Leonard W. Abreu

MODTRAN2: suitability for remote sensing

MODTRAN2 (1992) is the most recent version of MODTRAN, the Moderate Resolution Atmospheric Radiance and Transmittance Model, first released by the Geophysics Directorate, Phillips Laboratory, in 1990. It encompasses all the capabilities of LOWTRAN 7, the historic 20 cm-1 resolution radiance code, but incorporates a much more sensitive molecular band model with 2 cm-1 resolution. For inversion algorithm applications, MODTRAN2 must prove to be sufficiently accurate when calculating layer- specific perturbations. First steps in establishing this capability have recently been accomplished. DREV (Defence Research Establishment Valcartier, Canada), in conjunction with the Geophysics Directorate, has taken measurements with a surface-based Bomem interferometer (approximately 1 cm-1 resolution), with full supporting sonde profiles (z, T, p, and relative humidity). This suggests that the derivative matrices, typically required for inversion algorithms, may be readily (and rapidly) calculated using MODTRAN whenever its spectral resolution is adequate.

History of one family of atmospheric radiative transfer codes

Beginning in the early 1970s, the then Air Force Cambridge Research Laboratory initiated a program to develop computer-based atmospheric radiative transfer algorithms. The first attempts were translations of graphical procedures described in a 1970 report on The Optical Properties of the Atmosphere, based on empirical transmission functions and effective absorption coefficients derived primarily from controlled laboratory transmittance measurements. The fact that spectrally-averaged atmospheric transmittance (T) does not obey the Beer-Lambert Law (T equals exp(-(sigma) (DOT)(eta) ), where (sigma) is a species absorption cross section, independent of (eta) , the species column amount along the path) at any but the finest spectral resolution was already well known. Band models to describe this gross behavior were developed in the 1950s and 60s. Thus began LOWTRAN, the Low Resolution Transmittance Code, first released in 1972. This limited initial effort has how progressed to a set of codes and related algorithms (including line-of-sight spectral geometry, direct and scattered radiance and irradiance, non-local thermodynamic equilibrium, etc.) that contain thousands of coding lines, hundreds of subroutines, and improved accuracy, efficiency, and, ultimately, accessibility. This review will include LOWTRAN, HITRAN (atlas of high-resolution molecular spectroscopic data), FASCODE (Fast Atmospheric Signature Code), and MODTRAN (Moderate Resolution Transmittance Code), their permutations, validations, and applications, particularly as related to passive remote sensing and energy deposition.

Reviewing atmospheric radiative transfer modeling: new developments in high- and moderate-resolution FASCODE/FASE and MODTRAN

Spectrally uniform treatment of the atmospheric radiative transfer (RI) problem has been approached through two different techniques - very high resolution line-by-line (LBL) algorithms and lower resolution band models (BM). Each has its advantages and specific applications. However, if commonality and validation of a specific pair of RI approaches is to be mutually maintained, then these codes must be continually reevaluated against both measurements and other models.

Atmospheric Attenuation In The 30 To 300 GHz Region Using RADTRAN And MWTRAN

Radiative transfer in the earths atmosphere is modeled by a computer code called RADTRAN. RADTRAN may be used to model atmospheric transmission and emission in the frequency range of 30-300 GHz. Two versions of this computer code exist: the first is RADTRAN which incorporates six model clear atmospheres, six cloud models, six rain models and eight humidity models to model worldwide atmospheric conditions; the second is MWTRAN which is approximately one-sixth of the physical size of RADTRAN. Both RADTRAN and MWTRAN allow the researcher to read in any model data in any format.

Millimeter Wave Propagation Modeling

Atmospheric models of fog, clouds and rain are described. These models are typical for mid-latitude temperate regions of the globe. A computer code for incorporating the models into a new efficient computer algorithm of the AFGL HITRAN series named FASCOD1 has been completed. The computer models presented allow calculation of atmospheric transmission or attenuation for millimeter and submillimeter waves (1-34 cm-1 or 1-1000 GHz). Four (4) fog models, eight (8) cloud types and rainfall rates from 1-150 mm hr available. All models consider hydrometeors as having temperatures between 0°C and 40°C and permit arbitrary input of atmospheric parameters and geometry (slant range).