From dusk till dawn: the role of the atmospheric limb in observations of Earth’s energy imbalance

The global mean outgoing radiation from the Earth system is typically visualised as a flow of energy in the radial direction. In the context of satellite-based observations, the picture is more complex, as each element in the field of view contributes with radiance through its own characteristic angular distribution. For instruments in low Earth orbit that are designed to observe Earth’s energy imbalance (EEI), such as the wide field-of-view cameras and radiometer onboard the proposed Earth Climate Observatory (ECO) mission, fluxes of the order of 0.1 W/m² are significant. Under such strict requirements, it is essential to capture the complete angular distribution of the radiation that leaves each element to prevent systematic errors in the estimated imbalance. In particular, a significant portion of the outgoing irradiance leaves the Earth's atmosphere above the horizon via the atmospheric limb. In this presentation, we explore the magnitude and characteristics of the limb radiance and investigate its dependence on solar conditions, surface properties, and stratospheric aerosols by using the radiative transfer model SASKTRAN. We show that the total irradiance contribution from the atmospheric limb can reach up to 2 W/m² and that significant signal may originate from above 30 km tangent altitude. We further investigate the influence of upper atmospheric levels to the full irradiance measurement at satellite altitude and demonstrate that contributions from the upper stratosphere may be significant for EEI monitoring.