Implementing a process-based contrail parametrization in the Unified Model

     The global aviation fleet modifies cloudiness through contrail formation and their subsequent competition with natural cirrus for ambient water vapor, along with enhanced ice-nuclei concentrations from aircraft soot emissions. Contrails form in the upper troposphere at temperatures below 233 K and pressures below 300 hPa, when plume gases from jet engines, having appreciable water vapor content, saturate with respect to liquid water (Schmidt-Appleman Criterion, SAC). Realistic assessments of the aviation-induced modifications to global cloud cover demand improved representation of contrails and their interaction with background cloudiness in climate models. We have implemented a process-based parametrization of contrail cirrus, that applies to both young (≤ 5 h) and aged contrails, in the UK Met Office Unified Model, version 12.0. Contrail cirrus is introduced as a new prognostic cloud class, forming in the parametrized, fractional ice supersaturated area which then undergoes advection, depositional growth, sublimation and sedimentation. The proxy for the fractional supersaturated area is calculated using the same total water PDF as used for natural cirrus but with a different critical relative humidity, r_cc - a value at which part of the model grid box is at least ice-saturated. The persistence of contrails being allowed in the ice supersaturated areas, the simulated coverage is not confined to flight corridors, but is advected to air traffic free zones as well. The simulated annual mean global coverage due to young contrails is 0.13%, with the main traffic areas of Europe and North America having the maximum coverage. Similar to natural cirrus, the contrail ice particles reflect the solar short-wave (SW) radiation and trap outgoing long-wave (LW) radiation, thereby modifying the radiative balance of the Earth’s atmosphere. Contrail cirrus is radiatively active in the model with forcing studies enabled via a ‘double radiation call’ approach, wherein parallel runs of the radiation scheme ‘with’ (prognostic) and ‘without’ (diagnostic) the contrail radiative effects isolates the contrail-induced perturbations. Contrails are seen to induce a short-wave cooling and long-wave warming and the net (SW+LW) direct top-of-atmosphere radiative forcing by young contrails amounts globally to 0.5 mWm^-2, with the peak forcing seen along the main air traffic areas of North America, Europe and East Asia. The implementation of this process-based parametrization in the UM enables the simulation of the life cycle of persistent contrails, and can provide valuable insights to the aviation-induced modifications to the global cloud cover.