Impact of the atmospheric correction on infrared camera measurements

Monitoring clouds is necessary for many applications, such as aircraft navigation, astronomical observations and others. The height of the top and the bottom of the clouds can be retrieved from satellites and ground-based stations, respectively, by measuring their brightness temperature. In this context, ground-based infrared cameras offer interesting information about the spatial distribution of clouds and the height of their bases. Some atmospheric gases interact significantly with the radiation emitted by clouds, aerosols, atmospheric gases and the Earth’s surface, so an atmospheric correction is needed to obtain reliable estimates of a cloud base. In this study, the influence of water vapour and carbon dioxide on the downward radiance measured by a FLIR infrared camera on a height variable cloudy scenario is analyzed. The FLIR A325SC camera spectral response function is considered, and standard atmospheric profiles are used. The infrared absorption and emission of the profiles of water vapour and carbon dioxide is estimated by the Python package ‘RADIS’. The results show a positive net atmospheric effect on the downward radiance for all the standard atmospheric profiles considered, which indicates a higher emission contribution of the atmospheric gases compared to the absorption. However, the magnitude of the atmospheric effect significantly depends on the specific atmospheric profile. For example, the atmospheric net emission effect on the downward radiation for high clouds with a Tropical atmospheric profile is around 30 W/m²·sr, whereas for a Subarctic-Winter atmospheric profile is less than 8 W/ m²·sr. Main results show that the atmospheric effect notably depends also on the vertical gradient, being particularly high for the Tropical profile. Moreover, regarding a specific profile, the atmospheric correction becomes more important for high clouds than for medium or low clouds. Therefore, the atmospheric correction should not be neglected if accurate estimations of the cloud height are to be obtained.