PcModWIn 6

 A user just clicks on the inputs and selects the appropriate values from easy to understand prompts. More detail on any input can be viewed by clicking on the input label, which calls up a large Windows help file with a complete description of the prompt. Compare this with a typical ASCII input file required to run MODTRAN directly:
 T   6    1    0    0    0    0    0    0    0    0    0    0    0    0.000   0.00
F   2T   5   330.000 
    1    0    0    0    0    0     0.000     0.000     0.000     0.000     0.000
     6.000    10.000    30.000     0.000     0.000     0.000    0          0.000
  2050.000  2150.000     1.000     1.000 
    0 

 If you compile MODTRAN yourself and run it from a command line, you must hand edit text files like this and make sure numbers are lined up in the proper columns to satisfy the requirements of formatted FORTRAN input. Part of the very first line of this file corresponds to the inputs shown on the window above. It gets worse because as some inputs are set to certain values, new MODTRAN options are enabled, so new lines of inputs are required (in the correct order, of course). MODTRAN input files using complex options like user defined atmospheres can easily run to 100 lines or more in length. PcModWin will generate all of these files with proper formatting automatically for you. For most people, filling out an input window like the one above is easier than editing a text file like this!  

The most significant new feature in MODTRAN® 6 and hence PcModWin is the capability to do line-by-line (LBL) transmission and radiance calculations. This means MODTRAN® calculations are applicable for laser calculations at all pressures at a spectral resolution determined by the HITRAN database. This is a significant enhancement over MODTRAN 5 with the 0.1 to 0.2 cm-1 capability. Hence there is no longer the need to us FASCOD for LBL calculations. Please see test case 1.5.11 AnLBLTemplate below for instructions on LBL calculations

The other changes from previous versions of MODTRAN are the input file format, and the programming language used for the source code. Previously, the code was written in FORTRAN with formatted FORTRAN input/output files. The current version is a combination of C++ and FORTRAN source code. The input can be in either the older FORTRAN format, i.e. tape5 etc., or in JSON format.

The material here is taken from MODTRAN® 6.0.0 User's Manual (revision 5), performed under US Government Contract No. FA9453-12-C0262; Data Item A007, July 2016. A. Berk, J. van den Bosch, F. Hawes, T. Perkins, P.F. Conforti, G.P. Anderson, R.G. Kennett, and P.K. Acharya.

A summary of new features includes:

• Reformulating the band model parameters and radiative transfer formalism to increase the resolution of MODTRAN spectral calculations to 0.1 cm-1;
• Increasing the top-of-atmosphere solar database resolution to 0.1 cm-1;
• Introducing Fontenla top-of-atmosphere solar irradiance data; SUNp1med2irradwn_Normt.dat is now the default.
• Incorporating code interface changes between MODTRAN and DISORT to increase its speed and accuracy of multiple scattering calculations;
• Upgrading MODTRAN to perform spectral radiance computations for auxiliary molecules (by including their concentrations and spectral parameters) that are not part of the traditional MODTRAN database; band models are provided for all HITRAN molecular species;
• Incorporating the effect of a thin layer of water, which can either simply wet the ground or accumulate on it, on radiance computations;
• Adding the capabilities to model a boundary layer aerosol whose extinction coefficient obeys the Angstrom law or to modify the extinction of a model aerosol with an Angstrom law perturbation;
• Adding the capability to determine the spherical albedo and reflectance of the atmosphere and diffuse transmittance from a single MODTRAN run;
• Adding the ability to include only the solar contribution to multiple scattering and ignore the thermal component where it is not significant;
• Adding an option to write spectral output in binary, and a utility to convert the binary output to ASCII;
• Upgrading the MODTRAN-DISORT interface so that only a single parameter (MXCMU in routine disusr_mod.f90) needs to be modified to change the maximum number of streams available for DISORT runs.
• Adding dithering of the solar angle in cases where the DISORT particular solution to the solar problem was unstable.
• Adding option to save and re-use DISORT scattering data.
• Adding .wrn files, and modifying comments, warnings, and errors to have a common format.
• Adding separate flux and atmospheric correction data, .acd, binary output files.
• Adding option to model line center data via 2 pairs of absorption coefficient and line spacing band model parameters.
• Introducing a much improved algorithm for computing the finite bin Voigt transmittance.
• Adding option to use distinct temperature grid data for each auxiliary (Y) species.
• Adding atmospheric correction data to the channel spectra output.
• Adding path geometry output files named _pth (which can be used as path geometry input files by renaming the files without the underscore).
• Adding thermal scatter as a DISORT run spectral output.
• Replacing the spherical refractive geometry package with an iterative circular arc algorithm for improved accuracy.
• Adding H2-CH4 and CH4-CH4 collision induced absorption features for modeling extra- terrestrial planet atmospheres.

AnLBLTemplate

This file was added to the set of test cases to serve as a template for doing line-by-line (LBL) calculations. We recommend you start with this case and modify it to meet your specific requirements.

It is for a slant path geometry, using a Mid-Latitude atmospheric model. It calculates radiance with scattering over the 0.7 to 1.2 μm spectral interval (i.e. visible to near IR). It includes multiple scattering using the Isaac 2 stream algorithm. Because of scattering and spectral interval, this calculation may take several hours to complete.

For most applications, you will do calculations over a much smaller spectral interval which will significantly reduce the time. However, keep the following "rules of thumb" in mind:

1. Multiple scattering is very important in the visible spectral region;
2. Less important in the 1 to 3 μm region;
3. And NOT a contributor beyond 3.5 μms.

Consequently, we recommend:

1. Visible region, use multiple scattering, DISORT scattering algorithm with 4 or more streams;
2. 1-2 μm, use multiple scattering, DISORT scattering algorithm with 2 streams;
3. 2-3.5 μm, use multiple scattering, Issaac scattering algorithm with 2;
4. Beyond 3.5 μm, NO multiple scattering is needed.

An LBL calculation requires three major changes to a "standard", e.g. MODTRAN® 5 file as shown in the figures below.

Also, keep in mind that an LBL calculation may generate large files depending on the options selected. For example, the *_highres.csv file may have over 1,000,000 lines if you cover a 1000 cm-1 spectral region.

Three are three parameters that must be set to do an LBL calculations. They are:

1. Select "Line by Line" for the Calculation Option on the first input screen as shown in the figure immediately below.
2. Input the Number of LBL Spectral Points (NLBL) on the first input screen. This is also shown in the figure below. NLBL is the integer number of spectral points in each 0.1 cm-1 bin at which LBL calculations are performed. The default is 100 corresponding to 0.001 cm-1 spacing. 100 is used in this test case. The figure below with data from the AnLBLTemplate*_highres.csv file demonstrated the effect of setting NLBL = 100.