*GAS Section

1. The type of field is established by checking in the following order:
   1. GASFIL (ie if it is an existing file),
   2. * (ie if it is an asterisk character),
   3. IDX (ie if it can be read as an integer), and finally
   4. GAS (ie if it can be read as a string of up to 7 characters)
   A fatal error results if it is none of the above. However, since any unrecognised string is assumed to be a new cross-section molecule, the error may be deferred until proved otherwise.

2. GASFIL contents (apart from any comment records) are read field-by-field as if they were inserted directly into the *GAS section (see Data Files).

3. The wildcard character '*' can be used on its own or in conjunction with any other gases (in which case it will add any additional absorbers), but may only appear once in the *GAS section. The main use of this is if you do not know in advance which absorbers might be significant in a particular spectral range. The RFM has an internal database (in file optdat_dat.f90) of approximate optical depths for a zenith path through a standard atmosphere of most absorbing species in the range 0–20 000 cm^-1.
   • LIM: The gas wildcard character has optional 'optical strength' qualifier which can select a cut off for absorbers, eg '*(10)' is a reasonable value to exclude all undetectable absorbers. A value '*(0)' is equivalent to just '*'. The 'optical strength' parameter is notionally 15 + log10(max optical depth), so a value 15 corresponds to optical depth 1 (hence '*(10)' excludes all absorbers with optical depths < 1E-5 in a vertical path).
   • If the gas wildcard is used, an entry appears in the rfm.log file listing all absorbers found together with their 'optical strengths', i.e. you could just use the RFM to produce an ordered list of significant absorbers within any spectral range (performed by the subroutine gasall_sub.f90).

   Continuum

4. The CTMGAS qualifier is only applicable to molecules with continuum models: CO2, H2O, O2, N2; and the the CTM Flag (*FLG section) is also required to have any effect.
   • (CTM): Only the continuum component will be modelled for this molecule — all HITRAN line transitions will be omitted, so no other qualifiers may be used.
   • (NOCTM): Exclude the continuum component for this molecule. This may be placed anywhere in the qualifier string.

5. H2OWNG: The definition of the standard H2O continuum is that it includes the cumulative absorption of all line wings beyond 25 cm^-1 from the line centre. So to combine H2O line and continuum terms the standard procedure would be to calculate the optical depth contribution of each H2O line at ±25 cm^-1 from line centre, and subtract this value across the whole line shape between ±25 cm^-1. This is all handled automatically within the RFM, depending on whether or not the H2O continuum is included in the calculation.

   An issue arises when trying to model H2O absorption using look-up tables (TAB, LUT flags). Since the H2O continuum term has a strong dependence on p,T and H2O concentration it requires a high density of points to be modelled in a LUT. On the other hand, the continuum part is relatively simple to compute explicitly. So it is usually desirable to have H2O LUTs which contain only the truncated H2O line contributions which can then simply be added to the computed continuum term.

   The RFM anticipates this approach so, if calculating LUTs (TAB flag, without the CTM flag) it will subtract the 25 cm^-1 offset from all H2O lines, and if using LUTs (LUT flag, with the CTM flag) it will simply add these contributions to any H2O continuum term. The H2OWNG qualifier allows the user to alter this default behaviour, eg

   • H2O(SUB): calculate H2O lines with the 25 cm^-1 subtraction but without the continuum. This might be useful for a line-by-line calculation to verify the accuracy of spectra generated with a standard H2O .tab file
   • H2O(NOSUB): override-continuum subtraction with TAB flag. This could be used to force the RFM to generate H2O .tab files which include the continuum term.
   Note that the RFM itself has no way of distinguishing whether an H2O LUT has been calculated with or without the continuum term.
   • See also: RFM Look Up Tables

   Isotopologues (See RFM Handling of Isotopologues)

6. ISO: The Isotopologue qualifier may only be used for 'line' molecules (i.e. not cross-section molecules), those with HITRAN/RFM indices 1:99. The effect is to apply a selection criterion to the HITRAN database so that only transitions belonging to the specified isotopes are read in.

   The value refers to the HITRAN "local ID" nomenclature, ordered such that 1=most abundant.

   Negative values are interpreted as excluding particular isotopologues.

   For multiple isotopologues use eg H2O(1)(2)(3)(4) or H2O(-5)(-6)(-7) (obviously all indices must be either positive or negative)

   Vibrational Bands (See RFM Vibrational Level Assignments)

7. (LGQ:UGQ): The vibrational bands are identified by the RFM indices for each vibrational state, whose definition depends on the form of the molecule but 1=ground state in all cases. Either the lower or upper indicies numbers may be replaced by a wildcard character '*', indicating that all transitions with a particular upper/lower level are to be included. However, these are checked for logical consistency with other selections, eg you cannot have (1:*) and (1:4) in the qualifier list for the same molecule.

8. More complex selection rules can be applied by combining the (ISO) and (LGQ:UGQ) qualifiers. The general form of the qualifiers is [list_of_isotopes][list_of_bands], and implicitly the list starts with all isotopes and finishes with all bands; eg ...(1)(2)(3:4)(1:6)(3)(2:6)... selects bands (3:4) and (1:6) for isotopes (1) and (2), and band (2:6) for isotope (3). If the first qualifier(s) (or only qualifiers) are a list of bands, then these are selected for all isotopes. If the last qualifier(s) (or only qualifiers) are a list of isotopes, then all bands are selected for these isotopes.

*GAS H2O CO2 N2O CH4 HNO3 O3 ! MIPAS target species O2(CTM) N2(CTM) ! Oxygen and Nitrogen - continua only minor.gas ! a file of minor species *(10) ! Plus any other significant absorbers CO(1)(1:3) NO(1)(1:*) ! Only primary isotopes & specific bands