Upper-tropospheric slightly ice-subsaturated regions: frequency of occurrence and statistical evidence for the appearance of contrail cirrus

Microphysical, optical, and environmental properties of contrail cirrus and natural cirrus were investigated by applying a new, statistically based contrail-cirrus separation method to 14.7 h of cirrus cloud measurements (sampling frequency 1 Hz, max. similar to 290 m s(-1), total length of sampled in-cloud space similar to 15 000 km) during the airborne campaign ML-CIRRUS in central Europe and the northeast Atlantic flight corridor in spring 2014. We find that pure contrail cirrus appears frequently at the aircraft cruising altitude (CA) range with ambient pressure varying from 200 to 245 hPa. It exhibits a higher median ice particle number concentration (N-ice), a smaller median mass mean radius (R-ice), and lower median ice water content (IWC) (median: Nice=0.045 cm(-3), R-ice=16.6 mu m, IWC = 3.5 ppmv), and it is optically thinner (median extinction coefficient Ext = similar to 0.056 km(-1)) than the cirrus mixture of contrail cirrus, natural in situ-origin and liquid-origin cirrus found around the CA range (median: N-ice=0.038 cm(-3), R-ice=24.1 mu m, IWC = 8.3 ppmv, Ext =similar to 0.096 km(-1)). The lowest and thickest cirrus, consisting of a few large ice particles, are identified as pure natural liquid-origin cirrus (median: N-ice=0.018 cm(-3), R-ice=42.4 mu m, IWC = 21.7 ppmv, Ext = similar to 0.137 km(-1)). Furthermore, we observe that, in particular, contrail cirrus occurs more often in slightly ice-subsaturated instead of merely ice-saturated to supersaturated air as often assumed, thus indicating the possibility of enlarged contrail cirrus existence regions. The enlargement is estimated, based on IAGOS long-term observations of relative humidity with respect to ice (RHice) aboard passenger aircraft, to be approximately 10 % for Europe and the North Atlantic region, with the RHice threshold for contrail cirrus existence decreased from 100 % to 90 % RHice and a 4 h lifetime of contrail cirrus in slight ice subsaturation assumed. This increase may not only lead to a non-negligible change in contrail cirrus coverage and radiative forcing, but also affect the mitigation strategies of reducing contrails by rerouting flights.