On the Lifetimes of Persistent Contrails and Contrail Cirrus

Kurzfassung

Prediction of contrail cirrus persistence is highly problematic for models, in part due to a poor representation of intertwined microphysical and dynamical processes controlling contrail evolution in large‐scale ice‐supersaturated areas. Knowledge of contrail cirrus lifetimes is required to estimate their lifecycle‐average radiative effect, but lifetime statistics inferred from observations are incomplete, hampering model validation. Contrail cirrus ice crystal size distributions are mainly impacted by entrainment and plume dilution, ice deposition growth and sublimation, and gravitational settling. Changes in the size distributions are due to synoptic and mesoscale air motions that affect sign and magnitude of ice supersaturation experienced by contrail particles. Driven by internal gravity waves, rapid growth/sublimation cycles and sedimentation lead to a selection of ice crystal sizes enabling long lifetimes for persistent contrails whose evolution is not limited by synoptic warming. Wave‐induced mesoscale supersaturation fluctuations lead to a wide spectrum of lifetimes with mean values of 24 hr and maxima up to 16 hr in large‐scale ice‐saturated conditions. Even longer maximum lifetimes as seen in some observations are possible within deep ice‐supersaturated layers, enhanced microscale turbulent temperature fluctuations, and new ice formation at the top of contrail cirrus. We conceptualize lifetime statistics and define initial adjustment, intermediate stabilization, and final dissipation regimes. Our analysis of contrail cirrus lifetimes and ice crystal size distributions will aid future satellite analyses and model development.