Impact of parametrizing microphysical processes in the jet and vortex phase on contrail cirrus properties and radiative forcing

Abstract of the manuscript

Contrail ice nucleation and ice crystal loss during the vortex phase control ice crystal numbers in young contrails and can have a strong impact on the properties and the life cycle of contrail cirrus. For current soot number emissions, ice nucleation is controlled by the number of emitted soot particles and atmospheric conditions while ice crystal loss during the vortex phase depends predominantly on the nucleated ice crystal numbers and the relative humidity over ice. 

Initial ice crystal numbers after the vortex phase are close to the emitted soot particle number only for very low ambient temperatures (< 210 K) and for highly ice-supersaturated conditions. Higher temperatures and lower relative humidities lead to significantly decreased ice crystal numbers.

Global climate model simulations show that ice crystal numbers per fuel mass of young contrails are on average significantly lower than the soot number emission index. In the extratropics this is caused by a high ice crystal loss during the vortex phase and in the subtropics, tropics and at lower flight levels by decreased ice nucleation. Simulated ice crystal numbers per newly formed contrail length agree well with in-situ measurements over Central Europe within the variability of present-day soot number emissions. Our estimated global mean contrail cirrus radiative forcing (gRF) for the year 2006 is 44 mWm^-2, around 22% lower than estimated in a previous study. When reducing soot number emissions, the gRF decreases slightly weaker than proposed by a recent study due to the ice crystal loss during the vortex phase.