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A Correlated K-Distribution (CKD) tool generates CKD gas-optics models in a number of steps, some which may require human intervention. One of the most interesting parts of the CKDMIP project will be to understand how differences in how each step is performed feed through to differences in the accuracy of fluxes and heating rates. The page is an attempt to gather the necessary information about the CKD tools participating in CKDMIP, specifically:
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Choosing number of g points: The number of g points for a given gas depends on its transmission over the band for airmass = 2 computed for a test atmosphere (usually US62). There are two different versions, "full" and "reduced" with the numbers of g points as shown in the table below:
Atmosphere transmission | Ng "full" | Ng "reduced" |
0 < T ≤ 0.05 | 10 | 3 |
0.05 < T ≤ 0.5 | 20 | 5 |
0.5 < T ≤ 0.8 | 15 | 5 |
0.8 < T ≤ 0.9 | 10 | 5 |
0.9 < T ≤ 0.99 | 5 | 5 |
0.99 < T ≤ 0.999 | 3 | 2 |
0.999 < T ≤ 1.0 | 1 | 1 |
Partitioning g space for one gas: For each gas, the partition of g space is obtained by first setting a Gaussian quadrature grid over the log10(k) range for a reference pressure and temperature (most likely corresponding to an altitude where the absorption will be the most important in the atmosphere). This grid is then mapped to the g space and the corresponding g partition is then used for all other pressures and temperatures.
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Significant hand-tuning of k’s was done in many bands to increase the code’s accuracy. This is described further by Pincus et al. (2019).
PSLACKD
Implementation details: Fortran, C and IDL
Selecting band boundaries: There is no restriction on band width, except that the maximum band width is limited by the line-by-line code outputs that are used to generate k-distributions.
Reordering spectrum: Reordering is done separately at each pressure level. However, in order to select the same number of g points, determination of the number of g points is done using all pressure levels.
Choosing number of g points: It is automated using a common threshold of transmission for all pressure and temperature, and water vapor concentrations.
Partitioning g space for one gas: Gaussian quadrature is used for the shortwave and longwave.
Partitioning g space for multiple gases: We can handle both random overlap and separating major and minor gases. The submitted files used fixed minor gas concentrations. The concentration of minor gases is fixed in line-by-line computations to generate inputs. Two separate concentrations of the major gas are used so that the absorption cross section is computed by interpolation.
Computing absorption of one gas: Absorption cross sections are averaged in logarithmic space for pressure and concentration, linear for temperature.
Computing combined absorption of multiple gases: Minor gas absorption is included in line-by-line computations. The k-table is built with additional dimension of minor gas concentrations to allow the variation of the minor gas concentration.
Computing Planck function for each longwave g point: The Planck function is first computed for the band and then partitioned amongst the g points.
Computing incoming solar radiation for each shortwave g point: The solar constant for the band is weighted by the gaussian weight. The Coddington Insolation Spectrum is used.