Upper-tropospheric slightly ice-subsaturated regions: frequency of occurrence and statistical evidence for the appearance of contrail cirrus
<p>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. <span class="inline-formula&quo...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/23/2251/2023/acp-23-2251-2023.pdf |
| Summary: | <p>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. <span class="inline-formula">∼</span> 290 m s<span class="inline-formula"><sup>−1</sup></span>,
total length of sampled in-cloud space <span class="inline-formula">∼</span> 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 (<span class="inline-formula"><i>N</i><sub>ice</sub></span>), a smaller median mass mean radius
(<span class="inline-formula"><i>R</i><sub>ice</sub></span>), and lower median ice water content (IWC) (median: <span class="inline-formula"><i>N</i><sub>ice</sub>=0.045</span> cm<span class="inline-formula"><sup>−3</sup></span>, <span class="inline-formula"><i>R</i><sub>ice</sub>=16.6</span> <span class="inline-formula">µ</span>m, IWC <span class="inline-formula">=</span> 3.5 ppmv), and it is optically thinner (median extinction coefficient Ext <span class="inline-formula">=</span> <span class="inline-formula">∼</span> 0.056 km<span class="inline-formula"><sup>−1</sup></span>) than the cirrus mixture of contrail cirrus, natural in
situ-origin and liquid-origin cirrus found around the CA range (median:
<span class="inline-formula"><i>N</i><sub>ice</sub>=0.038</span> cm<span class="inline-formula"><sup>−3</sup></span>, <span class="inline-formula"><i>R</i><sub>ice</sub>=24.1</span> <span class="inline-formula">µ</span>m, IWC <span class="inline-formula">=</span> 8.3 ppmv, Ext <span class="inline-formula">=</span> <span class="inline-formula">∼</span> 0.096 km<span class="inline-formula"><sup>−1</sup></span>). The lowest and thickest
cirrus, consisting of a few large ice particles, are identified as pure
natural liquid-origin cirrus (median: <span class="inline-formula"><i>N</i><sub>ice</sub>=0.018</span> cm<span class="inline-formula"><sup>−3</sup></span>,
<span class="inline-formula"><i>R</i><sub>ice</sub>=42.4</span> <span class="inline-formula">µ</span>m, IWC <span class="inline-formula">=</span> 21.7 ppmv, Ext <span class="inline-formula">=</span> <span class="inline-formula">∼</span> 0.137 km<span class="inline-formula"><sup>−1</sup></span>). 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 (RH<span class="inline-formula"><sub>ice</sub></span>) aboard passenger aircraft, to be
approximately 10 % for Europe and the North Atlantic region, with the
RH<span class="inline-formula"><sub>ice</sub></span> threshold for contrail cirrus existence decreased from 100 % to
90 % RH<span class="inline-formula"><sub>ice</sub></span> 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |