Hilary E. Snell

Infrared Radiance Modeling by Optimal Spectral Sampling

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.

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 radiance modeling using the optimal spectral sampling (OSS) method

Optimal Spectral Sampling (OSS) is a new approach to radiative transfer modeling which addresses the need for algorithm speed, accuracy, and flexibility. The OSS technique allows for the rapid calculation of radiance for any class of multispectral, hyperspectral, or ultraspectral sensors at any spectral resolution operating in any region from microwave through UV wavelengths by selecting and appropriately weighting the monochromatic points that contribute over the sensor bandwidth. This allows for the calculation to be performed at a small number of spectral points while retaining the advantages of a monochromatic calculation such as exact treatment of multiple scattering and/or polarization. The OSS method is well suited for remote sensing applications which require extremely fast and accurate radiative transfer calculations: atmospheric compensation, spectral and spatial feature extraction, multi-sensor data fusion, sub-pixel spectral analysis, qualitative and quantitative spectral analysis, sensor design and data assimilation. The OSS was recently awarded a U.S. Patent (#6,584,405) and is currently used as part of the National Polar-Orbiting Operational Environmental Satellite System (NPOESS) CrIS, CMIS, and OMPS-IR environmental parameter retrieval algorithms. This paper describes the theoretical basis and development of OSS and shows examples of the application and validation of this technique for a variety of different sensor types and applications.

Simultaneous retrieval of middle atmospheric temperature and trace gas species volume mixing ratios from Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A)

We have developed an atmospheric temperature and constituent retrieval code using an efficient derivative of Fast Atmospheric Signature Code 3 coupled with the optimal estimation inversion method. This code provides arbitrary spectral resolution and instrument response function and was thoroughly tested with model generated “data” and then applied to the stratospheric Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) database. The CIRRIS 1A experiment flew aboard space shuttle mission Space Transportation System 39 in April 1991. It included a 1 cm−1 resolution midinfrared interferometer that made numerous scans of the earthlimb. A small subset of these included a survey of the stratosphere during the day at 50°–65°N latitude and during the night at 35°–50°S latitude. The retrieval code provides us with the error analysis tools necessary to determine the relative contributions of the measurements and any a priori information made available to the code. Temperature and constituent profiles are reported along with comparisons to other recent atmospheric measurements. The absence of sufficient information to retrieve certain species is also discussed.

Non-LTE effects on retrieval of temperature from the CO2 laser bands using CIRRIS 1A data

It is well known that in the upper stratosphere and lower mesosphere, the CO2 laser band (00011–10001) transitions are populated by both local thermodynamic equilibrium (LTE) and non-LTE mechanisms. Examination of infrared (IR) limb emission measurements from the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment clearly illustrates this effect. The presence of non-LTE emissions, if left unaccounted for, will severely degrade the retrieval of kinetic temperature profiles. We have applied a non-LTE compensation technique whereby the non-LTE component is modeled and the resulting non-LTE radiance is subtracted from the measured radiances. The kinetic temperature profile is then retrieved utilizing the modified data. As a means of validating this approach, we have compared the resulting temperature profiles to ones retrieved using the 792 cm−1 Q branch of CO2, the (11101–10002) transition, which is not significantly affected by non-LTE excitation.

NPOESS Cross-Track Infrared and Microwave Sounder Suite (CrIMSS) EDR Retrieval Algorithm and Its Performance Assessment

This paper presents a brief description and the current performance estimate of the Environmental Data Records (EDR) retrieval algorithm for the Cross-track Infrared and Microwave Sounder Suite (CrIMSS) to fly on the NPP/NPOESS satellites.

Atmospheric temperature, moisture, and trace gas retrieval algorithm for the NPOESS/NPP cross-track infrared microwave sounder suite (CrIMSS)

The Cross-track Infrared and Microwave Sounder Suite (CrIMSS) consists of a microwave radiometer and an infrared interferometer and is scheduled to fly on the NPP and NPOESS satellites. The sensors are designed for the accurate measurement of atmospheric pressure, temperature and moisture profiles. This paper presents an overview of the CrIMSS sensors and the retrieval algorithm. Validation of the algorithm with current satellite sounder data will also be presented.