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A very minor amount of hand-tuning of k’s was done in one or two bands.

RRTMGP

Reference(s): Pincus et al. (2019). Also, Mlawer et al. (1997) and Iacono et al. (2008) may have some useful information.

Implementation details: A series of bash scripts that execute FORTRAN code.  It is potentially suitable for others to use, but is in the process of being fixed up and archived so will be more useable in a few months.

Selecting band boundaries: The band boundaries were mostly inherited from RRTMG, with some minor tweaks. Therefore the criteria were the same as for RRTMG:  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 v12.8, line parameter file aer v 3.6 (itself based to a large extent on the HITRAN 2012 line file), and MT CKD 3.2.

Reordering spectrum: Reordering is done separately at each pair of pressure and temperature values.

Choosing number of g points: Fixed at 16 initially per band (RRTM) and then reduced (process underway) to as low as possible until accuracy is considered compromised.

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 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: 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 AER high-resolution solar spectrum is used, which has an envelope consistent with the NRLSSI2 medium-resolution solar spectrum, but has incorporated the high-resolution features of the Kurucz solar spectrum (in (roughly) the IR, visible, and UV) and the Toon solar transmittance spectrum (roughly NIR). The code includes the capability to represent solar variability through time-averaged or time-specific values of facular brightening and sunspot dimming.

Other relevant information...

Significant hand-tuning of k’s was done in one or two bands to increase the code’s accuracy. This is described further by Pincus et al. (2019).