Ground-based and balloon-borne far-infrared spectral observations to support the preparation of the FORUM mission

The measurements of the FIR (Far InfraRed) portion of the spectral radiance emitted by the Earth's atmosphere, the main goal of the FORUM mission, were primarily exploited by a few ground-based and air-borne prototypes. In this presentation, a review of the main results obtained in recent years from the field deployment of two Fourier transform spectroradiometers (FTS) developed at CNR, REFIR-PAD (Radiation Explorer in the Far InfraRed - Prototype for Applications and Development) and FIRMOS (Far-Infrared Radiation Mobile Observation System) is presented. REFIR-PAD is the first FTS where technology developments allowed uncooled high-accuracy operations over a wideband spectral region. The instrument performs spectral radiance measurements covering the 100 to 1500 cm^-1 range with a resolution of 0.4 cm^-1, at different viewing directions: nadir, zenith, limb and deep space view at +30° elevation angle in the balloon-application. The capability to provide a full spectral characterisation of the atmospheric OLR (Outgoing Long-wave Radiation) emission was assessed during a stratospheric balloon flight in 2005 from Teresina (Brazil). Subsequently, on three different ground-based campaigns, the capability for measuring DLR (Downward Long-wave Radiation) was also established. The OLR results from the 2005 flight made possible the retrieval of the atmospheric state with high precision, leading to a more accurate calculation of the integrated outgoing radiation flux. It was thereby demonstrated that spectral information can be used to infer the angular distribution of radiance. We have observed that the error of our flux measurement is about 1 W/m^2 and it is mainly due to the radiometric calibration uncertainty, rather than the random detector noise. This result proves the feasibility of climatological studies with instruments employing noisy uncooled detectors (Palchetti et al., 2008). The measured spectra have also been used to test the performance of the various water vapour spectroscopic databases (Di Roma et al. 2021). The results from the ground-based deployment of REFIR-PAD show the capability of these measurements to improve the radiative transfer codes used to simulate the spectral radiances in the FIR. The ECOWAR (Earth COoling by Water vapor Radiation) field campaign in the Italian Alps (Testa Grigia, 3500 m AMSL) and RHUBC-II (Radiative Heating in Underexplored Bands Campaign) from Chile (Cerro Toco, 5380 m AMSL) allowed us to revise and improve the MT_CKD continuum model mostly in the range 260-590 cm^-1 (Masiello et al., 2012, Mlawer et al., 2019). The permanent installation in 2011 of REFIR-PAD over the Antarctic plateau at Concordia Station (Dome-C, 3220 m AMSL), where it almost continuously measures the DLR, allowed us to extend the validation and verification of state-of-art water vapour continuum absorption models down to 180 cm^-1 (Liuzzi et al., 2014). REFIR-PAD measurements were also used to test machine learning algorithms for cloud identification and classification (Cossich et al., 2021) and to derive optical and microphysical properties of cirrus clouds (Maestri et al., 2019). The FIR spectral region allows to improve the quality of the retrieval of cirrus properties, which critically influence the Earth radiation budget and climate sensitivity (Maestri et al., 2014, Palchetti et al., 2016). After REFIR-PAD was permanently installed in Antarctica, in 2018 a new prototype FIRMOS was developed to support the preparation of the feasibility phase of the FORUM mission both with laboratory measurements and field applications. A ground-based campaign was performed at the Zugspitze summit, Germany (2962 m AMSL) that offers excellent facilities to provide an independent and complete characterisation of the observed atmospheric state. The instrument was operated for two months in November-December 2018 and January-February 2019 in favourable weather conditions. FIRMOS covers the 100-1000 cm^-1 range with a resolution of 0.36 cm^-1 and 36 s acquisition time. The calibrated measurements were validated using E-AERI (Extended-range Atmospheric Emitted Radiance Interferometer), an operational instrument installed permanently at the summit covering the 400 to 3000 cm^-1 spectral range with 0.6 cm^-1 of resolution (Sussmann et al., 2016). FIRMOS measurements were used to retrieve the atmospheric parameters with the KLIMA code (Del Bianco et al. 2014) in clear sky conditions and with the SACR code (Di Natale et al. 2020) for cloudy scenes. The retrieved vertical profiles and cloud parameters show a good agreement respectively with the standard products of the closest radio soundings and with the backscattering Lidar, operated from the nearby research station of the Schneefernerhaus, about 600 m away from the summit. During the campaign, FIRMOS was also used to measure the surface spectral emission of snow that is not well-characterised in the FIR range. Measurements were performed by adapting FIRMOS to observe in a slant direction close to nadir providing one of the first measurement capable to sound the FIR snow properties down to 180 cm^-1. The described results show the capability of spectroradiometers like REFIR-PAD and FIRMOS to characterise the radiative properties of water vapour, cirrus clouds and snow emissivity in the under-explored FIR spectral range. They also demonstrate that such instruments can be used to provide valuable datasets to the atmospheric modelling community as well as a methodology to improve the scientific readiness level of the FORUM mission.