Factors limiting contrail detection in satellite imagery

Abstract

Contrails (ice clouds, originally line-shaped after initiation by aircraft exhaust) provide 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 life cycle, which in turn need suitable observations to constrain them. Infrared imagers on geostationary satellites provide widespread contrail observations, with sufficient time resolution to observe the evolution of their 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 linear contrails in an otherwise clear sky against a homogeneous ocean background. Only (46±2) % of a modelled population of global contrail segments is found to be observable using current 2 km resolution satellite-borne imagers, even in this maximally observable case. By estimating the radiative forcing of individually modelled contrails, it is found that a significantly higher portion of contrail forcing is detectable using the same 2 km resolution imager - (72±2) % of instantaneous long-wave (LW) forcing - because more easily 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 life cycle. 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 contrails where the observation conditions may vary by 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.