RFM Driver Table Sections

*GAS Section: List of Absorbers

17JUL17

Type
Primary Section#4

Description
List of absorbing species required for calculations, plus optional qualifiers to select particular bands, isotopes and continuum handling.

Format
Multiple fields, arbitrary order for molecules.
FieldTypeDescriptionRange
FILGAS C*200 Name of a .gas file
GAS C*7 Chemical Formula. See List of Absorbers
IDX I HITRAN/RFM index of molecule See List of Absorbers, or '*'

Various 'qualifiers', contained in a pair of brackets (...), may be appended immediately after (no spaces) the GAS or IDX fields and their order may be significant.
QualifierTypeDescriptionRange
(ISO) I HITRAN Isotope index 1:(N.isotopes for molecule)
(LGQ:UGQ) 2I Lower, Upper HITRAN Global Quantum indices 1:999 or '*'
(CTMGAS) C*5 Continuum treatment for gas 'CTM' or 'NOCTM'
(LIM) I Threshold for '*' absorbers 0:99
(H2OWNG) I Treatment of H2O 25cm-1 offset 'SUB' or 'NOSUB'

Notes
  1. The type of field is identified by first checking to see if it is a .gas filename (FILGAS), then if it is a '*' character, then if it can be interpreted as a molecule.
  2. 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, and not in any .gas file. 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.inc optdat_dat.f90 for RFM v5 ) of approximate optical depths for a zenith path through a standard atmosphere of all absorbing species in the range 0-20 000 cm-1.
  3. The gas wildcard character has optional 'optical strength' qualifier (LIM) 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).
  4. If the gas wildcard is used, an entry appears in the rfm.runlog 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 (the subroutine gasall.for does this task gasall_sub.f90 for RFM v5 ).
  5. The (CTM) and (NOCTM) qualifiers are only applicable to molecules with continuum models: CO2, H2O, O2, N2. (CTM) can only be used in conjunction with the CTM Flag and means that only the continuum component will be selected for this molecule - all HITRAN line transitions will be omitted (so no other qualifiers may be used). The (NOCTM) qualifier indicates that the continuum component is to be switched off for this particular molecule. Without the CTM Flag it has no effect, but generates a warning message in the rfm.runlog file. (NOCTM) can be placed anywhere in the qualifier string.
  6. The Isotope (ISO) and band (LGQ:UGQ) qualifiers 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 as the HITRAN database so that only transitions from the specified isotopes/transitions are read in. This is distinct from specifying different profiles for different isotopes which is achieved in the *ATM section.
  7. The isotope number (ISO) refers to the HITRAN nomenclature, ordered such that 1=most abundant. See List of recognised isotopes for each molecule. RFM v4.34 onwards: Negative values can also be used, interpreted as excluding particular isotopes.
  8. As special cases, the RFM accepts commonly used abbreviations 'HDO' and 'CH3D' as equivalent to 'H2O(4)' and 'CH4(3)'.
  9. The vibrational bands (LGQ:UGQ) are identified by the HITRAN Global Quantum 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 quantum 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 can't have (1:*) and (1:4) in the qualifier list for the same molecule.
  10. More complicated 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.
  11. H2OWNG (RFM v4.34 onwards)
    • The MT_CKD H2O continuum is defined to include the contributions of all H2O beyond ±25 cm-1 from the line centre, so to combine H2O lines with the continuum it necessary to subtract the value of the H2O lineshape evaluated at (ν-ν0)=25 cm-1 from the (Voigt) lineshape function of each H2O line, while without the continuum the full lineshape is used as normal.
    • A further complication arises when generating look-up tables using the TAB flag. Given the strong dependence of continuum on p,T and H2O concentration, it can only be adequately represented using a high density of tabulated points; on the other hand the explicit calculation is computationally cheap. In this case it is assumed that the LUTs are generated for the line-component of H2O absorption only (ie without the CTM flag), to be subsequently combined as a LUT file with a CTM-only calculation. So here the requirement is to subtract the 25 cm-1 offset from H2O lines in anticipation of subsequently combining with the continuum term.
    • So the default behaviour of the RFM is to subtract the 25 cm-1 offset if
      • EITHER modelling the H2O continuum, i.e. CTM flag enabled (but not subsequently disabled by the H2O(NOCTM) in the *GAS section).
      • OR if the TAB flag is enabled.
    • The H2OWNG qualifier allows the user to override this behaviour. For example
      • H2O(SUB) for a line-by-line calculation, without continuum, to verify the accuracy of spectra generated by an H2O .tab file
      • H2O(NOSUB), with continuum, to generate an H2O .tab file with continuum contributions included
    • These qualifiers can only be used with H2O lines
    • If the qualifier simply reproduces the default behaviour of the RFM a warning message is issued in the runlog file.

Example
*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, and specific bands

Bugs
Bug#114 (Fixed v4.35)
Bug#111 (Fixed v4.34)
Bug#90 (Fixed v4.32)
Bug#89 (Fixed v4.32)
Bug#87 (Fixed v4.32)
Bug#57 (Fixed v4.26)
Bug#14 (Fixed v4.11)
Bug#3 (Fixed v4.02)