Evidence of azimuthal anisotropy for the thermal infrared radiation leaving the Earth's atmosphere

The analysis of one year of Cloud and the Earth's Radiant Energy System (CERES) data from the Earth Observation System (EOS)-Terra satellite provides evidence that the longwave radiation escaping from the atmosphere exhibits significant variability according to the azimuthal angle of observation. A regional analysis of this variability shows that the anisotropy in azimuth is maximum over mountain and desert areas and under cloud-free conditions. A relative difference between north and south views of about 5% in annual average is observed over the Himalayan region in the 8-14 mum infrared (IR) window. The remote sensing community should be aware of this variability, in particular when analysing IR data provided by instruments on geostationary orbits. Indeed, in this case, the azimuthal anisotropy may lead to systematic overestimation of the outgoing longwave radiation and to biases on estimated quantities such as the surface temperature.