Contrail cirrus climate modelling: uncertainty, microphysical processes and mitigation pathways

Contrail cirrus is the largest contributor to aviation’s effective radiative forcing (ERF), making it a key mitigation target. However, current ERF estimates have a large ∼ 70% uncertainty due to limited understanding of the physical processes governing the contrail lifecycle, meaning that the potential of alternative fuels in contrail cirrus mitigation remains unclear.

This thesis addresses these challenges by: (i) developing contrail cirrus modelling capabilities in the UK Met Office Unified Model (UM); (ii) assessing the contrail cirrus ERF uncertainty due to host-model differences and initial contrail properties; (iii) quantifying the contrail cirrus ERF under alternative-fuel scenarios.

Model comparisons reveal that ice-supersaturation and microphysical processes drive discrepancies in contrail lifecycle and natural cloud responses. The UM single-moment scheme substantially underestimates contrail cirrus optical depths. Accounting for this yields a global-mean contrail cirrus ERF of 40.8 mW m−2 for 2018, ∼ 50% lower than the Community Atmosphere Model (CAM) estimate of 60.1 mW m−2, reflecting microphysics and radiation scheme differences.

Implementing the contrail parameterisation in the double-moment cloud scheme improves UM contrail cirrus modelling. Regional simulations indicate a contrail cirrus ERF of 0.93 W m−2 over Europe, consistent with existing estimates but highly sensitive to initial contrail properties. The seasonal cycle varies markedly, driven primarily by low-level cloud variability.

CAM simulations under future air-traffic scenarios indicate that contrail cirrus ERF might double from 2015 to 2050 under continued kerosene-only use. Switching to 100% sustainable aviation fuel (SAF) reduces ERF by ∼ 65%, while combining with liquid hydrogen (LH2) achieves ∼ 82% reductions, stemming from lower soot emissions producing fewer, larger ice crystals with shorter lifetimes and smaller optical depths.

In summary, reliable contrail cirrus ERF estimates require realistic ice-supersaturation fields, detailed representations of contrail microphysics, and well-constrained initial contrail properties. Scaling up SAF and LH2 use offers a promising pathway for mitigating contrail cirrus ERF.