Zero-dimensional contrail models could underpredict lifetime optical depth

Abstract

Proposed contrail avoidance schemes rely on being able to robustly predict which contrails cause the most climate warming. However, it has not yet been shown that different contrail models agree sufficiently to support the targeting of individual contrails by climate impact. To address this, we compare the most widespread contrail model, the Contrail Cirrus Prediction (CoCiP), to a higher-fidelity contrail model, the Aircraft Plume Chemistry, Emissions, and Microphysics Model (APCEMM), under parameterized meteorological conditions. The results show that the time-integrated total extinction (a proxy for climate impact) in APCEMM is 3.8 times that in CoCiP and that the models have opposite sensitivities of their time-integrated total extinction to relative humidity. We argue that these differences are due to the differing representations of the distribution of ice particles in space and in size across the contrails. The use of a monodisperse ice particle size distribution in a Gaussian plume means that CoCiP models the contrail exclusively as an accelerating, falling mass. The use of a spatially gridded and size-resolved aerosol scheme allows APCEMM to represent the separation of the precipitation plume from the contrail core, hence modeling behavior beyond the initial phase in which the contrail grows unconstrained. This behavior is consistent with prior large-eddy simulation studies, and it accounts for 92 % of the aggregate APCEMM time-integrated total extinction across all simulations. This suggests that models lacking a size-resolved aerosol scheme may underestimate the time-integrated total extinction. While a strategy avoiding a given proportion of persistent contrails in an unbiased way is still expected to yield a proportional reduction in the time-integrated total extinction, implementing strategies using contrail models to select the specific contrails to avoid may lead to fewer reductions in the time-integrated total extinction, primarily due to the current level of disagreement between models. We therefore recommend more research to establish confidence in model predictions at later contrail ages.