Validation of FASCOD3 and MODTRAN3: comparison of model calculations with interferometer observations from SPECTRE and ITRA

The Air Force has long maintained an `exact' accelerated line-by-line radiative transfer model, the Fast Atmospheric Signature CODE (FASCODE), appropriate for applications in both the laboratory and any arbitrary line-of-sight in the atmosphere. The current version of FASCODE, FASCOD3, is fully compatible with the HITRAN92 database, including access to the temperature-dependent cross sections for heavy molecules (e.g. CFC's etc.). Some new features of FASCOD3 are: line coupling algorithms for both 15 micron CO2 and the mm lines of O2; non-local thermodynamic equilibrium models; updated H2O continuum; multiple scattering capability; and laser options for lidar modeling applications. FASCOD3 is increasingly being used as a high resolution remote sensing data analysis tool from microwave and infrared (IR) to ultraviolet (UV) spectral ranges. The Moderate Resolution Atmospheric Radiance and Transmittance Model (MODTRAN) is a `first principle' band model with a nominal spectral resolution of 2.0 cm-1. Model parameters are derived directly from the HITRAN database. Standard 2-parameter Curtis-Godson approximations are used for H2O, CO2, etc., and 3-parameter Goody approximation is used for O3. The current version of MODTRAN, MODTRAN3, encompasses all the capabilities of LOWTRAN, and contains many important elements that many other band models do not incorporate, including: Voigt line shape; spherical geometry; solar and lunar source functions (irradiance); internal aerosol, clouds, and rain models; single and multiple scattering; default atmospheric profiles. Because of its speed advantage over FASCODE, about a factor of 100, and ease of use, MODTRAN3 has been and will continue to be an effective tool for atmospheric spectral heating/cooling rate calculations and atmospheric corrections in earth surface sensing and imaging. Because of the large user based of FASCODE and MODTRAN, it is very important to continue to improve and validate those codes. This paper presents the validation of FASCOD3 and MODTRAN3 in the context of SPECTRE and ITRA. Important considerations (such as water vapor continuum, frequency-dependent sea surface emissivity in the IR window region, and spectral resolution of MODTRAN3) in the comparison of model calculations with high resolution interferometer measurements will be discussed.