RFM Examples

Making/Using LUT files, simple case

28MAY21


Description

The following illustrates how to create a Look-Up Table (LUT) of absorption coefficients and use as an alternative to a line-by-line calculation, in this case to model the limb-radiance from part of the CO band.

Step 1: Create the LUT

*FLG section
TAB flag: switches RFM into LUT-generation mode
*SPC section
Sets the wavenumber axis for the LUT file
*ATM section
An atmospheric profile containing at least pressure, temperature and target molecule is always required in order to establish profiles of T and CO included in the LUT file format, but in this example the embedded profiles have no effect.
*DIM section
Sets the pressure and temperature axes for the LUT file

Driver Table

*HDR
28MAY21 RFM Example: Create CO LUT
*FLG
   TAB
*SPC 
   2150 2155 0.0005
*GAS
   CO
*ATM
   ../atm/usa.atm
*DIM
  -31 1000 0.01 14 180 310
*HIT
  ../hit/hitran_2016.bin
*END

Output Files

Step 2: Using the LUT

*FLG section
LUT flag: using LUTs for (some) molecular absorption data
*SPC section
The spectral range here need not match the LUT, but must be completely within it, otherwise the RFM reverts to a line-by-line calculation (or, here, would fail since no HITRAN data is supplied) The resolution need not match the LUT.
*GAS section
Although just CO is included here, other molecules can also be added as long as the spectroscopic data is supplied, eg via other LUT files or HITRAN data (*HIT section as above)
*LUT section
LUT file(s) to be used

Driver Table

*HDR
28MAY21 RFM Example: Use CO LUT
*FLG
   RAD LUT
*SPC 
   2150 2155 0.0005
*GAS
   CO
*ATM
   ../atm/usa.atm
*TAN
  10.0
*LUT
   tab_co.asc
*END

Output Spectrum

[plot.py]

Orange line shows around 1% difference compared with line-by-line calculation, which would be reduced with finer spacing for p,T axes.