The spectrally and vertically decomposed outgoing longwave radiation and its climate trends in the far-infrared

The outgoing longwave radiation (OLR) at the top of the atmosphere is a critical component of the Earth's radiation energy budget. A substantial portion of the OLR energy lies in the far-infrared (FIR) spectrum, which has not been directly measured for understanding weather and climate variations. Several satellite projects under development, including the Thin Ice Cloud in Far Infrared Experiment (TICFIRE, Blanchet et al. 2011) funded by the Canadian Space Agency, the Polar Radiant Energy in the Far Infrared Experiment (PREFIRE, L’Ecuyer et al., 2021) of U.S. NASA, and the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM, Palchetti et al., 2020) of the European Space Agency, are being developed to fill this observation gap. Given that the FIR observation data is not available yet, we use simulations to acquire prior knowledge of the climatological mean distribution of the OLR in FIR, by using a rapid radiative transfer model, RRTMG, to simulate spectrally decomposed OLR in different spectral bands from global instantaneous atmospheric profiles of the fifth generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5). Based on the radiative transfer equation, we dissect the OLR by attributing its distribution and variation to spectrally and vertically decomposed contributions of the atmosphere and Earth surface. Our results disclose that the relatively higher far-infrared fraction of the OLR in polar region is due to stronger surface contribution and identify a minimum atmospheric contribution layer around the tropopause. On the other hand, the variability of the spectrally decomposed OLR field is dissected with the aid of a new set of radiative sensitivity kernels. This analysis discovers that the non-cloud longwave climate feedback, as well as its inter-climate model discrepancy, mainly results from the upper tropospheric thermodynamic fields (temperature and water vapor) and their effects on the FIR radiation.