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Computing incoming solar radiation for each shortwave g point: The incoming solar spectral flux is accounted for in computing g as a weighting function of the spectral absorption coefficients.
RRTMG
Reference(s): Mlawer et al. (1997). Also Iacono et al. (2008) has some useful information.
Implementation details: The scripts and code are in a combination of bash scripts and FORTRAN (including a Numerical Recipe sorting code). This software is not suitable to be used by others.
Selecting band boundaries: The main consideration is to keep the number of major absorbers to no more than two. The variation in the source function is also considered.
Line-by-line model: LBLRTM. The most recent version of RRTM_LW (3.3) uses LBLRTM_v6.0, an unreleased AER line file for H2O and HITRAN 1996 for other species, and the MT_CKD_2.5 continuum. The most recent version of RRTMG_LW (5.0) uses the same, except it uses CKD_2.4. (An upcoming version (5.1) uses MT_CKD_2.5.) RRTM_SW and RRTMG_SW use LBLRTM_5.21, HITRAN 1996, and CKD_2.1.
Reordering spectrum: Reordering is done separately at each pair of pressure and temperature values.
Choosing number of g points: The number is fixed at 16 initially per band (RRTM) and then reduced manually for RRTMG to differing values in each band such that an integrated and per band clear-sky accuracy of about 1 Wm-2 for flux and 0.1 K/day for heating rate is retained relative to RRTM.
Partitioning g space for one gas: The partitioning is the same in each 16 g-point band. (See Table 2 in Mlawer et al. 1997.) A partitioning into ~10 is chosen by Gaussian quadrature, and then the last segment is partitioned more finely to resolve the ‘elbow’ in k(g) that many bands have.
Partitioning g space for multiple gases: The g-values are the same for single and multiple gases. The major gases (max two) are combined into a single effective gas using the ‘binary species parameter’ and the contributions of minor gases are added to total using a similar mapping.
Computing absorption of one gas: How are the LBL spectral absorption values averaged to obtain the absorption by one gas in one g point, e.g. logarithmic averaging? How is the pressure/temperature/concentration dependence of absorption handled in the gas-optics model generated, e.g. parametrically or as a look-up table?
Straight averaging of LBL optical depths is performed to obtain the absorption of one gas in one g point, then divided by column amount (effective column amount in the case of major species). The pressure/temperature/concentration dependence of absorption is handled in the gas-optics model as a look-up table.
Computing combined absorption of multiple gases: Major gases treated as one gas, minor gases added in using consistent mapping.
Computing Planck function for each longwave g point: A mapping consistent with the one that defines the k-distribution is applied to get the fraction of the band’s total Planck associated with each g-point. This is multiplied by the total Planck for the band (for that layer or level).
Computing incoming solar radiation for each shortwave g point: A mapping consistent with the one that defines the k-distribution is applied to get the total solar irradiance associated with each g-point. This mapping is applied at a pressure level where ~10% of the incoming irradiance is absorbed (at surface layer if less than 10% is absorbed in the column. The Kurucz solar spectrum is used in all versions of RRTM_SW and versions of RRTMG_SW earlier than RRTMG_SW_v4.0, and the NRLSSI2 solar function, including the capability to represent solar variability through time-averaged or time-specific values of facular brightening and sunspot dimming, is included in all versions of RRTMG_SW_v4.0 and later.
Other relevant information...
A very minor amount of hand-tuning of k’s was done in one or two bands.