Global Modeling of the Contrail and Contrail Cirrus Climate Impact

conditions, their direct radiative effect is mainly determined by coverage and optical depth. The microphysical properties of contrail cirrus likely differ from those of most natural cirrus, at least during the initial stages of the contrail cirrus life cycle (Heymsfield et al. 2010). Contrails form and persist in air that is ice saturated, whereas natural cirrus usually requires high ice supersaturation to form (Jensen et al. 2001). This difference implies that in a substantial fraction of the upper troposphere contrail cirrus can persist in supersaturated air that is cloud free, thus increasing high cloud coverage. Contrails and contrail cirrus existing above, below, or within clouds change the column optical depth and radiative fluxes. They may also indirectly affect radiation by changing the moisture budget of the upper troposphere, and therefore the microphysical properties and the frequency of occurrence of natural cirrus. Global radiative forcing resulting from persistent linear contrails has been calculated using either reanalyses data in combination with radiative transfer codes or within a climate model. It is estimated to amount to 0.012 W m−2 for 2005 (42% of the radiative forcing of the accumulated aviation CO2 emissions), with an uncertainty range of 0.005–0.026 W m−2 and with a low level of scientific understanding (Lee Modeling the physical processes governing the life cycle of