Factors limiting contrail detection in satellite imagery

Abstract. Contrails (clouds produced by aircraft exhaust) have a significant warming contribution to the overall climate impact of aviation. This makes reducing them a key target for future climate strategies in the sector. Identifying pathways for contrail reduction requires accurate models of contrail formation and lifecycle, which in turn need suitable observations to constrain them. Infrared imagers on geostationary satellites provide widespread, time-resolved observations of the evolution of contrail properties. However, contrails are often narrow and optically thin, which makes them challenging for satellites to identify. Quantifying the impact of contrail properties on observability is essential to determine the extent to which satellite observations can be used to constrain contrail models and to assess the climate impact of aviation. In this work, contrail observability is tested by applying a simple contrail detection algorithm to synthetic images of contrails in an otherwise-clear sky against a homogeneous ocean background. Only (46 ± 2) % of a modelled population of global contrail segments are found to be observable using current 2 km resolution instruments, even in this maximally-observable case. A significantly higher portion of contrail forcing is detectable using the same imager—(82 ± 2) % of instantaneous longwave forcing—because observable contrails have a larger climate impact. This detection efficiency could be partly improved by using a higher-resolution infrared imager, which would also allow contrails to be detected earlier in their lifecycle. However, even this instrument would still miss the large fraction of contrails that are too optically thin to be detected. These results support the use of contrail detection and lifetime observations from existing satellite imagers to draw conclusions about the relative radiative importance of different contrails under near-ideal conditions. However, there is a highlighted need to assess the observability of specific contrails depending on the observation requirements of a given application. These observability factors are shown to change in response to climate action, demonstrating a need to consider the properties of the observing system when assessing the impacts of proposed mitigation strategies.