Question Regarding the Calculation of Layer Pressure for Precise Optical Depth Simulation

[satpyscene]

Dear LBLRTM Developers,

I hope this message finds you well.

I am writing to seek clarification on a specific calculation within LBLRTM, as it is critical for my research. My primary goal is to accurately simulate the optical depth (which I will later convert to absorption cross-sections) at a specific, single target pressure and temperature.

However, LBLRTM's TAPE5 input requires me to define an atmospheric layer by specifying its upper and lower boundaries (pressures and temperatures), rather than a single point. This presents a challenge: if I cannot determine how LBLRTM calculates the representative pressure of this layer from the given boundaries, I will not be able to set up my simulation to ensure the calculation is performed precisely at my intended target pressure.

To understand this, I have been examining the representative layer pressure reported in the TAPE28 output file. Through my own testing, I've made the following observations:

The layer pressure is not the simple arithmetic mean of the two boundary pressures.

It also does not appear to be the logarithmic mean of the boundary pressures.

Interestingly, when I use the same two boundary pressures but provide different boundary temperatures, the resulting layer pressure in TAPE28 changes. This leads me to believe that the calculation is temperature-dependent.

Could you please clarify the specific method or formula LBLRTM uses to derive this single representative layer pressure from the given boundary pressures and temperatures?

Understanding this calculation is the crucial step for me to correctly configure my boundaries to simulate conditions at my exact target pressure. I would be very grateful if you could point me to the relevant section in the documentation or the specific subroutine in the source code where this is handled.

I have attached my TAPE5 file for your reference to provide a concrete example of my setup.

Thank you for your time and for maintaining this valuable tool.

Best regards,

satpyscene

TAPE5:
“
$Gas=CO2 P=14025Pa T=298.65K RH=0%
HI=1 F4=1 CN=0 AE=0 EM=0 SC=0 FI=0 PL=0 TS=0 AM=1 MG=0 LA=0 OD=1 XS=0 00 00
4201.681 4545.455 REJ=0 1.000E-02
0 2 -2 1 1 7 1 0 0.000 0.000
140.190 140.310 180.000
140.310 140.190
-2 GENERATED PROFILE
0.000 140.310 298.650 AA L HAAAAAA
0.00000000E+00 1.00000000E+02 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00
0.000 140.190 298.650 AA L HAAAAAA
0.00000000E+00 1.00000000E+02 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00
-1.0
$ Transfer to ASCII plotting data
HI=0 F4=0 CN=0 AE=0 EM=0 SC=0 FI=0 PL=1 TS=0 AM=0 MG=0 LA=0 OD=0 XS=0 0 0

4201.6807 4545.4545 10.2000 100.0000 5 0 12 0 1.000 0 0 0
0.0000 1.2000 7.020 0.200 4 0 1 1 0 0 0 3 27
4201.6807 4545.4545 10.2000 100.0000 5 0 12 0 1.000 0 0 0
0.0000 1.2000 7.020 0.200 4 0 1 1 0 0 0 3 28
-1.0
%%%%%
”

[pernak18]

@satpyscene it's been a while for me, but i'll give it a shot. there are two ways to run LBL -- on layers or on levels. you've tried the latter and want the former. i would suggest you take a look at Record 2.1 in the HTML documentation (that file, assuming you have the docs/html directory somewhere locally, needs to be opened in a web browser for it to display properly). essentially, you want IATM=0, not IATM=1. then you provide layer specs with records 2.1, 2.1.1, and 2.1.2

give that a try. in the meantime, i can try to dig up any examples i might have. again, it's been a little while for me, and i usually worked with IATM=1 as well

@jrmascio @mjiacono did i mess any of that explanation up?

[pernak18]

yeah, so here's a snippet (4 out of 27 layers) of my TAPE5 where i used IATM=0:

$                                                                               
 HI=1 F4=1 CN=1 AE=0 EM=1 SC=3 FI=0 PL=0 TS=0 AM=0 MG 0 LA=0 OD=0 XS=1    0    0    1
   050.00    1950.00       0.0       0.0       0.0       0.0    0.0000     0.000    0            0.0    1
1
1.07
   000.000      0.00       0.0       0.0      0.00       0.0       0.0     
 1 27    7  1.000000    User defined H1=    5.38 H2=   68.00 ANG=   0.000 LEN= 0                                        
   525.4290        265.00              3   5.383 529.390 267.51  5.500 521.476 264.27                                   
  8.1268656e-04  3.8999982E-04  3.8735559E-08  3.1999986E-07  1.2976621E-07  1.7000590E-06  2.0899991E-01  1.3273959E+23
   513.1512        262.95              3                         5.750 504.828 261.72                                   
  7.7661790e-04  3.8999998E-04  3.9149464E-08  3.1999998E-07  1.2950105E-07  1.7000706E-06  2.0899999E-01  2.7913389E+23
   496.7056        260.90              3                         6.000 488.555 259.94                                   
  6.2218481e-04  3.8999998E-04  3.9722709E-08  3.1999998E-07  1.2898444E-07  1.7000267E-06  2.0899999E-01  2.7233299E+23
   480.6890        258.54              3                         6.250 472.816 257.03                                   
  5.9786972e-04  3.8999998E-04  4.0303712E-08  3.1999998E-07  1.2823557E-07  1.6998905E-06  2.0899999E-01  2.6593037E+23

note we provide layer and level information, and only the lower bound of the first layer is needed explicitly (LBL assumes high er layers' lower bound is the upper bound of the previous layer), but the molecule amounts are layer averages.

[mjiacono]

One thing you could try is to keep your TAPE5 file (using IATM=1) and make use of the fact that you have set IPUNCH=1 in Record 3.1. That setting will create a TAPE7 output file, which formats the input atmosphere (showing layers and levels) so that it can be used as the input atmosphere in a TAPE5 with IATM=0. You can then slightly modify the level pressures in your TAPE5, and see what happens to the layer pressure in the resulting TAPE7. If you iterate on this process, you should be able to adjust your levels so that the layer pressure comes out where you want it to be. You can also make the adjacent layer and level temperatures isothermal so those don't change. You can then either use your modified TAPE5 to run the OD output, or you can install the TAPE7 information into a new TAPE5 that is set with IATM=0.