Infrared Interferometer Spectrometer Research Articles

Global evaluation of fast radiative transfer model coefficients for early meteorological satellite sensors

Abstract. RTTOV (the Radiative Transfer for TOVS code, where TOVS is the TIROS Operational Vertical Sounder) coefficients are evaluated using a large, independent dataset of 25 000 atmospheric model profiles as a robust test of the diverse 83 training profiles typically used. The study is carried out for nine historical satellite instruments: the InfraRed Interferometer Spectrometer D (IRIS-D), Satellite Infrared Spectrometer B (SIRS-B), Medium Resolution Infrared Radiometer (MRIR) and High Resolution Infrared Radiometer (HRIR) for the infrared part of the spectrum, and the Microwave Sounding Unit (MSU), Special Sensor Microwave Imager (SSM/I), Special Sensor Microwave – Humidity (SSM/T-2), Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager/Sounder (SSMI/S) for the microwave. Simulated channel brightness temperatures show similar statistics for both the independent and the 83-profile datasets, confirming that it is acceptable to validate the RTTOV coefficients with the same profiles used to generate the coefficients. Differences between the RTTOV and the line-by-line models are highest in water vapour channels, where mean values can reach up to 0.4 ± 0.2 K for the infrared and 0.04 ± 0.13 K for the microwave. Examination of the latitudinal dependence of the bias reveals different patterns of variability for similar channels on different instruments, such as the channel centred at 679 cm−1 on both IRIS-D and SIRS-B, showing the importance of the specification of the instrumental spectral response functions (ISRFs). Maximum differences of up to several kelvin are associated with extremely non-typical profiles, such as those in polar or very hot regions.

Patricia Ann Straat (1936–2020)

Straat was a leading astrobiologist. In addition to working on the Infrared Interferometer Spectrometer aboard the Mariner 9 spacecraft, she was co-investigator on the Labelled-Release Experiment aboard NASA's Viking Landers.

Optical Properties of Cyanoacetylene Ices in the Far- to Near-infrared with Direct Relevance to Titan's Stratospheric Ice Clouds

Abstract Cyanoacetylene (HC3N) ice has been observed in Titan’s stratosphere by both Voyager 1's InfraRed Interferometer Spectrometer (IRIS) and Cassini's Composite InfraRed Spectrometer (CIRS), and it is likely prevalent in other objects in our solar system and exoplanetary systems as well. While previous experimental studies targeting Titan’s stratospheric clouds have determined the optical properties of HC3N ice in the infrared (IR) spectral range, those thin ice films were formed by annealing processes, which contradicts the formation mechanism of Titan’s stratospheric ice clouds. As a result, optical constants of HC3N ices, experimentally created in a similar manner to the way they are formed in Titan’s stratosphere, are crucial. Here we experimentally measured absorbance spectra of HC3N thin ice films from the near- to far-IR spectral region (50–8000 cm−1; 200–1.25 μm) formed via direct vapor deposition at 30, 50, 70, 90, 110, and 113 K. The corresponding optical constants at all temperatures were also computed, resulting in the largest continuous IR spectral range available for HC3N ice. New tentative peak assignments for spectral features in the near-IR are also reported, thereby further enhancing the inventory of optical constants available for HC3N ice spanning the near- to far-IR spectral range.

Martian atmospheric particulate spectral end-members recovery from PFS and IRIS data

We present an application of a multivariate analyses technique on data returned by the Planetary Fourier Spectrometer (PFS) instrument on board the ESA’s Mars Express (MEX) spacecraft in order to separate the atmospheric contribution from the observed radiation. We observe that Thermal/Far Infrared spectra returned from Mars, covering almost a whole martian year, can be represented by a linear model using a limited set of end-member spectra. We identify the end-members as the suspended mineral dust and water ice clouds, but no surface signature was found. We improve previous studies performed with data from the Thermal Emission Spectrometer (TES) thanks to the higher spectral resolution of PFS. This allows for distinguishing narrow gaseous bands present in the martian atmosphere. Furthermore, the comparison of results from PFS and TES with data collected in 1971 by the Mariner 9 Infrared Interferometer Spectrometer (IRIS) shows an atmospheric dust component with similar spectral behavior. This might indicate homogeneity of the dust source regions over a time period of more than 30years.

SEASONAL DISAPPEARANCE OF FAR-INFRARED HAZE IN TITAN'S STRATOSPHERE

A far-infrared emission band attributed to volatile or refractory haze in Titan's stratosphere has been decreasing in intensity since Cassini's arrival in 2004. The 220 cm(sup -1) feature, first seen by the Voyager Infrared Interferometer Spectrometer, has only been found in Titan's winter polar region. The emission peaks at about 140 km altitude near the winter stratospheric temperature minimum. Observations recorded over the period 2004-2012 by the Composite Infrared Spectrometer on Cassini show a decrease in the intensity of this feature by about a factor of four. Possible seasonal causes of this decline are an increase in photolytic destruction of source chemicals at high altitude, a lessening of condensation as solar heating increased, or a weakening of downwelling of vapors. As of early 2012, the 220 cm(sup -1) haze has not yet been detected in the south. The haze composition is unknown, but its decrease is similar to that of HC3N gas in Titan's polar stratosphere, pointing to a nitrile origin.

Comparison of Spectrally Resolved Outgoing Longwave Radiation over the Tropical Pacific between 1970 and 2003 Using IRIS, IMG, and AIRS

AbstractThe observation of changes in the earth’s spectrally resolved outgoing longwave radiation (OLR) provides a direct method of determining changes in the radiative forcing of the climate system. An earlier study showed that satellite-observed changes in the clear-sky outgoing longwave spectrum between 1997 and 1970 from the Infrared Interferometer Spectrometer (IRIS) and Interferometic Monitor of Greenhouse Gases (IMG) instruments could be related to changes in greenhouse gas composition. The authors present a new study that extends this to 2003, through the first use of a new, independent source of global atmospheric infrared spectra, from the Atmospheric Infrared Sounder (AIRS) experiment. AIRS is a dispersion grating spectrometer, while the other two were Fourier transform spectrometers, and this is taken into account in the analysis. The observed difference spectrum between the years 2003 and 1970 generally shows the signatures of greenhouse gas forcing, and also shows the sensitivity of the signatures to interannual variations in temperature. The new 2003 data support the conclusions found in the earlier work, though, interestingly, the methane (CH4) Q branch centered at 1304 cm−1 exhibits more complex behavior, showing a decrease in intensity in the difference spectrum between 1997 and 2003. Sensitivity analysis indicates that this is due to changes in temperature structure, superposed on an underlying increase in CH4. Radiative transfer calculations based on reanalysis data are used to simulate the changes in the OLR spectrum; limitations in such data and possible variations that could account for several observed effects are discussed.

Successful Mars remote sensors, MO THEMIS and MER Mini-TES

This paper describes results of the calibration of the miniature thermal emission spectrometer (Mini-TES) and the thermal emission imaging system (THEMIS) built by Raytheon Santa Barbara Remote Sensing (SBRS) under contract to Arizona State University (ASU). This paper also serves as an update to an earlier paper (Silverman et al., 2003) for mission description and instrument designs (Schueler et al., 2003). A major goal of the Mars exploration program is to help determine whether life ever existed on Mars via detailed in situ studies and surface sample return. It is essential to identify landing sites with the highest probability of containing samples indicative of early pre-biotic or biotic environments. Of particular interest are aqueous and/or hydrothermal environments in which life could have existed, or regions of current near-surface water or heat sources [Exobiology_Working_Group, 1995, An Exobiological Strategy for Mars Exploration, NASA Headquarters]. The search requires detailed geologic mapping and accurate interpretations of site composition and history in a global context. THEMIS and Mini-TES were designed to do this and builds upon a wealth of data from previous experiments. Previous experiments include the Mariner 6 / 7 Mars infrared radiometer (MIR) and infrared spectrometer [G.C. Pimentel, P.B. Forney, K.C. Herr, Evidence about hydrate and solid water in the martian surface from the 1969 Mariner infrared spectrometer, Journal of Geophysical Research 79(11) (1974) 1623–1634], the Mariner 9 infrared interferometer spectrometer (IRIS) [B. Conrath, R. Curran, R. Hanel, V. Kunde, W. Maguire, J. Pearl, J. Pirraglia, J. Walker, Atmospheric and surface properties of Mars obtained by infrared spectroscopy on Mariner 9, Journal of Geophysical Research 78 (1973) 4267–4278], the Viking infrared thermal mapper (IRTM) [H.H. Kieffer, T.Z. Martin, A.R. Peterfreund, B.M. Jakosky, E.D. Miner, F.D. Palluconi, Thermal and albedo mapping of Mars during the Viking primary mission, Journal of Geophysical Research 82 (1977) 4249–4292], the Phobos Termoscan [A.S. Selivanov, M.K. Naraeva, A.S. Panfilov, Y.M. Gektin, V.D. Kharlamov, A.V. Romanov, D.A. Fomin, Y.Y. Miroshnichenko, Thermal imaging of the surface of Mars, Nature, 341 (1989) 593–595], and the continuing Mars global surveyor (MGS) mission using the Mars orbiter camera (MOC) [M.C. Malin, K.S. Edgett, Mars global surveyor Mars orbiter camera: interplanetary cruise through primary mission, Journal of Geophysical Research 106 (2001) 23, 429–23, 570] and MGS thermal emission spectrometer (TES) [P.R. Christensen, J.L. Bandfield, V.E. Hamilton, S.W. Ruff, H.H. Kieffer, T. Titus, M.C. Malin, R.V. Morris, M.D. Lane, R.N. Clark, B.M. Jakosky, M.T. Mellon, J.C. Pearl, B.J. Conrath, M.D. Smith, R.T. Clancy, R.O. Kuzmin, T. Roush, G.L. Mehall, N. Gorelick, K. Bender, K. Murray, S. Dason, E. Greene, S.H. Silverman, M. Greenfield, The Mars global surveyor thermal emission spectrometer experiment: investigation description and surface science results, Journal of Geophysical Research 106 (2001a) 23, 823–23, 871]. TES has collected hyperspectral images (up to 286 spectral bands from 6– 50 μ m ) of the entire martian surface, providing an initial global reconnaissance of mineralogy and thermophysical properties [J.L. Bandfield, Global mineral distributions on Mars, Journal of Geophysical Research 107 (2002) 10.1029/2001JE001510; S.W. Ruff, P.R. Christensen, Bright and dark regions on Mars: particle size and mineralogical characteristics based on thermal emission spectrometer data, Journal of Geophysical Research, 2002, in press]. By covering the key 6.3– 15.0 μ m region in both TES and THEMIS, it is possible to combine TES fine spectral resolution with THEMIS fine spatial resolution to achieve a global mineralogic inventory at the spatial scales necessary for detailed geologic studies within the Odyssey data resources. Mini-TES is a single detector Fourier transform spectrometer (FTS), covering the spectral range 5– 29 μ m at 10 cm - 1 spectral resolution. Launched in June 2003, one Mini-TES instrument will fly to Mars aboard each of the two missions of NASA's Mars Exploration Rover Project (MER), named Spirit and Opportunity. The first Mini-TES unit was required to meet a two-year development schedule with proven, flight-tested instrumentation. Therefore, SBRS designed Mini-TES based on proven heritage from the successful MGS TES. THEMIS is based on “bolt-together” pushbroom optics and uncooled silicon microbolometer focal plane array (FPA) technology. Sometimes dubbed “Mars Landsat,” THEMIS was launched in 2001 on Mars Odyssey, and provides guidance for future lander missions now in preparation for launch. Advanced materials and optical machining allow THEMIS low-scatter, reflective, wide field-of-view (WFOV) pushbroom optics for relatively long dwell-time compared to narrow FOV optics requiring cross-track scanning for equivalent spatial resolution. This allows uncooled silicon microbolometer FPAs, with less signal sensitivity than cryogenically cooled photo-diode FPAs, to meet the THEMIS sensitivity requirements. Instrument design, performance, integration, as well as details of the calibration are discussed. Full instrument and calibration details are available in the Journal of Geophysical Research Mini-TES and THEMIS papers by Christensen et al.

STRATOSPHERIC IMAGES OF JUPITER DERIVED FROM HYDROCARBON EMISSIONS IN VOYAGER 1 AND 2 IRIS SPECTRA

Spectroscopic data obtained by the Infrared Interferometer Spectrometer (IRIS) aboard Voyager 1 and 2 have been re-visited. Using the spectroscopic data and footprints of the IRIS aperture on the planet, we constructed images of the stratosphere of Jupiter at the emission bands of hydrocarbons including , and . Thermal emission from the hydrocarbons on Jupiter originates from a broad region of the stratosphere extending from 1 to 10 millibars. We averaged the data using a bin of 20 degrees of longitude and latitudes in order to increase signal-to-noise ratios. The resultant images show interesting wave structure in Jupiter`s stratosphere. Fourier transform analyses of these images yield wavenumbers 5 - 7 at mid-Northern and mid-Southern latitudes, and these results are different from those resulted from previous ground-based observations and recent Cassini CIRS, suggesting temporal variations on the stratospheric infrared pattern. The comparisons of the Voyager 1 and 2 spectra also show evidence of temporal intensity variations not only on the infrared hydrocarbon polar brightenings of hydrocarbon emissions but also on the stratospheric infrared structure in the temperate regions of Jupiter over the 4 month period between the two Voyager encounters. Short running title: Stratospheric Images of Jupiter derived from Voyager IRIS Spectra.

Mineralogy of Martian atmospheric dust inferred from thermal infrared spectra of aerosols

We have utilized optical constants derived from thermal infrared spectra acquired by the Mariner 9 Infrared Interferometer Spectrometer (M9 IRIS) and Mars Global Surveyor Thermal Emission Spectrometer (MGS TES), during periods of relatively high dust/low water ice opacity, to generate synthetic transmission and emission spectra of the atmospheric dust. Using libraries of transmission and emission spectra of common rock‐forming minerals and their alteration products, we applied a linear deconvolution algorithm to the spectra to model the dust mineralogy. Models of the M9 IRIS transmission data generally are of poor quality, and the mineralogical results of these fits are unlikely to be valid. Fits to the TES transmission spectrum and TES and IRIS emission spectra are of better quality and suggest that the dust mineralogy is dominated by framework silicates (feldspar and/or zeolite) with carbonate, sulfate, pyroxene, and olivine, identified near, but below, generalized detection limits. Evidence for phyllosilicates is not robust. Our results, combined with those from in situ measurements and other visible to infrared spectral studies, suggest that framework silicates (probably feldspar) dominate the dust mineralogy, with lesser amounts of olivine, pyroxene, amorphous material, hematite, and magnetite, and that the dust largely is the product of mechanical weathering of basaltic rocks with minor chemical alteration.

Observations of the Infrared Outgoing Spectrum of the Earth from Space: The Effects of Temporal and Spatial Sampling

Abstract A recent comparison between data taken by two different satellite instruments, the Interferometric Monitor of Greenhouse Gases (IMG) that flew in 1997 and the Infrared Interferometer Spectrometer (IRIS) that flew in 1970, showed evidence of a change in the clear-sky greenhouse radiative forcing due to the increase in greenhouse gas concentrations between those years. A possibly even more intriguing question is whether the data can be used to extract unambiguous information about the radiative feedback processes that accompany such a change of forcing, especially cloud feedback. This paper is an investigation of this question, with particular reference to the uncertainties introduced into the differences between IMG and IRIS spectra due to their different patterns of temporal and spatial sampling. This has been approached by modeling the sampling problem, using high-resolution proxy scenes of top-of-the-atmosphere 11-μm brightness temperature, TB11, taken from International Satellite Cloud Climato...

The impact of instrument field of view on measurements of cloudy-sky spectral radiances from space: application to IRIS and IMG

Spatially resolved images from the MODerate Resolution Imaging Spectrometer (MODIS) instrument are used to investigate the impact of a change in spatial field of view, from that typical of the Nimbus 4 Infrared Interferometer Spectrometer (IRIS) to that of the Interferometric Monitor for Greenhouse Gases (IMG), upon the spectral outgoing longwave radiation (OLR). Considering all-sky conditions it is found that for a typical tropical scene, approximately 150 paired measurements are required to obtain agreement to within ±2 K in the average brightness temperature ( T B), in the most transparent window channels. At mid-latitudes, the reduced scene variability means that fewer observations are required to meet the same criterion. For clear- and cloudy-sky separation a simple threshold technique based on the window T B and underlying sea-surface temperature tends to result in a systematic underestimate of the average cloudy T B by the larger field of view. A better estimate can be obtained by applying a double threshold to discriminate against the most mixed scenes.

Upper bound on Io's heat flow

Analysis of the temperatures and areas of Io's thermal anomalies yields an upper bound on the total heat flow. An extended distribution function allows an assessment of the heat flow from undetected, cooler (but larger) anomalies and predicts a limiting temperature of ∼90–95 K for the surface. This value is in agreement with measured Voyager infrared interferometer spectrometer and Galileo Photo‐Polarimeter Radiometer nighttime “minimum temperatures.” In addition, the lack of dependence on both latitude and time of night for these observed temperatures can be explained by cooling lavas on a global scale. We consider the extreme case that Io may be covered completely by lava in various stages of cooling to the exclusion of any thermally passive ‘background’ (excepting the few high mountains). Such a distribution of volcanic thermal anomalies up to the size of Io itself yields the first upper bound for heat flow, 13.5 W m−2. This corresponds to a total global, radiated power of 5.6×1014 W.

Spectral anomalies in the 11 and 12 μm region from the Mariner Mars 7 Infrared Spectrometer

Two hundred‐forty infrared spectra acquired by the 1969 Mariner Mars 7 Infrared Spectrometer (IRS), spanning the wavelength region 1.8–14.4 μm (5550–690 cm−1), have recently been recovered and calibrated in both wavelength and intensity. An examination of these IRS spectra has revealed absorptions at 11.25 and 12.5 μm that have not previously been reported for Mars. A search of the literature and spectral data bases shows that materials that exhibit a doublet at 11.25 and 12.5 μm are rare. In this paper we examine potential causes for these features and include a detailed discussion of carbonates, goethite, CO2 ice, and water ice. CO2 ice and water ice measured in transmission do not exhibit bands that match those recorded at 11.25 and 12.5 μm for Mars, which indicates that CO2 or water ice clouds are not the source of these features. Since these bands show no clear correlation with atmospheric path length, they are most likely caused by a surface material. In the IRS database they appear to be exceptionally intense in the western part of the Hellas basin. Goethite exhibits bands that are a good spectral match, but confirming whether goethite causes the features will require additional studies of the 20–50 μm region. These studies will require laboratory measurements of weathering coatings and an examination of spectra recorded of Mars by the 1971 Mariner Mars Infrared Interferometer Spectrometer (IRIS; 5–50 μm, 2000–200 cm−1) and the 1996 Thermal Emission Spectrometer (TES; 6–50 μm, 1667–200 cm−1).

The Martian Atmosphere During the Viking Mission, I: Infrared Measurements of Atmospheric Temperatures Revisited

The Viking Infrared Thermal Mapper 15-μm channel brightness temperature observations (IRTM T 15) provide extensive spatial and temporal coverage of martian atmospheric temperatures on diurnal to seasonal time scales. The 15-μm channel was designed so that these temperatures would be representative of a deep layer of atmosphere centered at 0.5 mb (∼25 km). Our re-examination of the IRTM data indicates that the 15-μm channel was additionally sensitive to surface radiance so that air temperature determinations (nominal T 15) are significantly biased when the thermal contrast between the surface and atmosphere is large. This bias is suggested by the strong correlation between the diurnal variation of tropical T 15 and surface temperatures for non-dust-storm conditions. We show that numerical modeling of the thermal tides provides a basis for distinguishing between the surface and atmospheric contributions to IRTM T 15 and thus allows the atmospheric component to be estimated. The resulting bias amounts to a ∼15-K offset for midday atmospheric temperatures at subsolar latitudes during relatively clear periods and is negligible at night. The proposed temperature correction results in close agreement between the stimulated and observed patterns of diurnal variation for conditions ranging from clear to dusty. A major consequence of this work is the improved definition of the diurnal, latitudinal, and seasonal variation of martian atmosphere temperatures during the Viking mission. An accounting for the surface temperature bias resolves much of the discrepancy between IRTM and corresponding microwave observations, indicating that there is relatively little interannual variability in global temperatures during the aphelion season ( L s ∼ 40°–100°). We find further support for this argument in a comparison with T 15 temperatures synthesized from Mariner 9 Infrared Interferometer Spectrometer spectra. The significantly reduced diurnal temperature variations in this season are consistent with the relatively clear atmosphere that is implied by the cooler temperatures. Cooler temperatures and reduced diurnal variation will likely be of significance for the modeling of water ice cloud dynamics in this season.

A Detection of Water Ice on Jupiter with Voyager IRIS

Possible water ice spectral signatures have been detected on Jupiter using Voyager Infrared Interferometer Spectrometer and Radiometer (IRIS) spectra taken near closest approach of the Voyager 1 and 2 missions. Comparison with simultaneous imaging data shows that these IRIS footprints approximately correspond to locations on the planet where deep convection is suspected. Of the sample of 7500+ spectra, approximately 1% show a water ice absorption feature near 44 μm. The infrared optical depths of the absorption features, between 0.18 and 0.44, indicate that large amounts of ice particles need not be present to produce the observed spectral signature. The water ice clouds would have visible optical depths on the order of unity and would not be distinguishable above a thick ammonia cloud deck, which can have much larger visible optical depths. These thin clouds yield about 1 precipitable micrometer of water, comparable to Martian Tharsis Ridge clouds.

IRIS Mariner 9 data revisited:: 1. An instrumental effect

Small spurious features are present in data from the Mariner 9 Infrared Interferometer Spectrometer (IRIS). These represent a low amplitude replication of the spectrum with a doubled wavenumber scale. This replication arises principally from an internal reflection of the interferogram at the input window. An algorithm is provided to correct for the effect, which is at the 2% level. We believe that the small error in the uncorrected spectra does not materially affect previous results; however, it may be significant for some future studies at short wavelengths. The IRIS spectra are also affected by a coding error in the original calibration that results in only positive radiances. This reduces the effectiveness of averaging spectra to improve the signal to noise ratio at small signal levels.

Variations in Martian surface composition and cloud occurrence determined from thermal infrared spectroscopy: Analysis of Viking and Mariner 9 data

Viking infrared thermal mapper (IRTM) and Mariner 9 infrared interferometer spectrometer (IRIS) data were examined for evidence of spatial variations in the composition of surface materials and the abundance of atmospheric condensates. The IRTM instrument collected broadband spectral radiance in bands centered at 7, 9, 11, and 20 μm. These measurements were converted to emissivities at 9, 11, and 20 μm and used to construct multispectral images at five seasons (Ls 90–180°, 180–360°, 0–90°, 90–180°, and 180–360°) and at local times of 6–8, 8–10, 10–12, 12–14, and 14–16 H (24 H equals one Martian day). During dusty periods (Ls 180–360°), these images clearly revealed the presence of suspended dust. During clear periods (Ls 0–180°) the seasonal and diurnal variations in water‐ice cloud occurrence could be mapped using the 11 μm IRTM band. Clouds tended to occur over the Tharsis region and the region between 0 and 60°W near the equator, and to evolve from dispersed hazes to more localized hazes during the day. As northern summer progressed during the Viking period, the northern clouds transitioned from a diffuse haze to a more concentrated zone of clouds in the equatorial region with a well‐defined northern boundary. During clear periods the IRTM multispectral data reveal significant absorptions at 9, 11, and 20 μm that are strongly correlated with surface albedo. These absorptions are produced by surface materials, and individual four‐point spectra show compositional variations within both dark and bright material types. The IRIS instrument collected high‐spectral resolution (2.4 cm−1) data from 6 to 50 μm. Unfortunately, only ∼85 daytime spectra out of the ∼21,000 total collected were acquired after the intense global dust storm of 1971 subsided (after Ls 330°). The close agreement of these spectra, convolved to IRTM resolution, with the IRTM surface spectra confirms that they also represent surface, rather than atmospheric, materials. The IRTM four‐point spectra show similarities with plagioclase feldspar minerals, clays, and pyroxenes, although the low spectral resolution of IRTM prevents definitive mineral identification. Carbonates do not compose a significant fraction of the dark‐region materials. The IRIS surface spectra show similarities with feldspars, clays, and pyroxenes in the 8–12 μm region but do not match any of these minerals well in the 18–25 μm region. Overall, the IRTM and IRIS data are consistent with previous suggestions of mafic to ultramafic compositions, but data provide stronger evidence that unweathered plagioclase feldspars may be significant components of the present surface. Together, the IRTM and IRIS reveal the presence of absorption features associated with surface materials which should permit detailed surface compositional mapping using future thermal IR spectral measurements.

A statistical method for testing a general circulation model with spectrally resolved satellite data

The motivation for this paper is to understand better the means available for testing climate models. Statistics of observed, outgoing, thermal spectra are compared with those predicted from a climate model, on the basis of data collected over a period of approximately 1 year. This is a powerful approach to testing a model with respect to processes internal to the atmosphere. These processes, which have characteristic timescales of less than a year, define the atmosphere's response to external forcing. Second‐moment statistics are particularly important for testing model variability, which is key to predicting the results of forcing the atmosphere, for example, by ocean surface temperature changes, increase of greenhouse gases, etc. Comparisons are presented between statistical data from the infrared interferometer spectrometer (IRIS), an orbiting fourier transform spectrometer, and spectra calculated using the medium‐resolution spectral code, MODTRAN, applied to the temperature and humidity profiles from a well‐known climate model. Ten months of IRIS data are available, and we have compared means, standard deviations, skew, and kurtosis of its spectrally resolved brightness temperature in three tropical regions for individual months and for a range of timescales. Also presented are comparisons of covariances using Empirical Orthogonal Functions (EOFs) calculated in frequency space. All data that are presented are based on radiance differences from two like spectra, which eliminates many of the errors generated by the use of MODTRAN and most of the errors due to calibration uncertainties in IRIS. Important differences (i.e., residuals) between the IRIS and the GCM statistics are found in comparisons, demonstrating that the spectral data can provide a severe test of many aspects of the variability of a general circulation model. We discuss some of the residuals and how they may be used to improve model performance in the context of an adjoint formalism. In the long run the only way to have confidence in the performance of a model is to subject it to as many discriminating comparisons with data as are practicable, and we present a good candidate.

High‐resolution spectroscopy of Mars at 3.7 and 8 μm: A sensitive search for H2O2, H2CO, HCl, and CH4, and detection of HDO

We report the first sensitive upper limits for H2O2, H2CO, and HCl, a sensitive upper limit for CH4, and a measured column abundance for HDO. A combination of the Kitt Peak National Observatory (KPNO) 4‐m telescope and Fourier transform spectrometer with the Goddard Space Flight Center (GSFC) postdisperser was used to observe low‐latitude regions of Mars in spectral ranges of 2650–2800 cm−1 and 1229–1237 cm−1 with resolving power v/δv=2.7×105 and 1.2×105, respectively. The main spectral features are lines of isotopic CO2 and HDO. A radiation transfer code has been developed which divides the atmosphere into 30 layers and the Voigt profile of each line in each layer into 60 intervals. This code couples the reflected solar and thermal radiations. A search for H2O2 at 1229–1237 cm−1 results in a 2‐σ upper limit of 30 ppb, close to the predictions of recent photochemical models. Our 2‐σ upper limit for HCl (<2 ppb) precludes any significant chlorine chemistry in the atmosphere. Our value for methane (70±50 ppb) is consistent with, but does not improve upon, earlier results from Mariner 9 infrared interferometer spectrometer (IRIS). Our 2‐σ upper limit for H2CO (3 ppb) is much smaller than recently claimed abundances. The observed HDO lines result in an abundance of (6.06±0.5)×1016 cm−2, which, when compared with that of H2O measured by the Viking orbiters at the same season and latitudes, yields an enrichment in the D/H ratio of a factor of 5.5±2 relative to Earth. The given uncertainty is mainly caused by possible variations of Mars' atmospheric water abundance.

Application of infrared interferometer spectrometer clear sky spectral radiance to investigations of climate variability

An investigation of clear sky spectral radiances and their applicability as a diagnostic for climate variability is described. Global observations of outgoing longwave radiation during 1970 from the infrared interferometer spectrometer (IRIS) aboard Nimbus 4 cover nearly a full annual cycle, and the spectral content of the data provides unique information for examining regional and seasonal variations of spectral radiance. The IRIS radiances have been validated against a line‐by‐line radiative transfer model and are found to compare favorably with calculated radiances. A brightness temperature threshold technique is used to separate clear and cloudy spectra, and seasonal means of clear sky spectra are analyzed for tropical and northern midlatitude ocean areas. Brightness temperature standard deviations are also examined spectrally. Values of 1–2 K in the tropics and 2–3 K at midlatitudes in the 800–1200 cm−1 window region are consistent with observed ocean temperature variability. Spectral features in the standard deviations reflect the variation of surface temperature, tropospheric temperature, and water vapor in the tropics and the increased variability of ozone during winter in the northern hemisphere. Spectral differences between seasonal and annual mean outgoing radiance are associated with variations in atmospheric parameters, and a linear retrieval algorithm is used to quantify these changes in the tropics. Retrieved seasonal sea surface temperature (SST) differences are typically within 0.7 K of the observed SST differences. Interseasonal water column variations during this annual cycle are 5–10% over the tropical Pacific and Atlantic and 10–20% over the equatorial Indian Ocean. In general, the retrieved seasonal changes in temperature and water vapor parameters are consistent with the known conditions of the 1970 tropical ocean and atmosphere. These variations are detectable as signatures in outgoing spectral radiances and provide significant information relevant to climate change.