.tab File

RFM File Formats: Look-Up Table of Absorption Coefficient


Output(s), if TAB Flag enabled.
Optional Input(s) if LUT Flag enabled

Tabulated absorption coefficients Look-Up Table (LUT) files contain pretabulated absorption coefficients as an alternative to performing time-consuming line-by-line calculations. In principle these are 4-dimensional tables of

ln ( k ( ν, p, T, q ) )
where k m2/kmole Absorption cross-section
ν cm-1 Wavenumber
p hPa Pressure
T K Temperature
q % Volume Mixing Ratio scale factor
In practice the q-axis, which allows for self-broadening, is seldom used (i.e., a single value set to 100%).

As well as axis values, the file contains embedded temperature and VMR profiles represented on the pressure-axis. The temperature profile allows for an alternative interpretation of the temperature axis as relative temperatures applied at each tabulated pressure. The VMR profile is used in conjunction with the VMR scale factor axis to establish actual VMR-axis values at each tabulated pressure.

Such LUTs can be generated using the RFM, and further manipulated with separate specific software.

Default is tab_[s][g].asc where [s] is the spectral range and [g] is the gas
Different names may be specified in the *OUT section of the Driver Table.

The file consists of a header section, then sets of records corresponding to lnki,*,*,*) for each tabulated wavenumber νi. For plain text files the Data Record may extend over several file records but for binary files it is a single record.
File Structure
Header Section
!CMNT (Optional) Comment record(s)
Format_ID File format ID record
Mol_ID   NWno   Wno1   Wno2   WnoD   NPTV   NPre   NTem   NVSF Molecule ID and dimensions record
Pre(1) Pre(2) ... Pressure axis values (multiple records)
... Pre(NPre)
TPr(1) TPr(2) ... Temperature profile (multiple records)
... TPr(NPre)
VPr(1) VPr(2) ... VMR profile (multiple records)
... VPr(NPre)
Tem(1) Tem(2) ... Temperature axis values (multiple records)
... Tem(NTem)
VSF(1) VSF(2) ... VMR Scale Factor axis values (multiple records)
Data Section
For i = 1, NWno
    Wno(i)   K(i,1,1,1) K(i,2,1,1) ... Wavenumber, then NPTV tabulated absorption coefficient (multiple records)
    ... K(i,NPre,NTem,NVSF)
Field Type Description Units Range
!CMNT C80 Comment record[1]
Format_ID R File format identifier 1.0[2]
Mol_ID R HITRAN molecule ID [.Isotope#] [3] ≥ 1
NWno I No. spectral points/data records in file ≥ 2
Wno1 D Lowest/First wavenumber in file cm-1 0 – Wno2
Wno2 D Highest/Last wavenumber in file cm-1 > Wno1
WnoD D Wavenumber (minimum) increment [4] cm-1 > 0
NPTV I Product of (NPre × NTem × NVSF) [5] ≥ 1
NPre I No. pressure-axis values ≥ 1
NTem I No. temperature-axis values[6] |NTem| ≥ 1
NVSF I No. VSF-axis (q-axis) values 1 (usually)
Pre(:) R (NPre) Pressure axis values[7] hPa 0 – ∼1000
TPrf(:) R (NPre) Temperature profile values [7] K > 0
VPrf(:) R (NPre) VMR profile values [7] ppmv 0 – 106
Tem(:) R (NTem) Temperature axis values [6] K > 0 (if Abs)
VSF(:) R (NVSF) VMR scale factor axis values [8] % 100.0 expected
Wno(NWno) D Wavenumber axis value cm-1 > 0
K(:,:) R (NWno,NPTV) ln(k) values, k is absoption coefficient m2/kmole ≥ −99[9]
Type: I=Integer; R=Real; D=Double Precision; Cn=character string, length n.

  1. !CMNT: Comment records are identified by the first character being '!'.

  2. Format_ID: Currently only a single version of the file format is defined.

  3. Mol_ID: The tabulated molecule is indicated by an integer value representing one the HITRAN indices, with RFM extensions Optionally, a second digit (or digits) can be added after a decimal point indicating a specific isotopologue, using the HITRAN "local ID" definitions, e.g. 2.1 for CO2 626, 2.10 for CO2 838

  4. WnoD: Although the wavenumbers are on arbitrary grid, it is assumed that in practice this is actually a subset of an uniformly-spaced original grid.

  5. NPTV: This number is just provided for convenience, e.g. when skipping over data records when individual axis dimensions are irrelevant

  6. Tem(NTem): The temperature axis values can either be absolute, indicated by a positive NTem value, or relative to the embedded temperature profile TPrf, indcated by a negative NTem value.

  7. Pre(NPre): The pressure axis also forms the vertical axis of the 'embedded' profiles of temperature and VMR. The embedded temperature profile is only used to interpret a relative temperature axis, and the embedded VMR only used if there is more than one VSF-axis value.

  8. NVSF, VSF: this axis allows for self-broadening having a significant impact on the absorption coefficient. Usually it won't, so a single value of 100.0% is expected.

  9. K: given the units, maximum values of ln(k) are still likely to be negative. Where the original k value is too small to convert to ln(k), a floor value of -99 is set.

tab_co.asc (CO absorption coefficients in range 2150–2160 cm-1)