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Volume 16, issue 18
Atmos. Chem. Phys., 16, 11649-11669, 2016
https://doi.org/10.5194/acp-16-11649-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Twenty-five years of operations of the Network for the Detection...

Atmos. Chem. Phys., 16, 11649-11669, 2016
https://doi.org/10.5194/acp-16-11649-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Sep 2016

Research article | 21 Sep 2016

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

Ralf Sussmann, Andreas Reichert, and Markus Rettinger Ralf Sussmann et al.
  • Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany

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; 2964ma.s.l.) a long-term radiative closure experiment designed to cover the infrared spectrum between 400 and 7800cm−1 (1.28–25µm). As a benefit for such experiments, the atmospheric states at the Zugspitze frequently comprise very low integrated water vapor (IWV; minimum = 0.1mm, median = 2.3mm) and very low aerosol optical depth (AOD = 0.0024–0.0032 at 7800cm−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 (400–580cm−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 (Reichert et al., 2016).

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Quantitative knowledge of infrared absorption by water vapor is crucial for remote sensing and climate simulations. Under cold atmospheric conditions this information can only be attained via field experiments. Therefore, we set upa radiative closure study covering 1.28–25 µm at the Zugspitze (47° N, 2964 m a.s.l.). We describe the setup and elaborate on the radiance uncertainty budget. Water vapor continuum quantification in the far infrared shows consistency with the MT_CKD continuum model.
Quantitative knowledge of infrared absorption by water vapor is crucial for remote sensing and...
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