Impact of Parametrizing Microphysical Processes in the Jet and Vortex Phase on Contrail Cirrus Properties and Radiative Forcing

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 the vortex phase loss depends predominantly on the nucleated ice crystal numbers and the ambient relative humidity. 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 initial contrail ice crystal numbers per fuel mass are on average 50%-65% decreased relative to the soot number emission index in the extratropics and more in tropics. In the extratropics, this is mainly caused by a high ice crystal loss during the vortex phase and in the (sub)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 (RF) for the year 2006 is 44 (31-49) mWm(-2), around 22% lower than estimated in a previous study. When reducing soot number emissions by 80%, RF decreases by 41%, slightly less than suggested by a recent study. Plain Language Summary Contrail cirrus are known to be a major contribution to the aviation climate impact connected with a large uncertainty. Earlier research has shown that the ice crystal number in newly formed contrails has a large impact on the average contrail cirrus climate impact. But the properties of newly formed contrails are not well captured within the climate models. We have improved the representation of the contrail formation processes in our contrail cirrus module within the ECHAM climate model by including parameterizations for contrail ice nucleation and the ice crystal survival in the vortex phase. We could show that young contrail properties agree well with available campaign measurements over central Europe, given the large variability in soot number emissions, when matching geographical locations, cruise level, and atmospheric variables. The improvements within our contrail cirrus parameterization lead to a decrease in our estimate of contrail cirrus radiative forcing by slightly more than 20% relative to our earlier estimates in which we prescribed constant initial ice crystal numbers. Furthermore, our improved model indicates that the decrease in the contrail cirrus climate impact due to introducing biofuels, which lead to a decrease in soot number emissions, is slightly smaller than estimated earlier.

Automated summary

Contrail ice nucleation and ice crystal loss during the vortex phase have significant effects on the properties and life cycle of contrail cirrus. The number of emitted soot particles and atmospheric conditions control ice nucleation, while the nucleated ice crystal numbers and ambient relative humidity determine the vortex phase loss. Global climate model simulations show a decrease in initial contrail ice crystal numbers compared to soot number emissions due to high ice crystal loss during the vortex phase and decreased ice nucleation in different regions. The estimated global mean contrail cirrus radiative forcing is lower than previous studies, while reducing soot number emissions can lead to a decrease in radiative forcing.