Clear-sky outgoing longwave radiation (OLR) is computed using the Atmospheric and Environmental Research (AER), Inc., Rapid Radiative Transfer Model (RRTM) for comparison with the observations of the Clouds and the Earth's Radiant Energy System (CERES) for both ocean and land scenes. CERES clear-sky OLR is in agreement with RRTM model calculations to 0.2% accuracy using best estimate radiosondes (BE) launched coincident with NASA Aqua overpasses at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site and 0.8% using retrieved profiles of temperature, water vapor, ozone, and surface parameters from the Atmospheric Infrared Sounder (AIRS) on the Aqua platform. A partial flux analysis using AIRS radiances implies an accuracy for the RRTM model in the far infrared of 0.4% (about 0.5 W/m(2)) for wave numbers less than 650 cm(-1) (wavelengths greater than 15.4 mu m). CERES minus model biases over clear-sky ocean are similar to previously published results. Ordering the results according to the magnitude of the measured minus model mean bias for nighttime, tropical, ocean gives: +0.57 +/- 1.9 W/m(2) (Dessler/Fu-Liou), +0.83 +/- 1.5 W/m(2) (Huang/MODTRAN5), +1.6 +/- 1.6 W/m(2) (Moy/RRTM), +3.7 +/- 2.1 W/m(2) (Dessler/Chou). Comparison of observed minus modeled OLR over land are included in this study. Excluding nonfrozen ocean, a mean difference over land of +2.0 W/m(2) for nighttime cases and +1.0 W/m(2) for daytime cases is found where the land classes are weighted inversely by their standard error. The nighttime bias is quite consistent across all the land classes. The daytime bias shows less consistency with a tendency toward larger CERES minus AIRS RRTM OLR bias for the land classes with smaller vegetation fraction. Comparison of clear-sky CERES and AIRS RRTM OLR over cold snow-/ice-covered surfaces (mainly in the polar regions) is complicated by the use of the MODIS cloud mask in the identification of the clear CERES footprints used in the comparison. Clear scenes over cold surfaces can be identified more reliably in the daytime, for which the comparison between CERES and AIRS RRTM is better than 1.2 W/m(2) indicating good agreement.

The Mixed-Phase Arctic Cloud Experiment (M-PACE) was conducted from 27 September through 22 October 2004 over the Department of Energy's Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) on the North Slope of Alaska. The primary objectives were to collect a dataset suitable to study interactions between microphysics, dynamics, and radiative transfer in mixed-phase Arctic clouds, and to develop/evaluate cloud property retrievals from surface- and satellite-based remote sensing instruments. Observations taken during the 1977/98 Surface Heat and Energy Budget of the Arctic (SHEBA) experiment revealed that Arctic clouds frequently consist of one (or more) liquid layers precipitating ice. M-PACE sought to investigate the physical processes of these clouds by utilizing two aircraft (an in situ aircraft to characterize the microphysical properties of the clouds and a remote sensing aircraft to constraint the upwelling radiation) over the ACRF site on the North Slope of Alaska. The measurements successfully documented the microphysical structure of Arctic mixed-phase clouds, with multiple in situ profiles collected in both single- and multilayer clouds over two ground-based remote sensing sites. Liquid was found in clouds with cloud-top temperatures as cold as -30 degrees C, with the, coldest cloud-top temperature warmer than -40 degrees C sampled by the aircraft. Remote sensing instruments suggest that ice was present in low concentrations, mostly concentrated in precipitation shafts, although there are indications of light ice precipitation present below the optically thick single-layer clouds. The prevalence of liquid down to these low temperatures potentially could be explained by the relatively low measured ice nuclei concentrations.

The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS), developed for the NASA New Millennium Program (NMP) Earth Observing-3 (EO-3) mission, has recently completed a series of uplooking atmospheric measurements. The GIFTS development demonstrates a series of new sensor and data processing technologies that can significantly expand geostationary meteorological observational capability. The resulting increase in forecasting accuracy and atmospheric model development utilizing this hyperspectral data is demonstrated by the uplooking data. The GIFTS sensor is an imaging FTS with programmable spectral resolution and spatial scene selection, allowing spectral resolution and area coverage to be traded in near-real time. Due to funding limitations, the GIFTS sensor module was completed as an engineering demonstration unit that can be upgraded to flight quality. This paper reviews the GIFTS system design considerations and the technology utilized to enable a nearly two order performance increase over the existing GOES sounder and shows its capability. While not designed as an operational sensor, GIFTS EDU provides a flexible and accurate testbed for the new products the hyperspectral era will bring. Efforts to find funding to upgrade and demonstrate this amazing sensor in space are continuing.

The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) was developed for the NASA New Millennium Program (NMP) Earth Observing-3 (EO-3) mission. This paper discusses the GIFTS measurement requirements and the technology utilized by the GIFTS sensor to provide the required system performance. Also presented are preliminary results from the recently completed calibration of the instrument. The GIFTS NMP mission challenge was to demonstrate new and emerging sensor and data processing technologies to make revolutionary improvements in meteorological observational capability and forecasting accuracy using atmospheric imaging and hyperspectral sounding methods. The GIFTS sensor is an imaging FTS with programmable spectral resolution and spatial scene selection, allowing radiometric accuracy and atmospheric sounding precision to be traded in near-real time for area coverage. System sensitivity is achieved through the use of a cryogenic Michelson interferometer and two large-area, IR focal plane detector arrays. Due to funding limitations, the GIFTS sensor module was completed as an engineering demonstration unit, which can be upgraded for flight qualification. Capability to meet the next generation geosynchronous sounding requirements has been successfully demonstrated through thermal vacuum testing and rigorous IR calibration activities.

Comparisons between observed AIRS radiances and radiances computed from coincident in situ profile data are used to validate the accuracy of the AIRS radiative transfer algorithm (RTA) used in version 4 processing at Goddard Space Flight Center. In situ data sources include balloon‐borne measurements with RS‐90 sensors and frost point hygrometers and Raman lidar measurements of atmospheric water vapor. Estimates of the RTA accuracy vary with wave number region but approach 0.2 K in mid‐ to lower‐tropospheric temperature and water vapor sounding channels. Temperature channel radiance biases using ECMWF forecast/analysis products are shown to be essentially identical to those observed with coincident sonde observations, with somewhat higher biases in water vapor channels. Some empirical adjustments to the RTA channel‐averaged absorption coefficients were required to achieve these stated accuracies.

The Atmospheric Infrared Sounder (AIRS) is the first of a new generation of advanced satellite‐based atmospheric sounders with the capability of obtaining high–vertical resolution profiles of temperature and water vapor. The high‐accuracy retrieval goals of AIRS (e.g., 1 K RMS in 1 km layers below 100 mbar for air temperature, 10% RMS in 2 km layers below 100 mbar for water vapor concentration), combined with the large temporal and spatial variability of the atmosphere and difficulties in making accurate measurements of the atmospheric state, necessitate careful and detailed validation using well‐characterized ground‐based sites. As part of ongoing AIRS Science Team efforts and a collaborative effort between the NASA Earth Observing System (EOS) project and the Department of Energy Atmospheric Radiation Measurement (ARM) program, data from various ARM and other observations are used to create best estimates of the atmospheric state at the Aqua overpass times. The resulting validation data set is an ensemble of temperature and water vapor profiles created from radiosondes launched at the approximate Aqua overpass times, interpolated to the exact overpass time using time continuous ground‐based profiles, adjusted to account for spatial gradients within the Advanced Microwave Sounding Unit (AMSU) footprints, and supplemented with limited cloud observations. Estimates of the spectral surface infrared emissivity and local skin temperatures are also constructed. Relying on the developed ARM infrastructure and previous and ongoing characterization studies of the ARM measurements, the data set provides a good combination of statistics and accuracy which is essential for assessment of the advanced sounder products. Combined with the collocated AIRS observations, the products are being used to study observed minus calculated AIRS spectra, aimed at evaluation of the AIRS forward radiative transfer model, AIRS observed radiances, and temperature and water vapor profile retrievals. This paper provides an introduction to the ARM site best estimate validation products and characterizes the accuracy of the AIRS team version 4 atmospheric temperature and water vapor retrievals using the ARM products. The AIRS retrievals over tropical ocean are found to have very good accuracy for both temperature and water vapor, with RMS errors approaching the theoretical expectation for clear sky conditions, while retrievals over a midlatitude land site have poorer performance. The results demonstrate the importance of using specialized “truth” sites for accurate assessment of the advanced sounder performance and motivate the continued refinement of the AIRS science team retrieval algorithm, particularly for retrievals over land.

AIRS and MODIS on the EOS Aqua spacecraft collect global observations of the Earth's upwelling infrared radiance for numerous remote sensing and climate related applications. This paper presents comparisons of the AIRS and MODIS radiance observations and illustrates the utility of using high–spectral resolution observations to create a highly accurate assessment of broadband sensor calibration. In the analysis, the high–spectral resolution AIRS spectra are reduced to MODIS spectral resolution, and the high–spatial resolution MODIS data are reduced to AIRS spatial resolution for global data collected on 6 September 2002 and 18 February 2004. Spatially uniform scenes are selected, and the observed differences are characterized as a function of several parameters including scene temperature, sensor scan (view) angle, and solar zenith angle. The comparisons are in general very good with respect to the expected radiometric accuracies of the sensors, with mean brightness temperature differences of 0.1 K or less for many of the MODIS bands. Uncertainties of these determinations range from near 0 K for window region bands to as large as 0.2 K for other bands. For MODIS water vapor bands 27 (6.8 μm) and 28 (7.3 μm) and temperature sounding bands 34 (13.7 μm), 35 (13.9 μm), and 36 (14.2 μm), the differences exhibit a dependence on scene temperature, with peak differences exceeding 1 K for bands 27 and 36. Differences as a function of scan angle are 0.4 K or less for all bands, and scan angles but clear trends are defined. Results for the 2 days demonstrate good reproducibility with changes in mean differences of 0.1 K or less for most bands.

We expand upon methods for retrieving thermal infrared cooling rate profiles, originally developed by Liou and Xue (1988) through application to the inversion of the stratospheric cooling rate produced by carbon dioxide (CO2) and a formal description of the associated error budget. Specifically, we infer lower‐ and mid‐stratospheric cooling rates from the CO2 ν2 band on the basis of selected spectral channels and available data from the Atmospheric Infrared Sounder (AIRS). In order to establish the validity of our results, we compare our retrievals to those calculated from a forward radiative transfer program using retrieved temperature data from spectra taken by the Scanning High‐Resolution Interferometer Sounder (S‐HIS) on two aircraft campaigns: the Mixed‐Phase Arctic Cloud Experiment (MPACE) and the Aura Validation Experiment (AVE) both in Fall, 2004. Reasonable and consistent comparisons are illustrated, revealing that spectral radiance data taken by high‐resolution infrared sounders can be used to determine the vertical distribution of radiative cooling due to CO2.

The ability to accurately validate high–spectral resolution infrared radiance measurements from space using comparisons with a high‐altitude aircraft spectrometer has been successfully demonstrated. The demonstration is based on a 21 November 2002 underflight of the AIRS on the NASA Aqua spacecraft by the Scanning‐HIS on the NASA ER‐2 high‐altitude aircraft. A comparison technique which accounts for the different viewing geometries and spectral characteristics of the two sensors is introduced, and accurate comparisons are made for AIRS channels throughout the infrared spectrum. Resulting brightness temperature differences are found to be 0.2 K or less for most channels. Both the AIRS and the Scanning‐HIS calibrations are expected to be very accurate (formal 3‐sigma estimates are better than 1 K absolute brightness temperature for a wide range of scene temperatures), because high spectral resolution offers inherent advantages for absolute calibration and because they make use of high‐emissivity cavity blackbodies as onboard radiometric references. AIRS also has the added advantage of a cold space view, and the Scanning‐HIS calibration has recently benefited from the availability of a zenith view from high‐altitude flights. Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for assuring the long‐term consistency and accuracy of climate observations, including those from the NASA EOS spacecraft (Terra, Aqua and Aura) and the new complement of NPOESS operational instruments. The validation role for accurately calibrated aircraft spectrometers also includes application to broadband instruments and linking the calibrations of similar instruments on different spacecraft. It is expected that aircraft flights of the Scanning‐HIS and its close cousin the NPOESS Airborne Sounder Test Bed (NAST) will be used to check the long‐term stability of AIRS and the NPOESS operational follow‐on sounder, the Cross‐track Infrared Sounder (CrIS), over the life of the missions.

A revolutionary satellite weather forecasting instrument, called the "GIFTS" which stands for the "Geostationary Imaging Fourier Transform Spectrometer", was recently completed and successfully tested in a space chamber at the Utah State University's Space Dynamics Laboratory. The GIFTS was originally proposed by the NASA Langley Research Center, the University of Wisconsin, and the Utah State University and selected for flight demonstration as NASA's New Millennium Program (NMP) Earth Observing-3 (EO-3) mission, which was unfortunately cancelled in 2004. GIFTS is like a digital 3-d movie camera that, when mounted on a geostationary satellite, would provide from space a revolutionary four-dimensional view of the Earth's atmosphere. GIFTS will measure the distribution, change, and movement of atmospheric moisture, temperature, and certain pollutant gases, such as carbon monoxide and ozone. The observation of the convergence of invisible water vapor, and the change of atmospheric temperature, provides meteorologists with the observations needed to predict where, and when, severe thunderstorms, and possibly tornados, would occur, before they are visible on radar or in satellite cloud imagery. The ability of GIFTS to observe the motion of moisture and clouds at different altitudes enables atmospheric winds to be observed over vast, and otherwise data sparse, oceanic regions of the globe. These wind observations would provide the means to greatly improve the forecast of where tropical storms and hurricanes will move and where and when they will come ashore (i.e., their landfall position and time). GIFTS, if flown into geostationary orbit, would provide about 80,000 vertical profiles per minute, each one like a low vertical resolution (1-2km) weather balloon sounding, but with a spacing of 4 km. GIFTS is a revolutionary atmospheric sensing tool. A glimpse of the science measurement capabilities of GIFTS is provided through airborne measurements with the NPOESS Airborne Sounding Testbed - Interferometer (NAST-I).

Analyses of Cross-track Infrared Sounder Engineering Development Unit data collected while in bench test and thermal vacuum conditions is presented. Focus is placed on accuracy of the sensor’s spectral lineshape and spectral calibration determination.

This study will present results of an effort to validate the accuracy of the cloud-cleared radiance products of the Atmospheric Infrared Sounder (AIRS) using observations from a variety of aircraft based sensors. The AIRS cloud-clear radiances are a product of the Level 2 ground processing software developed by the NASA AIRS science team. The cloud-clear radiance represents an estimate of the infrared upwelling spectrum at the top of the atmosphere for a cloud free atmosphere. This study concentrates on observations collected during the Pacific THORpex experiment, conducted in February-March 2003. NASA ER-2 aircraft based observations from the Scanning-High Resolution Interferometer Sounder (S-HIS) and the MODIS Airborne Simulator (MAS) are used in this validation effort.

The Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) sensor has been designed to provide highly accurate radiometric and spectral radiances in order to meet the requirements of remote sensing of atmospheric motion from a geostationary orbit. The GIFTS sensor was developed under NASA New Millenium Program funding to demonstrate the tracking of infrared water vapor features in the atmosphere with high vertical resolution. A calibration concept has been developed for the GIFTS instrument design which meets the top level requirement to measure brightness temperature to better than 1 K. The in-flight radiometric calibration is performed using views of two on-board blackbody sources along with cold space. For the GIFTS design, the spectral calibration is established by the highly stable diode laser used as the reference for interferogram sampling, and verified with comparisons to atmospheric absorption line positions. The status of the GIFTS on-orbit calibration approach is described and accuracy estimates are provided. GIFTS is a collaborative activity among NASA Langley Research Center, Utah State Space Dynamics Laboratory, and the University of Wisconsin Space Science and Engineering Center.

Development in the mid 80s of the High-resolution Interferometer Sounder (HIS) instrument for the high altitude NASA ER2 aircraft demonstrated the capability for advanced atmospheric temperature and water vapor sounding and set the stage for new satellite instruments that are now becoming a reality [AIRS(2002), CrIS(2006), IASI(2006), GIFTS(200?), HES(2013)]. Follow-on developments at the University of Wisconsin that employ Fourier Transform Infrared (FTIR) for Earth observations include the ground-based Atmospheric Emitted Radiance Interferometer (AERI) and the new Scanning HIS aircraft instrument.The Scanning HIS is a smaller version of the original HIS that uses cross-track scanning to enhance spatial coverage. Scanning HIS and its close cousin, the NPOESS Airborne Sounder Testbed (NAST), are being used for satellite instrument validation and for atmospheric research. A novel detector configuration on Scanning HIS allows the incorporation of a single focal plane and cooler with three or four spectral bands that view the same spot on the ground. The calibration accuracy of the S-HIS and results from recent field campaigns are presented, including validation comparisons with the NASA EOS infrared observations (AIRS and MODIS).Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for assuring the Iona-term consistency and accuracy of climate observations, including those from the NASA EOS spacecrafts (Terra, Aqua and Aura) and the new complement of NPOESS operational instruments. It is expected that aircraft flights of the S-HIS and the NAST will be used to check the long-term stability of AIRS and the NPOESS operational follow-on sounder, the Cross-track Infrared Sounder (CrIS), over the life of the mission.

Abstract Research funded by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program has led to significant improvements in longwave radiative transfer modeling over the last decade. These improvements, which have generally come in small incremental changes, were made primarily in the water vapor self- and foreign-broadened continuum and the water vapor absorption line parameters. These changes, when taken as a whole, result in up to a 6 W m−2 improvement in the modeled clear-sky downwelling longwave radiative flux at the surface and significantly better agreement with spectral observations. This paper provides an overview of the history of ARM with regard to clear-sky longwave radiative transfer, and analyzes remaining related uncertainties in the ARM state-of-the-art Line-by-Line Radiative Transfer Model (LBLRTM). A quality measurement experiment (QME) for the downwelling infrared radiance at the ARM Southern Great Plains site has been ongoing since 1994. This experiment has three...

The Atmospheric Emitted Radiance Interferometer (AERI) instrument was developed for the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program by the University of Wisconsin Space Science and Engineering Center (UW-SSEC). The infrared emission spectra measured by the instrument have the sensitivity and absolute accuracy needed for atmospheric remote sensing and climate studies. The instrument design is described in a companion paper. This paper describes in detail the measured performance characteristics of the AERI instruments built for the ARM Program. In particular, the AERI systems achieve an absolute radiometric calibration of better than 1% (3s) of ambient radiance, with a reproducibility of better than 0.2%. The knowledge of the AERI spectral calibration is better than 1.5 ppm (1s) in the wavenumber range 400‐ 3000 cm21.

The ability to accurately validate high spectral resolution infrared radiance measurements from space using comparisons with aircraft spectrometer observations has been successfully demonstrated. The demonstration is based on an under-flight of the Atmospheric Infrared Sounder (AIRS) on the NASA Aqua spacecraft by the Scanning High resolution Interferometer Sounder (S-HIS) on the NASA ER-2 high altitude aircraft on 21 November 2002 and resulted in brightness temperature differences approaching 0.1K for most of the spectrum. This paper presents the details of this AIRS/S-HIS validation case and also presents comparisons of Aqua AIRS and Moderate Resolution Imaging Spectroradiometer (MODIS) radiance observations. Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for assuring the long-term consistency and accuracy of climate observations. It is expected that aircraft flights of the S-HIS and its close cousin the National Polar Orbiting Environmental Satellite System (NPOESS) Atmospheric Sounder Testbed (NAST) will be used to check the long-term stability of the NASA EOS spacecrafts (Terra, Aqua and Aura) and the follow-on complement of operational instruments, including the Cross-track Infrared Sounder (CrIS).

A ground-based Fourier transform spectrometer has been developed to measure the atmospheric downwelling infrared radiance spectrum at the earth's surface with high absolute accuracy. The Atmospheric Emitted Radiance Interferometer (AERI) instrument was designed and fabricated by the University of Wisconsin Space Science and Engineering Center (UW-SSEC) for the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program. This paper emphasizes the key features of the UW-SSEC instrument design that contribute to meeting the AERI instrument requirements for the ARM Program. These features include a highly accurate radiometric calibration system, an instrument controller that provides continuous and autonomous operation, an extensive data acquisition system for monitoring calibration temperatures and instrument health, and a real-time data processing system. In particular, focus is placed on design issues crucial to meeting the ARM requirements for radiometric calibration, spectral calibration, noise performance, and operational reliability. The detailed performance characteristics of the AERI instruments built for the ARM Program are described in a companion paper.

This paper describes the application of principal component analysis to reduce the random noise present in the hyperspectral infrared observations. Within a set of spectral observations the number of components needed to characterize the atmosphere is far less than the number of wavelengths observed, typically by a factor between 50 and 70. The higher‐order components, which mainly serve to characterize noise, can be eliminated along with the noise that they characterize. The results obtained depend on the variability of the selected sets of observations and on specific instrument characteristics such as spectral resolution and noise statistics. For a set of 10,000 Fourier transform spectrometer (FTS) simulated spectra, whose standard deviation is about 10% of the mean, we were able to obtain noise reduction factors between 5 and 8. Results obtained from real FTS, with standard deviation of about 10% of the mean, indicated practical noise reduction between 5 and 6. To avoid loss of information in the presence of highly deviant observations, it is necessary to use a conservative number of principal components higher than the optimum to maximum noise reduction. However, even then, noise reduction factors of 4 are still achievable.