Infrared Radiance Modeling by Optimal Spectral Sampling

Abstract

This paper describes a rapid and accurate technique for the numerical modeling of band transmittances and radiances in media with nonhomogeneous thermodynamic properties (i.e., temperature and pressure), containing a mixture of absorbing gases with variable concentrations. The optimal spectral sampling (OSS) method has been designed specifically for the modeling of radiances measured by sounding radiometers in the infrared and has been extended to the microwave; it is applicable also through the visible and ultraviolet spectrum. OSS is particularly well suited for remote sensing applications and for the assimilation of satellite observations in numerical weather prediction models. The novel OSS approach is an extension of the exponential sum fitting of transmittances technique in that channel-average radiative transfer is obtained from a weighted sum of monochromatic calculations. The fact that OSS is fundamentally a monochromatic method provides the ability to accurately treat surface reflectance and spectral variations of the Planck function and surface emissivity within the channel passband, given that the proper training is applied. In addition, the method is readily coupled to multiple scattering calculations, an important factor for treating cloudy radiances. The OSS method is directly applicable to nonpositive instrument line shapes such as unapodized or weakly apodized interferometric measurements. Among the advantages of the OSS method is that its numerical accuracy, with respect to a reference line-by-line model, is selectable, allowing the model to provide whatever balance of accuracy and computational speed is optimal for a particular application. Generally only a few monochromatic points are required to model channel radiances with a brightness temperature accuracy of 0.05 K, and computation of Jacobians in a monochromatic radiative transfer scheme is straightforward. These efficiencies yield execution speeds that compare favorably to those achieved with other existing, less accurate parameterizations.

Cross-track Infrared Sounder (CrIS) satellite observations of tropospheric ammonia

“…The b a holds the corresponding a priori values, and K b = ∂L/∂b is the Jacobian describing the dependency of the forward model radiance L on the vector b. The fast forward model OSS-CrIS (Moncet et al, 2008), which is built from the Line-By-Line Radiative Transfer Model (LBLRTM) (Clough et al, 2005;Shephard et al, 2009;Alvarado et al, 2013), is used for these retrievals.…”

Validation of the CrIS fast physical NH<sub>3</sub> retrieval with ground-based FTIR

“…In this study we use the NH 3 retrieval as described by . The retrieval is based on an optimal estimation approach (Rodgers, 2000) that minimizes the differences between CrIS spectral radiances and simulated forward model radiances computed from the Optimal Spectral Sampling method (OSS) OSS-CrIS (Moncet et al, 2008), which is built from the well-validated Line-By-Line Radiative Transfer Model (LBLRTM) (Clough et al, 2005;Shephard et al, 2009;Alvarado et al, 2013) and uses the HITRAN database (Rothman et al, 2013) for its spectral lines. The fast computational speed of OSS facilitates the operational production of CrIS-retrieved (level 2) products using an optimal estimation retrieval approach (Moncet et al, 2005).…”

Simulation of nitrate, sulfate, and ammonium aerosols over the United States

“…TES is a high-resolution (0.06 cm −1 ) Fourier transform spectrometer onboard NASA's Aura satellite, in a sun-synchronous orbit with measurements at 01:30 and 13:30 LT. The spectrometer measures infrared radiation, and NH 3 concentrations are retrieved using optimal estimation methods Rodgers, 2000) with the Line-By-Line Radiative Tansfer Model (LBLRTM) and the fast forward model (OSS-TES) Moncet et al, 2008;Shephard et al, 2009). The ammonia data used in this study are from the TES Lite data product, Version 5 (http://avdc.gsfc.nasa.gov/index.…”