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.

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 hours of cirrus cloud measurements 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. They exhibit 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 they are 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-hour 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.

<p class="western" align="justify">Water vapor is an essential component for regulating the Earth's radiation budget. To realistically determine the global radiation budget, an accurate description of the water vapor distribution in the upper troposphere and lower stratosphere (UTLS) is therefore indispensable. For example, small changes in water vapor concentration can lead to significant changes in local radiative forcing, especially in the dry lower stratosphere. The change in this region can be even stronger if condensed water in the form of ice clouds is present instead of solely water vapor.</p> <p class="western" align="justify">The formation and evolution of ice clouds is crucially determined by the saturation ratio over ice (Si). Ice crystals can only form (and grow) at supersaturated conditions (i.e. Si>1), i.e. in so-called ice supersaturated regions (ISSRs), which also constitute potential regions for the formation and existence of persistent aircraft contrails. Knowing and precisely forecasting the occurrence of ISSRs can help reducing the contribution of aviation to man-made climate change, as contrails usually have a warming effect on the climate.</p> <p class="western" align="justify">Ice supersaturation is often observed in the UTLS. However, despite their importance, the large-scale three-dimensional structure of ISSRs is widely unknown. Therefore, we present a three-dimensional climatology of ice supersaturation in the UTLS over the North Atlantic for the years 2010 to 2019. This climatology is based on the recent ERA5 reanalysis data set of the European Center for Medium Weather Forecast (ECMWF), which explicitly allows ice supersaturation in cloud-free conditions. To quantify the quality of the ERA5 data set with respect to ice supersaturation, we use the long-term in-situ measurements of the European Research Infrastructure ’In-service Aircraft for a Global Observing System’ (IAGOS; www.iagos.org) (Petzold et al., 2015).</p>

<p>The decade, 2012 to 2021, was the warmest on record, with the global mean near-surface air temperature in the most recent seven years, 2015 to 2021, keeping hitting record high. Europe, with an increase of 1.94 to 2.01 °C in the mean annual temperature since pre-industry level, is warming much faster than the global average (1.11 to 1.14 °C) (https://www.eea.europa.eu/ims/global-and-european-temperatures). Surface warming disrupts upper-air temperature, which will affect the humidity fields in the upper troposphere. Both ambient temperature and relative humidity with respect to ice (RH<sub>ice</sub>) are key factors determining the formation and persistence of contrail cirrus clouds, which exert a net warming radiative forcing among aviation emissions (Lee et al., 2009; Lee et al., 2021).</p> <p>Previous studies have provided insights into the long-term trend and seasonal variability of upper-air temperature and relative humidity using airborne, radiosonde and reanalysis datasets (Petzold et al., 2020; Chen et al., 2015; Philandras et al., 2017; Essa et al., 2022). The variation of RH<sub>ice </sub>in relation to the changing upper tropospheric temperature because of surface warming has, however, rarely been investigated.</p> <p>In this work, we use the temperature and RH<sub>ice</sub> measurements over Western Europe from the European research infrastructure IAGOS (In-Service Aircraft for a Global Observing System; www.iagos.org) to study how the upper-air temperature and RH<sub>ice</sub> distributions in the warmest seven years have changed seasonally and regionally compared to the IAGOS-MOZAIC period, 1995 to 2010, when the surface warming was not so drastic. We will focus on whether the occurrence frequency of contrail forming regions, <em>i.e.</em>, slightly ice subsaturated to supersaturated regions in the upper troposphere, would be affected by the increasing warming climate, which could promote our understanding of contrail mitigation.</p> <p>[Note: This work is carried out under the frame of EU H2020 Research and Innovation Action “Advancing the Science for Aviation and Climate (ACACIA)”, Grant Agreement No. 875036.]</p>

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