Spectral outgoing longwave radiation derived from the Polar Radiant Energy in the Far Infrared Experiment (PREFIRE) Measurements: Algorithms and Validations

The far-infrared (>16 µm) spectrum plays a crucial role in Earth's radiation budget but was rarely observed from space before 2024. Two Polar Radiant Energy in the Far Infrared Experiment (PREFIRE) satellites, PREFIRE2 (SAT-2) and PREFIRE1 (SAT-1), were launched in 2024. They measure spectrally resolved radiances from 5 to 53 µm at 0.84 µm spectral resolution. Deriving spectral outgoing longwave radiation (OLR) and tracking its changes relative to other polar climate factors are key focuses of the PREFIRE mission. Here we describe a set of algorithms developed to estimate spectral OLR from the PREFIRE L1B-calibrated radiances. Spectral angular distribution models were developed to invert spectral radiance to spectral flux. For the spectral region not covered by the PREFIRE measurements, a principal-component-based linear regression scheme is used to estimate the spectral flux. When the clear-sky spectral OLRs derived from the PREFIRE SAT-2 measurements are compared with counterparts derived from in-situ measurements, the mean differences in broadband and far-IR OLR are 7.5 Wm-2 and 3.4 Wm-2, respectively. The results are also evaluated against collocated broadband and other spectral satellite measurements. The difference (mean ± standard deviation) between the collocated PREFIRE SAT-2 and CERES broadband OLR is 2.8±5.7 Wm-2 and 2.7±6.5 Wm-2 for clear-sky and cloudy-sky footprints, respectively. Similar agreements can be seen from the comparisons with spectral OLR derived from AIRS and CrIS measurements. In general, spectral OLR derived from SAT-2 has better agreement with other collocated measurements than that from SAT-1, which can be attributed to the higher noisy performance of the SAT-1 instrument.