A set of 26 clear-sky, co-located, infrared data from NASA's space-based, downward looking Atmospheric Infrared Sounder (AIRS)1 and its Clouds and the Earth's Radiant Energy System (CERES)2 have been paired with the DOE's Southern Great Plains (SGP)3 ground-based, upward looking Atmospheric Emitted Radiance Interferometer (AERI)4. These data sets have then been simulated using the MODTRAN®55 (MOD5) radiative transfer code with standard auxiliary 'truth' data as input. Of particular interest is the impact, if any, of the large Ground Sampling Distance (GSD) of AIRS and CERES (minimum radii of approximately 13 and 26 km, respectively) vs. the soda-straw up-looking mode of AERI. The smaller Moderate Resolution Imaging Spectroradiometer (MODIS)6 GSD (<1 km) provides estimated distributions of land-type and albedo within the larger footprints. The SGP's coincident vertical profile sondes and Aeronet7 retrievals, along with other satellite data [Ozone Monitoring Instrument (OMI)8] constrain the surface type, column ozone and aerosol optical depth that existed during the measurement events. Initial MOD5 calculations, using these ancillary data as input, have replicated the AERI and AIRS measurements to within an average difference of ~1% over their entire spectral range. Using MODIS visible albedo9 retrievals to extend the albedo into the long wave (LW) only slightly improved the statistical comparison between the CERES and MOD5 broadband LW radiances agreement, from 3% to ~1.5%, while increasing the variance. While these results are not sufficient for specific instrument inversion algorithms, they suggest some confidence in the generic use of MODTRAN®5 to help integrate and spectrally extend assorted data sets for sensitivity studies of Climate Change, where the estimated required sensitivity is <1%.

Infrared spectrometer data from the space-based downward looking NASA Atmospheric Infrared Sounder (AIRS) and from the ground-based upward looking Atmospherically Emitted Radiance Interferometer (AERI) are used in this study. Spatially and temporally concurrent cloud free data from these spectrometers are correlated, analyzed, and compared to MODTRAN®5 simulated data. The effects of optical depth, water vapor, ozone, carbon dioxide, and methane on infrared remote sensing applications are characterized.

Testing MODTRATM5 (MOD5) capabilities against NASA satellite state-of-the-art radiance and irradiance measurements has recently been undertaken. New solar data have been acquired from the SORCE satellite team, providing measurements of variability over solar rotation cycles, plus an ultra-narrow calculation for a new solar source irradiance, extending over the full MOD5 spectral range. Additionally, a MOD5-AIRS analysis has been undertaken with appropriate channel response functions. Thus, MOD5 can serve as a surrogate for a variety of perturbation studies, including two different modes for including variations in the solar source function, Io: (1) ultra-high spectral resolution and (2) with and without solar rotation. The comparison of AIRS-related MOD5 calculations, against a suite of 'surrogate' data generated by other radiative transfer algorithms, all based upon simulations supplied by the AIRS community, provide validation in the Long Wave Infrared (LWIR). All ~2400 AIRS instrument spectral response functions (ISRFs) are expected to be supplied with MODTRANTM5. These validation studies show MOD5 replicates line-by-line (LBL) brightness temperatures (BT) for 30 sets of atmospheric profiles to approximately -0.02°K average offset and <1.0°K RMS.

The MODTRAN (TM) 5 radiation transport (RT) model is a major advancement over earlier versions of the MODTRAN (TM) atmospheric transmittance and radiance model. New model features include (1) finer spectral resolution via the Spectrally Enhanced Resolution MODTRAN (TM) (SERTRAN) molecular band model, (2) a fully coupled treatment of auxiliary molecular species, and (3) a rapid, high fidelity multiple scattering (MS) option. The finer spectral resolution improves model accuracy especially in the mid- and long-wave infrared atmospheric windows; the auxiliary species option permits the addition of any or all of the suite of HITRAN molecular line species, along with default and user-defined profile specification; and the MS option makes feasible the calculation of Vis-NIR databases that include high-fidelity scattered radiances. Validations of the new band model algorithms against line-by-line (LBL) codes have proven successful.

The MODTRAN5 radiation transport (RT) model is a major advancement over earlier versions of the MODTRAN atmospheric transmittance and radiance model. New model features include (1) finer spectral resolution via the Spectrally Enhanced Resolution MODTRAN (SERTRAN) molecular band model, (2) a fully coupled treatment of auxiliary molecular species, and (3) a rapid, high fidelity multiple scattering (MS) option. The finer spectral resolution improves model accuracy especially in the mid- and long-wave infrared atmospheric windows; the auxiliary species option permits the addition of any or all of the suite of HITRAN molecular line species, along with default and user-defined profile specification; and the MS option makes feasible the calculation of Vis-NIR databases that include high-fidelity scattered radiances. Validations of the new band model algorithms against line-by-line (LBL) codes have proven successful.

The MODTRAN 1 cm(-1) band model has been reformulated for application to higher spectral resolution. Molecular line center absorption is still determined from finite spectral bin equivalent widths but is now partitioned between the bin containing the molecular transition and its nearest neighbor bin. Also, the equivalent width calculation has been upgraded to retain to maintain high accuracy at the increased spectral resolution. The MODTRAN Lorentz line tail spectral bin absorption coefficient data has been replaced by a more general and accurate Pade approximant for Voigt line tails, and higher order pressure dependencies are now modeled. Initial comparisons to the FASE model and to measurement data are presented.

The new MODTRAN4 band model, with its Correlated-k (CK) Beer’s Law algorithm, can efi e.g. DISORT. The second validation step centers on comparisons against a variety of measurements, mostly airborne visible and IR up-welling radiances, including both clear and clouded skies. However, in this instance, only simulations and sensitivity studies will be presented.

MODTRAN, the Moderate Resolution Atmospheric Radiance and Transmittance Model, encompasses all the capabilities of LOWTRAN 7, the widely used 20 cm(exp -1) resolution radiance code, but incorporates a much more sensitive molecular band model with 2 cm(exp -1) resolution. MODTRAN contains many important elements that other band model based radiative transfer codes do not incorporate. It shares with FASCODE: spherical geometry, single and multiple scattering default atmospheric profile descriptors (gases, aerosols, clouds, fogs, and rain), and molecular continua (H2O, CO2, O3, O2, N2). In addition, it can calculate the solar/lunar direct and scattered radiation. MODTRAN3 was released to the general public in November 1994. It has several important features that the previous version, MODTRAN2, does not have. Chloro-fluorocarbon (CFC) and related heavy molecules (whose spectroscopic properties first appear on the HITRAN92 data base as temperature-dependent cross sections) have been incorporated into pseudo-band models, with provision for using both default and user supplied profiles. The addition of SO2 and O2 in the UV, along with upgraded ozone Chappuis bands in the visible is also part of MODTRAN3. An improved multiple scattering algorithm, the DIScrete Ordinate Radiative Transfer (DISORT) has also been incorporated into MODTRAN3. MODTRAN is very fast: simple timing runs of MODTRAN3 vs. FASCOD3 show an improvement of more than a factor of 100 for a typical 500 cm(exp -1) spectral interval and comparable vertical layering. Speed is an important consideration in heating/cooling rates calculations, where a large number of radiative transfer calculations are needed. The MODTRAN3 used in this study is based on HITRAN92, but as mentioned, above, it will be upgraded to HITRAN94 upon its release at the end of 1994. MODTRAN has been adopted by: some researchers in the AVIRIS program as one radiative transfer code to derive surface reflectance from AVIRIS measurements. The accuracy of the code is very important because any errors in the radiative transfer calculation will directly translate into errors in the derived surface reflectance. In this paper, the new solar irradiance calculated by Kurucz, which is adopted in MODTRAN3, will be presented. Recent validations of MODTRAN3 with airborne high resolution interferometer measurements over ocean will be discussed. Good agreeement between model calculations and measurements was achieved.

The Moderate Resolution Atmospheric Radiance and Transmittance Model (MODTRAN3) is the developmental version of MODTRAN and MODTRAN2. The Geophysics Directorate, Phillips Laboratory, released a beta version of this model in October 1994. It encompasses all the capabilities of LOWTRAN7, the historic 20 cm{sup -1} resolution (full width at half maximum, FWHM) radiance code, but incorporates a much more sensitive molecular band model with 2 cm{sup -1} resolution. The band model is based directly upon the HITRAN spectral parameters, including both temperature and pressure (line shape) dependencies. Validation against full Voigt line-by-line calculations (e.g., FASCODE) has shown excellent agreement. In addition, simple timing runs demonstrate potential improvement of more than a factor of 100 for a typical 500 cm{sup -1} spectral interval and comparable vertical layering. Not only is MODTRAN an excellent band model for {open_quotes}full path{close_quotes} calculations (that is, radiance and/or transmittance from point A to point B), but it replicates layer-specific quantities to a very high degree of accuracy. Such layer quantities, derived from ratios and differences of longer path MODTRAN calculations from point A to adjacent layer boundaries, can be used to provide inversion algorithm weighting functions or similarly formulated quantities. One of the most exciting new applicationsmore » is the rapid calculation of reliable IR cooling rates, including species, altitude, and spectral distinctions, as well as the standard spectrally integrated quantities. Comparisons with prior line-by-line cooling rate calculations are excellent, and the techniques can be extended to incorporate global climatologies of both standard and trace atmospheric species.« less

The U.S. Air Force has long maintained an `exact' accelerated line-by-line (LBL) 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 first version was released in 1978 with optimized Voigt line shape decomposition and layering algorithms; it had a speed advantage of about 100 over existing fixed frequency LBL codes. 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., chloro-fluorocarbons/CFCs, 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. Because of its speed over other LBL codes and extensive validations against measurements, FASCOD3 is increasingly being used as a high resolution remote sensing data analysis tool from microwave and infrared (IR) to ultraviolet (UV) spectral ranges.

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.

In order to develop a method for the retrieval of temperature and water vapor profiles from IR emission spectra, the Defence Research Establishment Valcartier (DREV) has recently acquired a new device referred to as the Double input Beam Interferometer Sounder (DBIS). This sounder has been conceived to match the needs encountered in many remote sensing scenarios. It is a portable device composed of a Fourier spectrometer and one or optionally two telescopes that measures the atmospheric emission in the IR region from 3 to 20 μm at a spectral resolution of 1 cm-1and at any prescribed viewing angles. In the summer 1992, the DBIS has been deployed successfully in a series of field experiments, producing spectral data of good quality for a variety of sky conditions. These spectral data are now being utilized as a guide for the development of an optimized inversion algorithm