the ice crystals and the ice water content.  (It is important to note that the generalized effective size is not equivalent to the effective mean size.)
Reference: Fu, Q., An accurate parameterization of the solar radiative properties of cirrus clouds for climate models.  J. Climate, 9, 2058-2082, 1996.
• 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.

Key differences between RRTMG_SW and RRTM_SW are:

• RRTMG_SW uses a two-stream algorithm for multiple scattering, and RRTM_SW uses DISORT; Reference: Oreopoulos and Barker (1999).
• RRTMG_SW uses reduced set of g-intervals (112) for integration over extinction in each band relative to full set of g-intervals used in RRTM_SW (224).
• RRTMG_SW 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); RRTM_SW does not include McICA and does not have cloud fraction capability; it is limited to clear or fully overcast cloud conditions.
• RRTMG_SW coding has been reformatted to use many FORTRAN90 features.
• RRTMG_SW can be used as a callable subroutine and adapted for use within global or regional models.
• RRTMG_SW can optionally read the required input data either from a netCDF file or from the original RRTM_SW source data statements.