Understanding sources of contrail cirrus radiative forcing uncertainty using a new diagnostic contrail scheme for the UK Earth System Model

Condensation trails (contrails) are aircraft-induced line-shaped high clouds, which may persist and grow to form contrail cirrus in ice supersaturated regions. Contrail cirrus is the largest known component of aviation radiative forcing, with a large uncertainty associated with troposphere water budgets and contrail radiative properties. In addition, due to the limited number of climate models able to simulate contrail cirrus, the uncertainty in the global contrail cirrus radiative forcing cannot be estimated statistically.

The aim of this work is to implement a contrail cirrus parameterisation in the UK Earth System Model, therefore providing a new independent estimate of the contrail cirrus radiative forcing to be used in future assessments of aviation climate impacts. The new diagnostic scheme is based on the processes governing contrail formation (Schmidt-Appleman Criteria) and persistence (ice supersaturation). Persistent contrails are then added to the model cloud fields, where they interact with and evolve alongside natural clouds.

We use ensemble runs of both nudged and free running model experiments to estimate the contrail cirrus cover and effective radiative forcing. Comparisons with a similar contrail scheme implemented in the NCAR Community Atmospheric Model (CAM) indicate the importance of the host climate model via (i) the host’s cloud microphysics scheme (e.g. single vs. double moment) and (ii) its ability to simulate realistic ice supersaturated regions. By providing a new independent assessment of the contrail cirrus radiative forcing, our work contributes to improving our understanding of aviation climate impacts and therefore potential mitigation strategies for current and future generation aircraft.