Abstract Contrail formation constitutes a large fraction of the aviation climate impact. But only part of the effect has been studied in detail, as research focused on contrail formation in cloud‐free air. Even though it is known that the impact of contrail formation within cirrus can be detected in space‐born remote sensing data, little research has dealt with the associated processes. Accordingly, the global climate impact of contrail cirrus is as yet unknown and the impact of contrail formation within cirrus is not yet included in aviation climate assessments. We use the high‐resolution ICON‐LEM, including a representation of contrail formation, to study the impact of contrail formation within cirrus clouds. We find that contrail formation leads to large peaks in ice crystal number concentration which eventually have an impact on the microphysical properties of large parts of the cirrus. As ice crystal sizes are initially very small, contrail formation reduces sedimentation and, therefore, affects the vertical distribution of cloud ice. Increases in the ice water path at the location of contrail formation lead to corresponding decreases in the larger vicinity which are most prominent at lower levels. Those decreases may be interpreted as fast cloud adjustment. Adjustments are non‐local in space and time. Their strength depends on the synoptic situation and varies spatially even within a cloud deck. Our work improves the process understanding of contrail formation within clouds and the associated cloud adjustments, important steps toward including this effect in aviation climate assessments and mitigation studies, such as contrail avoidance.
Verma et al. (2026) studied this question.