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Quantitative knowledge of water vapor radiative processes in the atmospherethroughout the terrestrial and solar infrared spectrum is still incompleteeven though this is crucial input to the radiation codes forming the core ofboth remote sensing methods and climate simulations. Beside laboratoryspectroscopy, ground-based remote sensing field studies in the context ofso-called radiative closure experiments are a powerful approach because thisis the only way to quantify water absorption under cold atmosphericconditions. For this purpose, we have set up at the Zugspitze(47.42° N, 10.98° E; 2964 m a.s.l.) a long-term radiativeclosure experiment designed to cover the infrared spectrum between 400 and7800 cm−1 (1.28–25 µm). As a benefit for such experiments,the atmospheric states at the Zugspitze frequently comprise very lowintegrated water vapor (IWV; minimum  =  0.1 mm, median  =  2.3 mm) andvery low aerosol optical depth (AOD  =  0.0024–0.0032 at 7800 cm−1at air mass 1). All instruments for radiance measurements andatmospheric-state measurements are described along with their measurementuncertainties. Based on all parameter uncertainties and the correspondingradiance Jacobians, a systematic residual radiance uncertainty budget hasbeen set up to characterize the sensitivity of the radiative closure over thewhole infrared spectral range. The dominant uncertainty contribution in thespectral windows used for far-infrared (FIR) continuum quantification is fromIWV uncertainties, while T profile uncertainties dominate in themid-infrared (MIR). Uncertainty contributions to near-infrared (NIR) radianceresiduals are dominated by water vapor line parameters in the vicinity of thestrong water vapor bands. The window regions in between these bands aredominated by solar Fourier transform infrared (FTIR) calibrationuncertainties at low NIR wavenumbers, while uncertainties due to AOD becomean increasing and dominant contribution towards higher NIR wavenumbers.Exceptions are methane or nitrous oxide bands in the NIR, where theassociated line parameter uncertainties dominate the overall uncertainty. As a first demonstration of the Zugspitze closure experiment, a water vaporcontinuum quantification in the FIR spectral region (400–580 cm−1) hasbeen performed. The resulting FIR foreign-continuum coefficients areconsistent with the MT_CKD 2.5.2 continuum model and also agree with themost recent atmospheric closure study carried out in Antarctica. Results fromthe first determination of the NIR water vapor continuum in a fieldexperiment are detailed in a companion paper (Reichert and Sussmann, 2016)while a novel NIR calibration scheme for the underlying FTIR measurements ofincoming solar radiance is presented in another companion paper (Reichert etal., 2016)

atmospheric chemistry and physics

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