Abstract
Abstract. Persistent contrails and natural cirrus clouds often coexist in the upper troposphere and contrails can be embedded within cirrus clouds. The present paper deals with some questions regarding the interaction of cirrus clouds and embedded contrails. I have selected only questions that can be answered by analytical means. I find that (1) the emission index for water vapour is only slightly changed when an aircraft crosses a cirrus cloud, (2) that contrail formation is not affected by an ambient cirrus, (3) that cirrus ice crystals entrained into the trailing wing tip vortex do not efficiently retard the sublimation of contrail ice crystals, and (4) that cirrus can start to dissolve an embedded contrail after a couple of hours by aggregation.
Highlights
Contrails and cirrus clouds do not have the same formation conditions, they often occur together in the same regions because the condition for cirrus formation, ice supersaturation, is required for contrail persistence
– How much do ice crystals sucked through an engine change the water vapour emission index?
2 How much do ice crystals sucked through an engine change the water vapour emission index?
Summary
Contrails and cirrus clouds do not have the same formation conditions, they often occur together in the same regions because the condition for cirrus formation, ice supersaturation, is required for contrail persistence. The ice sublimates and the resulting water vapour at engine exit adds to the water vapour resulting from burning kerosene; an enhanced emission index for water vapour results which could affect contrail formation An aircraft engine typically has an air fuel ratio mair/mfuel ≈ 70, and the emission index of water vapour for kerosene is EIH2O = 1.25. The NASA–AAFEX campaign (Anderson et al, 2011, table 2) gives a fuel H/C (hydrogen to carbon ratio) variation of more than 10% which might directly translate into a corresponding variation of EIH2O Compared to these variations, the contribution of evaporating ice from even the thickest cirrus clouds is negligible
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