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APPENDIX
MODTRAN 3 User Instructions
This description is based on Section 3 from the original LOWTRAN 7 Users Manual; plus the
"Readme" files on the MODTRAN 3 "ftp" site and the MODTRAN report:
Users Guide to LOWTRAN 7
AFGL-TR-88-0177
16 August 1988
MODTRAN: A Moderate Resolution Model
LOWTRAN 7 GL-TR-89-0122
30 April 1989
The scientific differences between the codes will be outlined in the Scientific Report due for
publication in early 1996.
3. INSTRUCTIONS FOR USING MODTRAN 3
The instructions for using MODTRAN 3 are similar to those for the earlier LOWTRAN 7
version. However, some new parameters have been added, necessitating the addition of one new card,
and minor modifications to two other cards. The new parameters (appearing on required CARD 1A),
are principally required to govern: (1) a second multiple scattering option (based on the multiple
stream DISORT algorithm, after Stamnes et al. 1988); (2) a new high resolution solar irradiance
(based on a full calculated irradiance, after Kurucz 1995) with an optional triangular smoothing
function; and (3) a "one-step" update to the CO mixing ratio. This last option is offered because, for
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historic calibration studies, the 330 ppmv for CO has been preserved in the code. However, CO
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increases by approximately 1/2 % per year, and is currently 355-360 ppmv.
The most important altered card is the vital CARD 4 from the LOWTRAN sequence, governing
the beginning and ending frequencies, the frequency step size on output, and spectral resolution (based
on a triangular slit function). The format for these variables has changed from real to integer values.
In addition, MODTRAN performs all of its calculations at 1 cm intervals (for frequencies between 0
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22000 cm ), independent of the choice of output step size and triangular resolution. The new
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MODTRAN 1 cm band models, the primary upgrade from LOWTRAN, provide much improved
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spectral accuracy and will still run very rapidly. For frequencies beyond the current band model upper
limit (22000 - 50000 cm ), MODTRAN uses a fixed 5 cm step size, with the transition occurring
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automatically. Note that the option to "incorrectly" request an un-smoothed large step size,
particularly as used in the solar regime, is no longer available.
