Retrieval of outgoing longwave radiation from COMS narrowband infrared imagery

Hourly outgoing longwave radiation (OLR) from the geostationary satellite Communication Oceanography Meteorological Satellite (COMS) has been retrieved since June 2010. The COMS OLR retrieval algorithms are based on regression analyses of radiative transfer simulations for spectral functions of COMS infrared channels. This study documents the accuracies of OLRs for future climate applications by making an intercomparison of four OLRs from one single-channel algorithm (OLR12.0 using the 12.0 mu m channel) and three multiple-channel algorithms (OLR10.8+12.0 using the 10.8 and 12.0 mu m channels; OLR6.7+10.8 using the 6.7 and 10.8 mu m channels; and OLRAll using the 6.7, 10.8, and 12.0 mu m channels). The COMS OLRs from these algorithms were validated with direct measurements of OLR from a broadband radiometer of the Clouds and Earth's Radiant Energy System (CERES) over the full COMS field of view [roughly (50A degrees S-50A degrees N, 70A degrees-170A degrees E)] during April 2011. Validation results show that the root-mean-square errors of COMS OLRs are 5-7 W m(-2), which indicates good agreement with CERES OLR over the vast domain. OLR6.7+10.8 and OLRAll have much smaller errors (similar to 6 W m(-2)) than OLR12.0 and OLR10.8+12.0 (similar to 8 W m(-2)). Moreover, the small errors of OLR6.7+10.8 and OLRAll are systematic and can be readily reduced through additional mean bias correction and/or radiance calibration. These results indicate a noteworthy role of the 6.7 mu m water vapor absorption channel in improving the accuracy of the OLRs. The dependence of the accuracy of COMS OLRs on various surface, atmospheric, and observational conditions is also discussed.