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The interaction of contrail-cirrus and natural cirrus formed by homogeneous nucleation is studied over up to ten hours by means of a LES model equipped with a Lagrangian ice microphysics module. A pre-existing contrail evolves in an ascending air mass and becomes surrounded by natural cirrus. Such scenarios are compared to scenarios, where the ascent stops, before natural cirrus formation sets in. It is found that in high updraught cases contrail spreading is inhibited by surrounding cirrus and contrail total extinction is strongly reduced. In a slow updraught the cirrus forms later around a mature contrail and the contrail's further evolution is not considerably perturbed by the cirrus. Analysing the simulated extinction coefficient of such a dual-origin ice cloud suggests that contrails becoming embedded in cirrus do not generally remain identifiable as such in observations. It is further demonstrated that cirrus ice crystals exist in large parts of the contrail. If the contrail is located in the middle of a moist layer and the surrounding cirrus, it can happen that the complete contrail is “contaminated” with cirrus ice crystals within several hours. Finally, contrail properties are computed with and without considering the co-existing cirrus ice crystals which aids the interpretation of potentially “contaminated” contrail observations.
Unterstraßer et al. (Fri,) studied this question.