RFM Examples

Making/Using LUT files, complex case

15DEC21

Description

For terrestrial atmospheres, water vapour is the only absorber where the self-broadening contribution is large enough to make the absorption coefficient a function of absorber concentration as well as p,T, so requiring use of the third axis of the LUT tables.

It is also anticipated that the H2O LUT will be used with the H2O continuum (the RFM default behaviour for H2O LUTs [Details ...]) which means that the CTM flag is not used in the driver table to generate the LUT, but is used when the LUT is an input.

This example also introduces an optional compression step to reduce the size of the LUT file before use.

• RFM Use of Look-Up Tales of Absorption Coefficient

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. Here it is anticipated that the the LUT will actually be used to simulate the range, 1215–1220 cm^-1 but add a margin of ±1 cm^-1 to allow for some spectral convolution. (NB: the spectral resolution here is set to 0.01 cm^-1 just to keep the file sizes small, but 0.001 cm^-1 would be more reasonable)

*ATM section

An atmospheric profile containing at least pressure, temperature and target molecule is always required in order to establish profiles of T(p) and H2O(p) included in the LUT file format which, in this case, are needed to interpret the axes.

*DIM section

Sets the pressure, temperature and VMR axes for the LUT file

• PCG Anticipating that the LUT will be required for calculations on the pressure levels in the file usa.atm, pressure interpolation errors can be minimised by using the Curtis-Godson equivalent pressures for each layer as the p-axis for the LUT
• 11 temperature axis points but since the lowest temperature is negative (-50) this means that the temperature axis is relative to the T(p) profile values. That is, for pressure axis value p_i the 11 temperature axis points have values T(p_i)-50, T(p_i)-40, ... T(p_i)+50,
• 4 VMR axis points, with values 50%, 100%, 150% and 200%. That is, for pressure axis value p_i, the 4 VMR axis points have values q(p_i)x0.5, q(p_i)x1, q(p_i)x1.5, q(p_i)x2.

Driver Table

*HDR
15DEC21 RFM Example: Create H2O LUT
*FLG
   TAB
*SPC 
   1214 1221 0.01  ! NB: too coarse for realistic spectra
*GAS
   H2O
*ATM
   ../atm/usa.atm
*DIM
  PCG 11 -50 50 4 50 200
*HIT
  ../hit/hitran_2016.bin
*END

RFM Output Files

• tab_h2o.asc
• rfm.log

Step 2: LUT Compression

There are two separate FORTRAN programs that can be used to compress the data by removing axis points where the values can be adequately represented by interpolation from the remaining axis points.

The first of these is tabcmp_v, which compresses in the wavenumber axis. User inputs in red

It has been assumed that an absorption error of 0.001 within any profile layer is acceptable, and that the LUT output will be convolved to a resolution of 0.1 cm^-1.

This took a few second to run (but increases with number of spectral points), and results in a file tab_h2o_v.asc of size 16Mb containing 478 instead of 701 spectral axis points.

Next use the program tabcmp_x to compress in the (p,T,q) axes.

This is rather quicker to run. 'Orig Npts' and 'New Npts' here refer to N(p) x N(T) x N(q) axis points, while for tabcmp_v they were spectral axis points. In this case just a single q-axis point was removed (150%) out of the four, hence the 25% reduction in size.

The new file tab_h2o_vx.asc is size 14Mb, less than a 10th of the original, and contains 478 (ν) x 51 (p) x 11 (T) x 3 (q) = 804 464 data points.

Step 3: Using the LUT

*FLG section

AVG flag: in this example the output spectra are reduced in resolution by convolution with a triangular function

CTM flag: add continuum absorption (just for H2O in this case)

LUT flag: using LUTs for (some) molecular absorption data

TRA flag: calculate transmittance spectra

ZEN flag: upward-viewing spectra, as observed from the ground

*SPC section

The spectral range is narrower than the LUT, although with the AVG flag an extra ±0.1 cm^-1 will be used.

*GAS section

In this example CH4 is added, but without a corresponding LUT, hence the need to include the HITRAN database in the *HIT section

*LUT section

H2O LUT file to be used

*FIN section

Ensure underlying spectral fine grid matches that used for LUT generation

Driver Table

*HDR
15DEC21 RFM Example: Use H2O LUT
*FLG
   AVG CTM FIN LUT TRA ZEN
*SPC
   1215 1220 0.1
*GAS
   H2O CH4
*ATM
   ../atm/usa.atm
*SEC
  1
*LUT
  tab_h2o_vx.asc
*HIT
   ../hit/hitran_2016.bin
*FIN
  0.01
*END

RFM Output Files

• tra_01000.asc
• rfm.log

Output Spectrum

The calculated spectrum is shown in blue, but the plot also shows the difference compared to spectra calculated line-by-line (ie without LUTs), using the original uncompressed LUT, and using the LUT after just the wavenumber compression stage (almost identical).

Only the strong line at 1218.5 cm^-1 is due to water vapour, the weaker lines are CH4, which is calculated line-by-line in all cases.