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a) The calibrated downwelling radiance ( L s + İ L ) measured with an Atmospheric Emitted Radiance Interferometer (AERI) at Eureka, Canada on 1 July 2008 at 0357 UTC, and the Planck functions of the temperature of the cold calibration-source, B( T c ), and ambient temperature, B( T a ). A few radiances having significant errors are circled. b) The real part of the ratio of uncalibrated difference spectra for the measurement shown in a). Bounds used to identify spectral data points with large errors are shown. Dots indicate points identified with a criteri RQLQWHUPVRIWKHUHODWLYHHUURULQWKHLQVWUXPHQWUHVSRQVLYLW\ı r / r . 

a) The calibrated downwelling radiance ( L s + İ L ) measured with an Atmospheric Emitted Radiance Interferometer (AERI) at Eureka, Canada on 1 July 2008 at 0357 UTC, and the Planck functions of the temperature of the cold calibration-source, B( T c ), and ambient temperature, B( T a ). A few radiances having significant errors are circled. b) The real part of the ratio of uncalibrated difference spectra for the measurement shown in a). Bounds used to identify spectral data points with large errors are shown. Dots indicate points identified with a criteri RQLQWHUPVRIWKHUHODWLYHHUURULQWKHLQVWUXPHQWUHVSRQVLYLW\ı r / r . 

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Spectra measured by remote-sensing Fourier transform infrared spectrometers are often calibrated using two calibration sources. At wavenumbers where the absorption coefficient is large, air within the optical path of the instrument can absorb most calibration-source signal, resulting in extreme errors. In this paper, a criterion in terms of the ins...

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... s c h c r c s 2 c r h c c s , (9) Reviewing Eq. (7), we see that the f 2 terms in Eq. (9) approach infinity as e r / r approaches $OWKRXJK WKH YDULDQFH LV WKXV IRUPDOO\ LQILQLWH IRU ı r / r << 1, | e r / r | is unlikely to ever approach values as large as one, and the variance can be approximated as finite [Eq. (8)]. +RZHYHU DV ı r / r gets larger, the probability of infinite f 2 increases; this corresponds to a probability of arbitrarily large errors in calibrated radiances. Outside the low-noise limit, RNW find that < İ L > z 0, but rather, L s H L 0.5 f r L h L c . (10) In RNW, < f r ! LV FDOFXODWHG QXPHULFDOO\ DQG IRXQG WR YDU\ IURP WR DV ı r / r increases from 0 to infinity (see Fig. 2 of RNW). Figure 3 of RNW shows that averages of calibrated radiances approach 0.5( L h + L c ) outside the instrument bandwidth and at specific points in- EDQGZKHUHı r / r is very large. In RNW, numerical calculations were used to determine that the threshold ́ r / r d 0.3 corresponds to a bias [ d ~10 5 ( L h + L c )] that is negligible compared to most error budgets, and a variance for 99.999% of measurements that is within 20% of the variance in the low-noise approximation. This threshold was used to check the bandwidth of an AERI and a satellite- borne instrument. The opposite condition, r / > 0.3, can be used to identify calibrated radiances at wavenumbers in- EDQG ZKHUH DEVRUSWLRQ E\ WUDFH JDVHV ZLWKLQ WKH LQVWUXPHQW¶V RSWLFDO SDWK makes accurate calibration impossible. The remainder of this paper focuses on these in-band wavenumbers for specific instruments. In this section, we show that the responsivity criterion can accurately identify low- responsivity wavenumbers. To put the responsivity criterion into context, we first discuss a PHWKRG WHUPHG WKH 3UDWLR FULWHULRQ ́ FXUUHQWO\ LQFOXGHG DV TXDO ity-control in the standard AERI processing [7], whose purpose is to identify large spikes in calibrated radiances (that lead to ringing when Fourier transforms are performed in further processing). [11] Here, we apply both criteria to measurements from two AERI instruments, one with low noise and one with relatively high noise. We also provide a method for avoiding biases that can occur when corrected radiances are averaged. We then show how Fourier transforms can cause errors to ring into neighboring wavenumbers, making it important to replace radiances that have large errors. Finally, we apply the responsivity criterion to a year-long field experiment at Eureka, Canada, showing that the number of times a given wavenumber is identified correlates well with the strength and proximity of the nearest strong line center in the absorption spectrum of water vapor. Figure 2a shows the calibrated downwelling radiance measured by an AERI at Eureka, Canada at 0357 UTC on 1 July 2008 [5]. The AERI instrument is described in general by Knuteson et al. [12]. This AERI has a standard detector and is sensitive from about 500 to 1800 cm 1 , exhibiting relatively low noise [13]. Absorption features due to CO 2 , O 3 , and H 2 O are labeled in the figure. I QWKH3DWPRVSKHULFZLQGRZ ́IURPWRFP 1 , there is little emission from trace gases, except O 3 . At the center of the CO 2 band (667 cm 1 DQGLQWKHȞ 2 band of water vapor (1300 to 1900 cm 1 ), the absorption coefficient can be strong enough that almost all source radiation is absorbed and re-emitted within the optical path of the instrument. A few radiances in the water- YDSRU Ȟ 2 band having large errors ( t 10 RU) are indicated (circles); these correspond to the extremely low responsivities indicated previously in Fig. 1. Since the emission is known to be strong at these wavenumbers, we expect that if accurate calibration were possible, then L s would be | B( T a ), where B indicates the Planck function and T a is the ambient temperature. An estimate of B( T a ) is shown on the figure, derived from the radiance observed between 672 and 682 cm 1, a region known to saturate close to (but outside) the instrument. Also shown is the Planck function of the temperature of the cold calibration blackbody, B( T c ), which is (approximately) the radiance emitted by the cold calibration-source. B( T c ) is greater than B( T a ) because the cold calibration source was warmed to slightly above-ambient temperatures. The ratio criterion identifies wavenumbers where the real part of the ratio of uncalibrated spectral differences, Re[( V s - V c )/( V h - V c )], falls outside a set of pre-determined bounds. The bounds are intended to be generous enough to filter out large spikes in calibrated radiances but not noisy data generally, and to be widely applicable geographically and with variations in instruments [11]. However, the uncalibrated sky spectrum ( V s ) is highly variable both spectrally and with atmospheric conditions. Figure 2b demonstrates how the ratio criterion is applied. A set of bounds is chosen, in this case at +1.5 and 1.5 (red dashed lines). The ratio is calculated for each scene spectrum. The blue curve in the figure indicates the ratio for the case study on 1 July 2008 at 3.94 UTC. Wavenumbers are identified where Re[( V s - V c )/( V h - V c )] > 1.5 or Re[( V s - V c )/( V h - V c )] < 1.5. For this case, a single point is identified above 1.5, while many points are identified below 1.5. In current AERI processing, calibrated radiances at these wavenumbers are then replaced with the radiance from the cold calibration source, B c . Over the course of the day, the ratio will change slightly at most wavenumbers as V s changes with atmospheric conditions and V changes with the temperature of the cold calibration- source ( h is generally constant because ...

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... Large radiance discrepancies occur, especially in the window band, and are found to mainly come from clear-sky scenes (see Figure B1 and discussions in Appendix B). This suggests that the discrepancies likely result from calibration (Rowe, Neshyba, Cox, & Walden, 2011; and other undetected errors (e.g., something in the FOV of one instrument but not the other). In order to avoid discarding meaningful data in the trend analysis, we simulate the clear-sky DLR spectra using a radiation model together with collocated atmospheric measurements and use these synthetic spectra as a reference to assign proper weights in combining the data of AERI-01 and AERI-C1, based on the findings of previous radiance closure studies (e.g., Turner et al., 2004) that demonstrated high accuracy in such synthetic spectra. ...
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... Downwelling infrared radiances were measured from the South Pole surface using the Polar Atmospheric Emitted Radiance Interferometer (PAERI) in December 2000 and during most of 2001. The PAERI measures downwelling radiance from ∼500 to 3,000 cm −1 at a resolution of 0.5 cm −1 and has been described and characterized in a number of papers (Rowe et al., 2006(Rowe et al., , 2009Rowe, Neshyba, Cox, & Walden, 2011;; see also references therein). Here we use PAERI measurements made on 2 February 2001. ...
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Clouds have a large effect on the radiation budget and represent a major source of uncertainty in climate models. Supercooled liquid clouds can exist at temperatures as low as 235 K, and the radiative effect of these clouds depends on the complex refractive index (CRI) of liquid water. Laboratory measurements have demonstrated that the liquid‐water CRI is temperature‐dependent, but corroboration with field measurements is difficult. Here we present measurements of the downwelling infrared radiance and in‐situ measurements of supercooled liquid water in a cloud at temperatures as low as 240 K, made at South Pole Station in 2001. These results demonstrate that including the temperature dependence of the liquid‐water CRI is essential for accurate calculations of radiative transfer through supercooled liquid clouds. Furthermore, we show that when cloud properties are retrieved from infrared radiances (using the spectral range 500–1,200 cm⁻¹) spurious ice may be retrieved if the 300 K CRI is used for cold liquid clouds (∼240 K). These results have implications for radiative transfer in climate models as well as for retrievals of cloud properties from infrared radiance spectra.
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The Extended-range Atmospheric Emitted Ra-diance Interferometer (E-AERI) is a moderate resolution (1 cm −1) Fourier transform infrared spectrometer for mea-suring the absolute downwelling infrared spectral radiance from the atmosphere between 400 and 3000 cm −1 . The ex-tended spectral range of the instrument permits monitoring of the 400–550 cm −1 (20–25 µm) region, where most of the infrared surface cooling currently occurs in the dry air of the Arctic. Spectra from the E-AERI have the potential to provide information about radiative balance, trace gases, and cloud properties in the Canadian high Arctic. Calibra-tion, performance evaluation, and certification of the E-AERI were performed at the University of Wisconsin Space Sci-ence and Engineering Centre from September to October 2008. The instrument was then installed at the Polar Envi-ronment Atmospheric Research Laboratory (PEARL) Ridge Lab (610 m altitude) at Eureka, Nunavut, in October 2008, where it acquired one year of data. Measurements are taken every seven minutes year-round, including polar night when the solar-viewing spectrometers at PEARL are not operated. A similar instrument, the University of Idaho's Polar AERI (P-AERI), was installed at the Zero-altitude PEARL Auxil-iary Laboratory (0PAL), 15 km away from the PEARL Ridge Lab, from March 2006 to June 2009. During the period of overlap, these two instruments provided calibrated radiance measurements from two altitudes. A fast line-by-line radia-tive transfer model is used to simulate the downwelling ra-diance at both altitudes; the largest differences (simulation-measurement) occur in spectral regions strongly influenced by atmospheric temperature and/or water vapour. The two AERI instruments at close proximity but located at two dif-ferent altitudes are well-suited for investigating cloud forc-ing. As an example, it is shown that a thin, low ice cloud resulted in a 6 % increase in irradiance. The presence of clouds creates a large surface radiative forcing in the Arctic, particularly in the 750–1200 cm −1 region where the down-welling radiance is several times greater than clear-sky radi-ances, which is significantly larger than in other more humid regions.