Moderate Spectral Resolution Research Articles

High spatial resolution hyperspectral camera based on a linear variable filter

A small, lightweight, and inexpensive hyperspectral camera based on a linear variable filter close to the focal plane array (FPA) is described. The use of a full-frame sensor allows large coverage with high spatial resolution at moderate spectral resolution. The spatial resolution has been maintained using a tilt/shift lens for chromatic focusing corrections. The trade-offs of positioning the filter relative to the FPA and varying the f-number have been studied. Calibration can correct for artifacts such as spectral filter variability. Reference spectra can be obtained using the same camera system by imaging targets over homogeneous areas. For textured surfaces, the different materials can be separated by using statistical methods. Accurate reconstruction of the sparse spectral image data is demonstrated.

Calibration ofHerschelSPIRE FTS observations at different spectral resolutions

The SPIRE Fourier Transform Spectrometer on board the Herschel Space Observatory had two standard spectral resolution modes for science observations: high resolution (HR) and low resolution (LR), which could also be performed in sequence (H+LR). A comparison of the HR and LR resolution spectra taken in this sequential mode, revealed a systematic discrepancy in the continuum level. Analysing the data at different stages during standard pipeline processing, demonstrates the telescope and instrument emission affect HR and H+LR observations in a systematically different way. The origin of this difference is found to lie in the variation of both the telescope and instrument response functions, while it is triggered by fast variation of the instrument temperatures. As it is not possible to trace the evolution of the response functions through auxiliary housekeeping parameters, the calibration cannot be corrected analytically. Therefore an empirical correction for LR spectra has been developed, which removes the systematic noise introduced by the variation of the response functions.

New methods for the retrieval of chlorophyll red fluorescence from hyperspectral satellite instruments: simulations and application to GOME-2 and SCIAMACHY

Abstract. Global satellite measurements of solar-induced fluorescence (SIF) from chlorophyll over land and ocean have proven useful for a number of different applications related to physiology, phenology, and productivity of plants and phytoplankton. Terrestrial chlorophyll fluorescence is emitted throughout the red and far-red spectrum, producing two broad peaks near 683 and 736 nm. From ocean surfaces, phytoplankton fluorescence emissions are entirely from the red region (683 nm peak). Studies using satellite-derived SIF over land have focused almost exclusively on measurements in the far red (wavelengths > 712 nm), since those are the most easily obtained with existing instrumentation. Here, we examine new ways to use existing hyperspectral satellite data sets to retrieve red SIF (wavelengths < 712 nm) over both land and ocean. Red SIF is thought to provide complementary information to that from the far red for terrestrial vegetation. The satellite instruments that we use were designed to make atmospheric trace-gas measurements and are therefore not optimal for observing SIF; they have coarse spatial resolution and only moderate spectral resolution (0.5 nm). Nevertheless, these instruments, the Global Ozone Monitoring Instrument 2 (GOME-2) and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), offer a unique opportunity to compare red and far-red terrestrial SIF at regional spatial scales. Terrestrial SIF has been estimated with ground-, aircraft-, or satellite-based instruments by measuring the filling-in of atmospheric and/or solar absorption spectral features by SIF. Our approach makes use of the oxygen (O2) γ band that is not affected by SIF. The SIF-free O2 γ band helps to estimate absorption within the spectrally variable O2 B band, which is filled in by red SIF. SIF also fills in the spectrally stable solar Fraunhofer lines (SFLs) at wavelengths both inside and just outside the O2 B band, which further helps to estimate red SIF emission. Our approach is then an extension of previous approaches applied to satellite data that utilized only the filling-in of SFLs by red SIF. We conducted retrievals of red SIF using an extensive database of simulated radiances covering a wide range of conditions. Our new algorithm produces good agreement between the simulated truth and retrievals and shows the potential of the O2 bands for noise reduction in red SIF retrievals as compared with approaches that rely solely on SFL filling. Biases seen with existing satellite data, most likely due to instrumental artifacts that vary in time, space, and with instrument, must be addressed in order to obtain reasonable results. Our 8-year record of red SIF observations over land with the GOME-2 allows for the first time reliable global mapping of monthly anomalies. These anomalies are shown to have similar spatiotemporal structure as those in the far red, particularly for drought-prone regions. There is a somewhat larger percentage response in the red as compared with the far red for these areas that are drought sensitive. We also demonstrate that good-quality ocean fluorescence line height retrievals can be achieved with GOME-2, SCIAMACHY, and similar instruments by utilizing the full complement of radiance measurements that span the red SIF emission feature.

DISTINGUISHING A HYPOTHETICAL ABIOTIC PLANET–MOON SYSTEM FROM A SINGLE INHABITED PLANET

ABSTRACT It has recently been suggested that an exomoon with a CH4 atmosphere, orbiting an abiotic Earth-mass planet with an O2-rich atmosphere, can produce a false positive biosignature at a low–moderate spectral resolution (R = λ/Δλ ≤ 2000). If this were true, inferring the presence of life on exoplanets will be beyond our reach in the next several decades. Here we use a line-by-line radiative transfer model to compute the relevant reflection spectrum between 1 and 3.3 μm. We show that it is possible to separate the combined spectra of such planet–moon systems from an inhabited planet by multiple-band NIR observations. We suggest that future observations near the 2.3 μm CH4 absorption band at a resolution of 100 and an SNR of 10 or more may be a good way to distinguish an abiotic planet–moon system from a inhabited single planet.

WSPEC: A Waveguide Filter-Bank Focal Plane Array Spectrometer for Millimeter Wave Astronomy and Cosmology

Imaging and spectroscopy at (sub-)millimeter wavelengths are key frontiers in astronomy and cosmology. Large area spectral surveys with moderate spectral resolution (R=50-200) will be used to characterize large scale structure and star formation through intensity mapping surveys in emission lines such as the CO rotational transitions. Such surveys will also be used to study the SZ effect, and will detect the emission lines and continuum spectrum of individual objects. WSPEC is an instrument proposed to target these science goals. It is a channelizing spectrometer realized in rectangular waveguide, fabricated using conventional high-precision metal machining. Each spectrometer is coupled to free space with a machined feed horn, and the devices are tiled into a 2D array to fill the focal plane of the telescope. The detectors will be aluminum Lumped-Element Kinetic Inductance Detectors (LEKIDs). To target the CO lines and SZ effect, we will have bands at 135-175 GHz and 190-250 GHz, each Nyquist-sampled at R~200 resolution. Here we discuss the instrument concept and design, and successful initial testing of a WR10 (i.e. 90 GHz) prototype spectrometer. We recently tested a WR5 (180 GHz) prototype to verify that the concept works at higher frequencies, and also designed a resonant backshort structure that may further increase the optical efficiency. We are making progress towards integrating a spectrometer with a LEKID array and deploying a prototype device to a telescope for first light.

Effect of self-apodization correction on Cross-track Infrared Sounder radiance noise.

Self-apodization correction increases noise of the radiance spectra measured by the Cross-track Infrared Sounder (CrIS) in three spectral bands. It also increases noise correlation between spectral channels. The degree of the noise effect depends on the spectral resolution, channel frequency, and detector position in the 3×3 detector grids on the focal planes. The effect is relatively small when CrIS is operated in the normal spectral resolution mode, but can reach to the level of a 76% noise increase and 52% noise correlation in high-frequency channels of the shortwave-band spectra from the corner detectors when CrIS is operated in the full spectral resolution mode. The noise impact of the Hamming apodization applied to CrIS spectra is also discussed.

SPATIALLY RESOLVED SPECTROSCOPY OF EUROPA: THE DISTINCT SPECTRUM OF LARGE-SCALE CHAOS

We present a comprehensive analysis of spatially resolved moderate spectral resolution near infrared spectra obtained with the adaptive optics system at the Keck Observatory. We identify three compositionally distinct end member regions: the trailing hemisphere bullseye, the leading hemisphere upper latitudes, and a third component associated with leading hemisphere chaos units. We interpret the composition of the three end member regions to be dominated by irradiation products, water ice, and evaporite deposits or salt brines, respectively. The third component is associated with geological features and distinct from the geography of irradiation, suggesting an endogenous identity. Identifying the endogenous composition is of particular interest for revealing the subsurface composition. However, its spectrum is not consistent with linear mixtures of the salt minerals previously considered relevant to Europa. The spectrum of this component is distinguished by distorted hydration features rather than distinct spectral features, indicating hydrated minerals but making unique identification difficult. In particular, it lacks features common to hydrated sulfate minerals, challenging the traditional view of an endogenous salty component dominated by Mg-sulfates. Chloride evaporite deposits are one possible alternative.

MAGI: A New High-Performance Airborne Thermal-Infrared Imaging Spectrometer for Earth Science Applications

A new airborne facility instrument for Earth science applications is introduced. The Mineral and Gas Identifier (MAGI) is a wide-swath (programmable up to ±42° off nadir) moderate spectral resolution thermal-infrared (TIR) imaging spectrometer that spans the 7.1- to 12.7- $\mu\mbox{m} $ spectral window in 32 uniform and contiguous channels. Its spectral resolution enables improved discrimination of rock and mineral types, greatly expanded gas-detection capability, and generally more accurate land-surface temperature retrievals. The instrument design arose from trade studies between spectral resolution, spectral range, and instrument sensitivity and has now been validated by flight data acquired with the completed sensor. It offers a potential prototype for future space-based TIR instruments, which will require much higher spectral resolution than is currently available in order to address more detailed climate, anthropogenic, and solid Earth science questions.

Measurement and Correction of Atmospheric Effects at Different Altitudes for Remote Sensing of Sun-Induced Fluorescence in Oxygen Absorption Bands

Under sunlight illumination, the shape of the atmospheric oxygen bands [O2-B (687 nm) and O2-A (760 nm)] of the vegetation radiance is modified by chlorophyll fluorescence as a result of the filling-in process. However, for far-range measurements, atmospheric effects also modify this shape. In this paper, measurements of the depth of O2-A and O2-B absorption bands have been performed at different altitudes, from 324 to 3123 m, over different targets, including bare soil and wheat fields. It is observed that bands depth increases significantly with altitude. In the O2-B band, the total magnitude of variation is on the same order of magnitude as the change induced by vegetation, while it is much greater in the O2-A band. Three main factors affect the measurements of band depth: 1) the transmittance of the air column between ground and sensor, 2) the path radiance, and 3) the additional contribution of the ground surface outside the field of view scattered by the atmosphere (adjacency effect). We used the MODerate spectral resolution atmospheric TRANsmittance algorithm (MODTRAN) 4 to compute separately these atmospheric and environmental terms, and we corrected airborne measurements to retrieve ground-level band depths. A study was conducted to assess the sensitivity of the absorption band depth to several atmospheric parameters. The magnitudes of these effects were compared to the band depth variation induced by fluorescence. We identified temperature, air pressure, and aerosols as the most critical parameters. After correction of airborne measurements from atmospheric effects, it is found that the variation of band depth with altitude is greatly reduced. The residual variation, particularly in the case of O2-B band over vegetation, is attributed to an inaccurate assessment of the environment contribution that is scattered by the atmosphere to the sensor. These results show that the MODTRAN 4 is able to simulate oxygen band depth variations at moderate spectral resolution (i.e., 0.5–1 nm) and can be used to compute atmospheric and environmental factors for corrections of airborne measurements of oxygen band profiles. We also showed that an accurate evaluation of the contribution of environment is needed to correct the band depth from atmospheric effects in the O2-B band.

Airborne Remote-Sensing Technologies Detect, Quantify Hydrocarbon Releases

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25984, “Crosscutting Airborne Remote-Sensing Technologies for Oil and Gas and Earth Science Applications,” by A.D. Aubrey, C. Frankenberg, R.O. Green, M.L. Eastwood, and D.R. Thompson, National Aeronautics and Space Administration Jet Propulsion Laboratory, California Institute of Technology, and A.K. Thorpe, University of California, Santa Barbara, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. Airborne imaging spectroscopy has evolved dramatically since the 1980s as a robust remote-sensing technique used to generate 2D maps of surface properties over large areas. Two recent applications are particularly relevant to the needs of the oil and gas sector and government: quantification of surficial hydrocarbon thickness in aquatic environments and mapping atmospheric greenhouse-gas components. These techniques provide valuable capabilities for monitoring petroleum seepage and for detection and quantification of fugitive emissions. Introduction The Jet Propulsion Laboratory (JPL), a National Aeronautics and Space Administration (NASA) federally funded research-and-development center operated by the California Institute of Technology, has been a pioneer in optical remote sensing since the 1980s. JPL capabilities include expertise across all project phases, including sensor design and construction, airborne experiment execution, and data generation driven by science and customer needs. JPL has particular expertise in imaging spectroscopy, a passive method to interrogate objects or surfaces without physical contact. Such remote sensing has traditionally been applied to investigation of surface composition in terrestrial environments. These surface compositions are characterized by use of a spectral library that includes the surface-reflectance or emissivity fingerprints of constituent materials. Airborne imaging spectrometers provide a powerful method to survey wide spatial extents with high-performance surface characterization because of the wide contiguous spectral range at moderate spectral resolution. Novel quantitative methods have emerged recently for both atmospheric gases and surficial oil on water.

Correcting Systematic Polarization Effects in Keck LRISp Spectropolarimetry to < 0.05%

Spectropolarimetric measurements at moderate spectral resolutions are effective tracers of stellar magnetic fields and circumstellar environments when signal-to-noise ratios (S/Ns) above 2000 can be achieved. The LRISp spectropolarimeter is capable of achieving these S/Ns on faint targets with the 10 m aperture of the Keck telescope, provided several instrumental artifacts can be suppressed. We describe here several methods to overcome instrumental error sources that are required to achieve these high S/Ns on LRISp. We explore high S/N techniques such as defocusing and slit-stepping during integration with high spectral and spatial oversampling. We find that the instrument flexure and interference fringes introduced by the achromatic retarders create artificial signals at 0.5% levels in the red channel which mimic real stellar signals and limit the sensitivity and calibration stability of LRISp. Careful spectral extraction and data filtering algorithms can remove these error sources. For faint targets and long exposures, cosmic ray hits are frequent and present a major limitation to the upgraded deep depletion red-channel CCD. These must be corrected to the same high S/N levels, requiring careful spectral extraction using iterative filtering algorithms. We demonstrate here characterization of these sources of instrumental polarization artifacts and present several methods used to successfully overcome these limitations. We have measured the linear to circular cross-talk and find it to be roughly 5%, consistent with the known instrument limitations. We show spectropolarimetric signals on brown dwarfs are clearly detectable at 0.2% amplitudes with sensitivities better than 0.05% at full spectral sampling in atomic and molecular bands. Future LRISp users can perform high-sensitivity observations with high-quality calibration when following the described algorithms.

SPECTRAL CONFUSION FOR COSMOLOGICAL SURVEYS OF REDSHIFTED C II EMISSION

Far-infrared cooling lines are ubiquitous features in the spectra of star-forming galaxies. Surveys of redshifted fine-structure lines provide a promising new tool to study structure formation and galactic evolution at redshifts including the epoch of reionization as well as the peak of star formation. Unlike neutral hydrogen surveys, where the 21 cm line is the only bright line, surveys of redshifted fine-structure lines suffer from confusion generated by line broadening, spectral overlap of different lines, and the crowding of sources with redshift. We use simulations to investigate the resulting spectral confusion and derive observing parameters to minimize these effects in pencil-beam surveys of redshifted far-IR line emission. We generate simulated spectra of the 17 brightest far-IR lines in galaxies, covering the 150–1300 μm wavelength region corresponding to redshifts 0 < z < 7, and develop a simple iterative algorithm that successfully identifies the 158 μm [C ii] line and other lines. Although the [C ii] line is a principal coolant for the interstellar medium, the assumption that the brightest observed lines in a given line of sight are always [C ii] lines is a poor approximation to the simulated spectra once other lines are included. Blind line identification requires detection of fainter companion lines from the same host galaxies, driving survey sensitivity requirements. The observations require moderate spectral resolution 700 < R < 4000 with angular resolution between 20'' and 10', sufficiently narrow to minimize confusion yet sufficiently large to include a statistically meaningful number of sources.

Interferometric broadband Fourier spectroscopy with a partially coherent gas-discharge extreme ultraviolet light source

Extreme ultraviolet (EUV) spectroscopy is a powerful tool for studying fundamental processes in plasmas as well as for spectral characterization of EUV light sources and EUV optics. However, a simultaneous measurement covering a broadband spectral range is difficult to realize. Here, we propose a method for interferometric broadband Fourier spectroscopy connecting soft x ray and visible spectral ranges with moderate spectral resolution. We present an analytical model to recover the spectrum from a double-slit interferogram. We apply our model for spectral characterization of a partially coherent gas-discharge EUV light source operated with different gases in the spectral range between 10 and 110 nm wavelengths. Our approach allows a simple and fast broadband spectroscopy with fully or partially spatially coherent light sources, for instance, to characterize out-of-band radiation in EUV lithography applications.

Lower atmosphere minor gas abundances as retrieved from Venus Express VIRTIS-M-IR data at 2.3 µm

Minor gas abundances in the lower atmosphere of Venus׳ southern hemisphere are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer aboard ESA’s Venus Express mission in the moderate spectral resolution infrared mapping channel (VIRTIS-M-IR, 1–5µm, FWHM=17nm). The entire usable data archive is utilized including only radiation spectra sampled at long detector exposure times (≥3.3s) during eight Venus solar days between April 2006 and October 2008. Combined radiative transfer and retrieval techniques (Haus et al., 2013; Haus et al., 2014) are applied for a simultaneous determination of total cloud opacity and H2O, CO, and OCS abundances from the 2.3µm atmospheric transparency window that sounds the altitude range between about 30 and 45km. A wavelength-dependent CO2 opacity correction is considered.Zonal averages of CO abundances at 35km increase by about 35% from (22.9±0.8) ppmv at equatorial latitudes to (31.0±2.1) ppmv at 65°S and then decrease to (29.4±2.4) ppmv at 80°S The±figures refer to the statistical variability of retrieved abundances. In accordance with earlier results, the observed latitudinal variation of tropospheric CO is consistent with a Hadley cell-like circulation. Dawn side CO abundances at high latitudes are slightly smaller than dusk side values by about 7%. The latitudinal distribution of OCS at 35km is anticorrelated with that of CO, ranging from about (1.15±0.2) ppmv at 65°S to (1.60±0.2) ppmv at low latitudes (poleward decrease of 28%). Zonal averages of H2O abundances near 35km slightly decrease toward the South Pole by about 10%, and the hemispheric average is (32.0±1.3) ppmv. A significant local time dependence of OCS and H2O is not observed. Detailed analyses of individual spectrum retrieval errors for different atmospheric models reveal that CO abundance results are reliable (error 4–7%), while H2O and OCS results have lower confidence (errors 30–47% and 41–86%, respectively). SO2 abundances cannot reliably be retrieved from VIRTIS-M-IR spectra.

ACCURATE ATMOSPHERIC PARAMETERS AT MODERATE RESOLUTION USING SPECTRAL INDICES: PRELIMINARY APPLICATION TO THE MARVELS SURVEY

Studies of Galactic chemical and dynamical evolution in the solar neighborhood depend on the availability of precise atmospheric parameters (Teff, [Fe/H] and log g) for solar-type stars. Many large-scale spectroscopic surveys operate at low to moderate spectral resolution for efficiency in observing large samples, which makes the stellar characterization difficult due to the high degree of blending of spectral features. While most surveys use spectral synthesis, in this work we employ an alternative method based on spectral indices to determine the atmospheric parameters of a sample of nearby FGK dwarfs and subgiants observed by the MARVELS survey at moderate resolving power (R~12,000). We have developed three codes to automatically normalize the observed spectra, measure the equivalent widths of the indices and, through the comparison of those with values calculated with pre-determined calibrations, derive the atmospheric parameters of the stars. The calibrations were built using a sample of 309 stars with precise stellar parameters obtained from the analysis of high-resolution FEROS spectra. A validation test of the method was conducted with a sample of 30 MARVELS targets that also have reliable atmospheric parameters from high-resolution spectroscopic analysis. Our approach was able to recover the parameters within 80 K for Teff, 0.05 dex for [Fe/H] and 0.15 dex for log g, values that are lower or equal to the typical external uncertainties found between different high-resolution analyzes. An additional test was performed with a subsample of 138 stars from the ELODIE stellar library and the literature atmospheric parameters were recovered within 125 K for Teff, 0.10 dex for [Fe/H] and 0.29 dex for log g. These results show that the spectral indices are a competitive tool to characterize stars with the intermediate resolution spectra.

Retrieval of Sea and Land Surface Temperature From SVISSR/FY-2C/D/E Measurements

This paper addresses the retrieval of sea surface temperature (SST) and land surface temperature (LST) from the measurements acquired by the Stretched Visible and Infrared Spin Scan Radiometer (SVISSR) on FengYun (FY) 2C/D/E satellites. First, the generalized split-window algorithms for SST and LST retrieval were developed using the moderate spectral resolution atmospheric transmittance algorithm and computer model (MODTRAN) fed with the parameter-adjusted standard model atmospheres. Then, the developed algorithms were applied to SST and LST retrieval from the SVISSR/FY-2C/D measurements in September 2007 and the SVISSR/FY-2E measurements in May 2010 over a large study area (latitude: 10° S-50° N; longitude: 60° E-130° E). Finally, the derived SVISSR/FY-2 SST and LST were, respectively, cross-validated with the MODIS/Terra SST and LST products. The results show the following: 1) the generalized split-window algorithms developed in this work are valid for both SST and LST retrieval from SVISSR/FY-2C/D/E measurements; 2) in contrast to the MODIS/Terra SST and LST products, the errors in nighttime retrieval are usually less than that in daytime retrieval, and the errors in SVISSR/FY-2 SST are generally less than the errors in SVISSR/FY-2 LST; and 3) for both daytime and nighttime, the total errors in SVISSR/FY-2C/D/E LST are, respectively, 0.3 ± 1.6 K, 0.3 ± 1.9 K, and 1.0 ± 1.8 K, while the total errors in SVISSR/FY-2C/D/E SST are, respectively, -0.6 ± 1.3 K, -0.2 ± 1.5 K, and 0.4 ± 1.8 K.

Airborne visualization and quantification of discrete methane sources in the environment

Airborne thermal-infrared (TIR) imaging spectrometry techniques have been used to detect and track methane and other gaseous emissions from a variety of discrete sources in diverse environmental settings, and to enable estimation of the strength of each plume. The high spatial resolution (1–2m) permits attribution of chemical plumes to their source, while the moderate spectral resolution (44nm across the 7.5–13.5μm TIR band) enables identification and quantification of the gaseous plume constituents, even when one is present in considerably greater concentration than the others. Raw imagery was quantitatively analyzed using matched filtering and adaptive coherence techniques. Experiments under controlled conditions demonstrated successful detection of methane point sources at release rates as low as 2.2kg/h (~1dm3/s at NTP).

The energy source and dynamics of infrared luminous galaxy ESO 148-IG002

ESO 148-IG002 represents a transformative stage of galaxy evolution, containing two galaxies at close separation which are currently coalescing into a single galaxy. We present integral field data of this galaxy from the ANU Wide Field Spectrograph (WiFeS). We analyse our integral field data using optical line ratio maps and velocity maps. We apply active galactic nucleus (AGN), star-burst and shock models to investigate the relative contribution from star-formation, shock excitation and AGN activity to the optical emission in this key merger stage. We find that ESO 148-IG002 has a flat metallicity gradient, consistent with a recent gas inflow. We separate the line emission maps into a star forming region with low velocity dispersion that spatially covers the whole system as well as a southern high velocity dispersion region with a coherent velocity pattern which could either be rotation or an AGN-driven outflow, showing little evidence for pure star formation. We show that the two overlapping galaxies can be separated using kinematic information, demonstrating the power of moderate spectral resolution integral field spectroscopy.

Information Content in the Oxygen &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$A$&lt;/tex-math&gt;&lt;/inline-formula&gt;-Band for the Retrieval of Macrophysical Cloud Parameters

Current and future satellite sensors provide measurements in and around the oxygen A-band on a global basis. These data are commonly used for the determination of cloud and aerosol properties. In this paper, we assess the information content in the oxygen A-band for the retrieval of macrophysical cloud parameters using precise radiative transfer simulations covering a wide range of geophysical conditions in conjunction with advance inversion techniques. The information content of the signal with respect to the retrieved parameters is analyzed in a stochastic framework using two common criteria: the degrees of freedom for a signal and the Shannon information content. It is found that oxygen A-band measurements with moderate spectral resolution (0.2 nm) provide two pieces of independent information that allow the accurate retrieval of cloud-top height together with either cloud optical thickness or cloud fraction. Additionally, our results confirm previous studies indicating that the retrieval of cloud geometrical thickness (CGT) from single-angle measurements is not reliable in this spectral region. Finally, a sensitivity study shows that the retrieval of macrophysical cloud parameters is slightly sensitive to the uncertainty in the CGT and very sensitive to the uncertainty in the surface albedo.

Imaging Fourier-transform spectrometer measurements of a turbulent nonpremixed jet flame.

This work presents recent measurements of a CH4/H2/N2 turbulent nonpremixed jet flame using an imaging Fourier-transform spectrometer (IFTS). Spatially resolved (128×192 pixels, 0.72 mm/pixel) mean radiance spectra were collected between 1800 cm(-1)≤ν˜≤4500 cm(-1) (2.22 μm≤λ≤5.55 μm) at moderate spectral resolution (δν=16 cm(-1), δλ=20 nm) spanning the visible flame. Higher spectral-resolution measurements (δν=0.25 cm(-1), δλ=0.3 nm) were also captured on a smaller window (8×192) at 20, 40, and 60 diameters above the jet exit and reveal the rotational fine structure associated with various vibrational transitions in CH4, CO2, CO, and H2O. These new imaging measurements compare favorably with existing spectra acquired at select flame locations, demonstrating the capability of IFTS for turbulent combustion studies.