The Zugspitze radiative closure experiment for quantifying water vapor absorption over the terrestrial and solar infrared – Part 1: Setup, uncertainty analysis, and assessment of far-infrared water vapor continuum

Abstract

Abstract. Quantitative knowledge of water vapor radiative processes in the atmosphere throughout the terrestrial and solar infrared spectrum is still incomplete even though this is crucial input to the radiation codes forming the core of both remote sensing methods and climate simulations. Beside laboratory spectroscopy, ground-based remote sensing field studies in the context of so-called radiative closure experiments are a powerful approach because this is the only way to quantify water absorption under cold atmospheric conditions. For this purpose, we have set up at the Zugspitze (47.42 • N, 10.98 • E; 2964 m a.s.l.) a long-term radiative closure experiment designed to cover the infrared spectrum between 400 and 7800 cm −1 (1.28-25 µm). As a benefit for such experiments, the atmospheric states at the Zugspitze frequently comprise very low integrated water vapor (IWV; minimum = 0.1 mm, median = 2.3 mm) and very low aerosol optical depth (AOD = 0.0024-0.0032 at 7800 cm −1 at air mass 1). All instruments for radiance measurements and atmospheric-state measurements are described along with their measurement uncertainties. Based on all parameter uncertainties and the corresponding radiance Jacobians, a systematic residual radiance uncertainty budget has been set up to characterize the sensitivity of the radiative closure over the whole infrared spectral range. The dominant uncertainty contribution in the spectral windows used for far-infrared (FIR) continuum quantification is from IWV uncertainties, while T profile uncertainties dominate in the mid-infrared (MIR). Uncertainty contributions to near-infrared (NIR) radiance residuals are dominated by water vapor line parameters in the vicinity of the strong water vapor bands. The window regions in between these bands are dominated by solar Fourier transform infrared (FTIR) calibration uncertainties at low NIR wavenumbers, while uncertainties due to AOD become an increasing and dominant contribution towards higher NIR wavenumbers. Exceptions are methane or nitrous oxide bands in the NIR, where the associated line parameter uncertainties dominate the overall uncertainty.As a first demonstration of the Zugspitze closure experiment, a water vapor continuum quantification in the FIR spectral region (400-580 cm −1 ) has been performed. The resulting FIR foreign-continuum coefficients are consistent with the MT_CKD 2.5.2 continuum model and also agree with the most recent atmospheric closure study carried out in Antarctica. Results from the first determination of the NIR water vapor continuum in a field experiment are detailed in a companion paper (Reichert and Sussmann, 2016) while a novel NIR calibration scheme for the underlying FTIR measurements of incoming solar radiance is presented in another companion paper .

The Zugspitze radiative closure experiment for quantifying water vapor absorption over the terrestrial and solar infrared – Part 3: Quantification of the mid- and near-infrared water vapor continuum in the 2500 to 7800 cm<sup>−1</sup> spectral range under atmospheric conditions

“…As outlined in the companion paper Part 2, recent studies on the NIR ESS have yielded results that feature differences of up to 5-10 % (see, e.g., Menang et al, 2013;Bolsée et al, 2014;Thuillier et al, 2014Thuillier et al, , 2015Weber, 2015). Furthermore, the number of solar lines differs significantly between, e.g., the ESS versions of Kurucz (2005) and Menang et al (2013).…”

Comment on essd-2020-377

“…At the Summit station, KIT operates an Extended-range Atmospheric Emitted Radiance Interferometer (E-AERI) system (Sussmann et al, 2016), manufactured by ABB Bomem Inc. (Quebec, Canada), which covers part of the far, mid and near infrared spectral regions. The spectrum of the Downwelling Longwave Radiation (DLR) was therefore measured in the FIR and mid-infrared spectral ranges from 100 to 1800 cm −1 (100-3.3 µm) using both FIRMOS, covering the 100-1000 cm −1 spectral range with 0.3 cm −1 of resolution, and the E-AERI, covering the 400-1800 cm −1 range with 0.5 cm −1 of resolution.…”

Analysis of Water Vapor Absorption in the Far‐Infrared and Submillimeter Regions Using Surface Radiometric Measurements From Extremely Dry Locations

“…Not shown in Figure 11 are the results of Sussmann et al (2016), which determined foreign continuum coefficients between 400 and 600 cm −1 . The coefficients determined in that work agree better with MT_CKD_2.8 than MT_CKD_3.0, although the MT_CKD_3.0 and Sussmann et al (2016) coefficients still generally agree in light of the uncertainties of the two continuum data sets. Between 350 and…”