Dissertation an der Fakult ̈at f ̈ur Physik der Ludwig–Maximilians–Universit ̈at Munchen angefertigt am Karlsruher Institut fur Technologie (KIT), Institut fur Meteorologie und Klimaforschung Atmospharische Umweltforschung (IMK-IFU) Garmisch-Partenkirchen vorgelegt von Andreas Reichert aus Kosching Munchen, 24.10.2016
The aim of this work is to reduce the uncertainties of atmospheric radiative transfer calculations by improving the quantitative knowledge of the water vapor continuum using atmospheric measurements. In addition to line absorption and emission, the water vapor continuum is responsible for a significant fraction of the interaction between infrared radiation and atmospheric water vapor. Due to the limitations of previous field and laboratory studies, there remains a lack of accurate measurements of the water vapor continuum throughout a significant fraction of the infrared spectral range, especially under atmospheric conditions. The consequential significant uncertainties in atmospheric radiative transfer calculations lead to possible inaccuracies in climate models and numerous remote sensing techniques. An accurate quantification of water vapor radiative processes is therefore of vital importance. The study presented in this thesis relies on a radiative closure experiment, i.e. a quantitative comparison of spectral radiance measurements with radiative transfer calculations in the spectral interval between 400 and 7800 cm−1 (1.3–25.0 μm). The experiment was set up at the Zugspitze (47.42◦N, 10.98◦E, 2964 m a.s.l.) high-altitude observatory and comprises thermal atmospheric emission spectra in the far infrared and solar FTIR (Fourier transform infrared) measurements covering the near infrared. Several new methods were developed to improve the sensitivity of the closure compared to previous studies and to be able to cover spectral intervals previously not accessible to atmospheric continuum studies, e.g. a new approach for the correction of sun-pointing inaccuracies in solar absorption spectrometry and for the radiometric calibration of near-infrared solar absorption spectra. The method for quantification and correction of systematic sun-pointing inaccuracies in solar absorption spectrometry presented in this work relies on subsequent measurements of the Doppler shift of solar lines with differing orientations of the solar rotation axis relative to the zenith direction. The proposed concept augments the sensitivity of the closure experiment by improving the accuracy of trace gas column measurements and the near-infrared radiometric calibration used in this study. The mispointing correction is demonstrated using measurement time series of dry-air column-averaged mole fractions of methane (XCH4), for which consistency of the XCH4 trend with results from the nearby Garmisch FTIR site (47.48◦N, 11.06◦E, 743 m a.s.l.) is restored by applying the correction. Water vapor continuum quantification from near-infrared solar absorption spectra requires sufficiently accurate radiometric calibration of the measured spectra. A new calibration approach presented in this work combines the Langley technique with spectral radiance measurements of a high-temperature blackbody calibration source. The calibration scheme provides a calibration accuracy of less than 1 % in window regions and up to 2 % within absorption bands. A validation of this calibration uncertainty estimate is performed by investigation of calibration self-consistency, which yields compatible results within the estimated errors for 91 % of the 2500 to 7800 cm−1-range. A second validation effort consists in a comparison of a set of calibrated spectra to radiative transfer model calculations, which are consistent within the estimated errors for 98 % of the spectral range. The continuum results in the far infrared, namely in the 400 to 580 cm−1 spectral range, are consistent with the widely used MT_CKD 2.5.2 (Mlawer et al., 2012) continuum model and with the findings of other recent atmospheric closure studies. Throughout most of the spectral range covered by the near-infrared section of the closure study, the results presented in this work constitute the first quantification of the water vapor continuum absorption under atmospheric conditions. The dry atmospheric conditions at the Zugspitze site enable continuum quantification even within water vapor absorption bands, while only upper limits for continuum absorption can be provided in the centers of window regions. Throughout 75 % of the 2500 to 7800 cm−1 spectral range, the Zugspitze results are agree within our estimated uncertainty with the MT_CKD 2.5.2-model. Notable exceptions are the 2800 to 3000 cm−1 and 4100 to 4200 cm−1 spectral ranges, where our measurements indicate about 5 times stronger continuum absorption than MT_CKD. The measurements are consistent with the laboratory measurements of Mondelain et al. (2015), which rely on cavity ring-down spectroscopy (CDRS), and the calorimetric-interferometric measurements of Bicknell et al. (2006). Compared to the recent FTIR laboratory studies of Ptashnik et al. (2012, 2013), our measurements indicate 2–5 times weaker continuum absorption under atmospheric conditions in the wings of water vapor absorption bands, namely in the 3200 to 3400 cm−1, 4050 to 4200 cm−1, and 6950 to 7050 cm−1 spectral regions. The results obtained in this work constitute a significant contribution to the characterization of the water vapor continuum under atmospheric conditions and thereby add to decrease the water vapor-related uncertainties in atmospheric radiative transfer calculations. Given that previously no results under atmospheric conditions were available in the near-infrared, the findings of this work are a valuable tool for the validation of the commonly used MT_CKD continuum model and allow resolving the inconsistencies between recent laboratory studies in this spectral range. Additionally, the experimental setup established in this work provides the foundation to address further key questions considering the water vapor continuum in the future. The findings of recent studies on the climate relevance of the water vapor continuum (Paynter and Ramaswamy, 2014; R̈adel et al., 2015) imply that the results presented in this thesis are expected to have a significant impact on climate models. The likely effects comprise an adjustment of the surface energy budget through a decrease in both latent and sensible heat and, as a consequence, a reduction of tropical convection and rainfall.
The room temperature self- and foreign-continua of water vapor have been measured near 4250 cm−1 with a newly developed high sensitivity cavity ring down spectrometer (CRDS). The typical sensitivity of the recordings is αmin ≈ 6 × 10−10 cm−1 which is two orders of magnitude better than previous Fourier transform spectroscopy (FTS) measurements in the spectral region. The investigated spectral interval is located in the low energy range of the important 2.1 μm atmospheric transparency window. Self-continuum cross-sections, CS, were retrieved from the quadratic dependence of the spectrum base line level measured for different water vapor pressures between 0 and 15 Torr, after subtraction of the local water monomer lines contribution calculated using HITRAN2012 line parameters. The CS values were determined with 5% accuracy for four spectral points between 4249.2 and 4257.3 cm−1. Their values of about 3.2 × 10−23 cm2 molecule−1 atm−1 are found 20% higher than predicted by the MT_CKD V2.5 model but two times weaker than reported in the literature using FTS. The foreign-continuum was evaluated by injecting various amounts of synthetic air in the CRDS cell while keeping the initial water vapor partial pressure constant. The foreign-continuum cross-section, CF, was retrieved from a linear fit of the spectrum base line level versus the air pressure. The obtained CF values are larger by a factor of 4.5 compared to the MT_CKD values and smaller by a factor of 1.7 compared to previous FTS values. As a result, for an atmosphere at room temperature with 60% relative humidity, the foreign-continuum contribution to the water continuum near 4250 cm−1 is found to be on the same order as the self-continuum contribution.
