Comparisons of Clear-Sky Outgoing Far-IR Flux Inferred from Satellite Observations and Computed from the Three Most Recent Reanalysis Products

The far IR spectrum plays an important role in Earth’s radiation budget and remote sensing. We compare the near-global (80ºS–80ºN) outgoing clear-sky far-IR flux inferred from the collocated AIRS & CERES observations in 2004 with the counterparts computed from reanalysis data sets sub-sampled along the same satellite trajectories. The three most recent reanalyses are examined: ECMWF ERA-interim, NASA MERRA, and NOAA/NCEP CFSR.  Following a previous study by Huang et al. (2008), clear-sky spectral angular distribution models (spectral ADMs) are developed for five of the CERES land surface scene types as well as for the  extra-tropical oceans. The outgoing longwave radiation (OLR) directly estimated from the AIRS  radiances using our algorithm agrees well with the OLR in collocated CERES SSF (Single Satellite Footprint) dataset. The daytime difference is 0.96±2.02 Wm-2 and the nighttime difference is 0.86±1.61 Wm-2 . To a large extent, the far-IR flux derived in this way agrees with those directly computed from three reanalyses. The near-global averaged differences between reanalyses and observations tend to be slightly positive (0.66-1.15%) over 0-400cm-1 and slightly negative (-0.89―-0.44%) over 400-600cm-1. For all three reanalyses, the spatial distributions of  such differences show the largest discrepancies over the high-elevation areas during the daytime but not during the nighttime, suggesting discrepancies in the diurnal variation of such areas among different data sets. The composite differences with respect to temperature or precipitable water suggest large discrepancies for cold and humid scenes.