LUT File Structure

22SEP22 Page reformatted
Contents

Overview

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.

File Structure

The file consists of a header section, then sets of records corresponding to k( νi,*,*,*) 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 Overview
File_Header_Section defines NWno, NPTV
For i = 1, NWno
    Data_Record containing (1+NPTV) data values

File Header Section
!CMNT Comment record(s) (optional)
Format_ID File format identifier (single record)
Mol_ID   NWno   Wno1   Wno2   WnoD   NPTV   NPre   NTem   NVSF Molecule ID and dimensions (single record)
Pre(1) Pre(2) ... Pre(NPre) Pressure axis values (multiple records)
TPr(1) TPr(2) ... TPr(NPre) Temperature profile (multiple records)
VPr(1) VPr(2) ... VPr(NPre) VMR profile (multiple records)
Tem(1) Tem(2) ... Tem(NTem) Temperature axis values (multiple records)
VSF(1) VSF(2) ... VSF(NVSF) VMR Scale Factor axis values (multiple records)
Data Record
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)

Data Dictionary

Field Type Description Units Range
!CMNT String*80 Comment record first character is '!'
Format_ID Float File format identifier 1.0 expected
K Float(NWno, NPre, NTem, NVSF) ln(k) where k is absoption coefficient m2/kmole ≥ −99, and <0 expected [1]
Mol_ID String*5 HITRAN molecule ID [.Isotope#] [2] ≥ 1
NPre Integer No. pressure-axis values [3] ≥ 1
NPTV Integer Product of NPre x NTem x NVSF [4] ≥ 1
NTem Integer No. temperature-axis values [5] ≥ 1 NTem (Abs) or ≤ -1 (Rel)
NVSF Integer No. VSF-axis (q-axis) values [6] 1 expected
NWno Integer No. spectral points/data records in file ≥ 2
Pre Float(NPre) Pressure axis values hPa 0 – ∼1000
Tem Float(NTem) Temperature axis values K > 0 (if Abs)
TPrf Float(NPre) Temperature profile values [3] K > 0
VPrf Float(NPre) VMR profile values [3] ppmv 0 – 106
VSF Float(NVSF) VMR scale factor axis values [6] % 100.0 expected
Wno Double(NWno) Wavenumber axis value cm-1 > 0
Wno1 Double Lowest/First wavenumber in file cm-1 0 – Wno2
Wno2 Double Highest/Last wavenumber in file cm-1 > Wno1
WnoD Double Wavenumber (minimum) increment [7] cm-1 > 0

Notes

  1. 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.

  2. 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

  3. 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.

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

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

  6. 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.

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

Document Changes

22SEP22 This page reformatted. Notes added.
06AUG22 Original