Contrail models lacking post-fallstreak behavior could underpredict lifetime optical depth

Abstract. Proposed optimized 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, CoCiP, to a higher-fidelity contrail model, APCEMM, under parametrized meteorological conditions. The results show that the lifetime optical depth (a proxy for climate impact) in APCEMM is 3.8 times that in CoCiP, and that the models have opposite sensitivities of their lifetime optical depth 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 – a fallstreak. 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 modelling behavior past the initial fallstreak phase. This behavior is consistent with prior large eddy simulation studies, and it accounts for 92 % of the aggregate APCEMM lifetime optical depth. This suggests that fallstreak-only simulation may underestimate contrail climate impact. While a strategy avoiding all contrail formation is still expected to yield a reduction in climate impact, implementing optimized strategies requires more research to establish confidence in model predictions.