Contributing factors to downward longwave radiation at the Earth's surface

To estimate contributions of water vapor (WV), carbon dioxide (CO₂), and clouds to longwave radiation, surface downward longwave irradiance (DLI) was evaluated by comparing observations with values calculated using data from vertical profiles of WV and clouds obtained from radiosonde observations at five Baseline Surface Radiation Network (BSRN) sites. The observed DLI was reproduced by calculation with an accuracy of 3.9 ± 4.4 W m^-2 for clear-sky conditions at all sites, but the accuracy was -7.7 ± 8.6 W m^-2 for overcast conditions. The individual contributions of WV, CO₂, and clouds to DLI were evaluated by removing these factors one by one from the normal condition including all of the factors (removal method) and by removing all factors except for one particular factor (addition method). The results indicate that the contributions of WV and clouds are relatively large, whereas the contribution of CO₂ is relatively small.