Observations of the downwelling far-infrared atmospheric emission at the Zugspitze observatory

Abstract. Measurements of the spectrum of the atmospheric emission in the far-infrared (FIR) range, between 100 and 667 cm−1 (100–15 micron), are scarce because of the detection complexity and of the strong absorption of air at ground level, preventing the sounding of the FIR from low altitude. Consequently, FIR measurements need to be made from high-altitude sites or on board airborne platforms or satellites. This paper describes the dataset of FIR spectral radiances of the atmosphere and snow surface emission, measured in the 100–1000 cm−1 range by the Far-Infrared Radiation Mobile Observation System (FIRMOS) instrument, during a 2-month campaign carried out from ground at 3000 m of altitude, on the top of Mount Zugspitze in the German Alps, in 2018–2019. This campaign is part of the preparatory activity of a new space FIR mission, named Far-infrared-Outgoing-Radiation Understanding and Monitoring (FORUM), which is under development by the European Space Agency (ESA). The dataset acquired during the campaign also includes all the additional measurements needed to provide a full characterisation of the observed atmospheric state and the local atmospheric and surface conditions. It includes co-located spectral measurements in the infrared range from 400 to 1800 cm−1; lidar backscatter profiles; radiosoundings of temperature, humidity and aerosol backscatter profiles; local weather parameters; and snow/ice microphysical properties. These measurements provide a unique dataset that can be used to perform radiative closure experiments to improve modelling parameters that in the FIR are not well characterised, such as water vapour spectroscopy, scattering properties of cirrus clouds and the FIR emissivity of the surface covered by snow. The consolidated dataset is freely available via the ESA campaign dataset website at https://doi.org/10.5270/ESA-38034ee .