Spectral files

Overview

Atmospheric radiative transfer spans an enormous range of frequencies, across which gaseous absorption coefficients vary by many orders of magnitude. The approach adopted in SOCRATES, and in general circulation models (GCMs) broadly, is to divide the solar or infra-red spectral region into a number of bands, across which all radiative quantities except gaseous absorption coefficients may be treated as uniform. More bands give more accuracy, but at greater computational cost; the appropriate balance depends on the application.

A key design choice in SOCRATES is that the frequency discretisation is not fixed in the code itself. Instead, it is set by an external file supplied at runtime: the spectral file. This separates the physical parametrisation data from the radiation solver, making the code flexible and easy to update. A spectral file generated for one version of SOCRATES remains compatible with future versions (though not necessarily the reverse, as new functionality may add new data blocks).

Role of the spectral file

The spectral file serves as the single source of truth for all spectrally dependent data used by the radiation code. This includes:

• The number and limits of spectral bands
• The gaseous absorbers active in each band and their absorption data
• Rayleigh scattering coefficients
• Planck function polynomial fits (longwave only)
• Parametrisations for the single-scattering properties of cloud droplets, ice crystals, and aerosols
• Continuum absorption coefficients

Because parametrisations that require spectrally dependent data can only be selected if such data are present in the spectral file, the choice of spectral file determines which physical parametrisations are available at runtime. Spectral files should therefore be selected with care to match the intended configuration.

Spectral bands

The spectral region is divided into bands within which all radiative quantities — except gaseous absorption coefficients — are treated as frequency-independent. The total flux is the sum of partial fluxes over bands (see Two-stream radiation code).

In some shortwave spectral files, certain bands share the same wavelength limits. These are not true spectral bands — they represent a computational splitting for efficiency, and only the sum of fluxes across such paired bands is physically meaningful. The excluded sub-ranges are specified in block 14 (see below).

Block structure

The spectral file is organised into numbered blocks, each corresponding to a different category of physical data. The flag l_present(i) is set to .TRUE. if block i is present; not all blocks are required for all calculations.

Gaseous absorption (block 5)

Gaseous absorption data are stored as k-distribution fits. For each active gas in each band, absorption coefficients are either:

• Scaled from a reference value using a pressure- and temperature-dependent scaling function (two functional forms are supported; see Two-stream radiation code), or
• Interpolated directly from a look-up table of coefficients at a grid of pressures and temperatures — now the preferred approach.

The first gas listed in block 4 for each band is the primary absorber; minor gases may be treated via the equivalent extinction approximation rather than full random overlap.

Cloud and ice parametrisations (blocks 10 and 12)

Blocks 10 and 12 may contain data for multiple types of droplet or ice crystal parametrisation within a single file. The term type is deliberately flexible — different types may represent:

• Different assumed size distributions
• Different spectral averaging methods (thin or thick averaging)
• Different crystal shapes (for ice)
• Different fitting functions (linear, Padé approximants, polynomial)

Type numbers are supplied at runtime and must be chosen to match the spectral file in use. The minimum and maximum particle sizes for which each parametrisation is valid are recorded alongside the coefficients.

Split bands (block 14)

In some files, a band's wavelength limits in block 1 encompass a wider range than is actually used, with a sub-range excluded by listing it in block 14. For example, if band 5 spans 8–12 µm but band 6 spans 10–11 µm and is excluded from band 5, then band 5 effectively covers 8–10 µm and 11–12 µm. Split bands are used purely for computational efficiency and are transparent to the solver.

Naming conventions

Although not formally required, a consistent naming convention is used for standard files:

When new functionality is added to a file without changing existing results, the filename is unchanged. When existing data are modified (changing results), a new filename is introduced so that older configurations remain reproducible.

Two utility programs handle format conversion between namelist and text formats: nml_spec (text → namelist) and spec_nml (namelist → text).

Generating and modifying spectral files

Spectral files are generated using the pre-processing suite of the offline Edwards–Slingo radiation code. Generation requires expertise in radiative transfer — most users will use standard provided files and should not modify them.

When a new requirement arises (e.g. adding a new absorbing gas, updating continuum data, or incorporating a new aerosol type), users should contact the radiation group rather than modifying files independently. Aerosol optical property data in particular are generated in consultation with aerosol modelling specialists.