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

Abstract

Abstract. 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. ∼ 290 m s−1, total length of sampled in-cloud space ∼ 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 (Nice), a smaller median mass mean radius (Rice), and lower median ice water content (IWC) (median: Nice=0.045 cm−3, Rice=16.6 µm, IWC = 3.5 ppmv), and it is optically thinner (median extinction coefficient Ext = ∼ 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: Nice=0.038 cm−3, Rice=24.1 µm, IWC = 8.3 ppmv, Ext = ∼ 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: Nice=0.018 cm−3, Rice=42.4 µm, IWC = 21.7 ppmv, Ext = ∼ 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.

Contrail radiative dependence on ice particle number concentration

“…Based on the differences in N ice and IWC between natural cirrus and contrails reported in the literature, our results show that a 60% IWC reduction, similar to the differences found by Li et al [21], produces a larger net RF reduction (∼55%), compared to that (∼35%) from the reported larger N ice values also reported for contrails. It is nevertheless concluded that the accuracy and representation of these variables combined needs to be improved in order to better estimate contrail RF and its dependence on the introduction of SAF.…”

“…IWC differences between contrails and natural cirrus have been also measured. Li et al [21] reported that countrail cirrus present smaller IWCs by around 60%, while Chauvigné et al [19] reported smaller contrail and contrail cirrus IWC values in all stages of contrails compared to natural cirrus, and larger N ice for young contrails suggesting the MH97 scheme better represents linear contrail N ice values. Based on these results, we reduced our previous estimate from 9 mW m −2 , reported in RL18 (represented here as case 3 for MH97), to 4 mW m −2 , based on case 1 of the MH97 scheme, which assumes an IWC reduction of 60% as reported by Li et al [21].…”

“…Li et al [21] reported that countrail cirrus present smaller IWCs by around 60%, while Chauvigné et al [19] reported smaller contrail and contrail cirrus IWC values in all stages of contrails compared to natural cirrus, and larger N ice for young contrails suggesting the MH97 scheme better represents linear contrail N ice values. Based on these results, we reduced our previous estimate from 9 mW m −2 , reported in RL18 (represented here as case 3 for MH97), to 4 mW m −2 , based on case 1 of the MH97 scheme, which assumes an IWC reduction of 60% as reported by Li et al [21]. This significant reduction evidences the importance that contrail measurement campaigns have on reducing global RF uncertainties by improving combined IWC and N ice climatological data compared to surrounding natural cirrus.…”

“…Given that the ice water path would also include an uncertainty linked to the physical thickness of the cloud layer, it seems reasonable to assume that the ±60% IWC uncertainty implemented here is similar to that between GCMs. Also, contrail cirrus are reported to have a 60% smaller IWC compared to cirrus under the same atmospheric conditions [21].…”

“…Despite the currently limited information about contrail N ice and IWC properties and their temporal evolution, young contrails have been shown to have higher N ice values than natural cirrus by an order of magnitude [19,20], with enhanced N ice values being found even after several hours compared to surrounding cirrus under similar atmospheric conditions [18]. A more recent study [21] found more modest N ice differences of around 50% between natural and contrail cirrus. Based on these findings here we also here estimate the calculated contrail net RF sensitivity to N ice and IWC differences.…”