Contrasting ice formation in Arctic clouds: surface-coupled vs. surface-decoupled clouds
<p>In the Arctic summer of 2017 (1 June to 16 July) measurements with the OCEANET-Atmosphere facility were performed during the <i>Polarstern</i> cruise PS106. OCEANET comprises amongst other instruments the multiwavelength polarization lidar Polly<span class="inline-formul...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/10357/2021/acp-21-10357-2021.pdf |
| Summary: | <p>In the Arctic summer of 2017 (1 June to 16 July) measurements with the
OCEANET-Atmosphere facility were performed during the <i>Polarstern</i> cruise
PS106. OCEANET comprises amongst other instruments the multiwavelength
polarization lidar Polly<span class="inline-formula"><sup>XT</sup></span>_OCEANET and for PS106 was complemented
with a vertically pointed 35 <span class="inline-formula">GHz</span> cloud radar. In the scope of the
presented study, the influence of cloud height and surface coupling on the
probability of clouds to contain and form ice is investigated. Polarimetric
lidar data were used for the detection of the cloud base and the identification
of the thermodynamic phase. Both radar and lidar were used to detect cloud
top. Radiosonde data were used to derive the thermodynamic structure of the
atmosphere and the clouds. The analyzed data set shows a significant impact of
the surface-coupling state on the probability of ice
formation. Surface-coupled clouds were identified by a quasi-constant
potential temperature profile from the surface up to liquid layer base. Within
the same minimum cloud temperature range, ice-containing clouds have been
observed more frequently than surface-decoupled clouds by a factor of up to 6
(temperature intervals between <span class="inline-formula">−</span>7.5 and <span class="inline-formula">−</span>5 <span class="inline-formula"><sup>∘</sup>C</span>, 164 vs. 27
analyzed intervals of 30 <span class="inline-formula">min</span>). The frequency of occurrence of
surface-coupled ice-containing clouds was found to be 2–3 times higher
(e.g., 82 <span class="inline-formula">%</span> vs. 35 <span class="inline-formula">%</span> between <span class="inline-formula">−</span>7.5 and
<span class="inline-formula">−</span>5 <span class="inline-formula"><sup>∘</sup>C</span>). These findings provide evidence that above
<span class="inline-formula">−</span>10 <span class="inline-formula"><sup>∘</sup>C</span> heterogeneous ice formation in Arctic mixed-phase
clouds occurs by a factor of 2–6 more often when the cloud layer is coupled
to the surface. In turn, for minimum cloud temperatures below
<span class="inline-formula">−</span>15 <span class="inline-formula"><sup>∘</sup>C</span>, the frequency of ice-containing clouds for coupled
and decoupled conditions approached the respective curve for the
central European site of Leipzig, Germany (51<span class="inline-formula"><sup>∘</sup></span> N,
12<span class="inline-formula"><sup>∘</sup></span> E). This corroborates the hypothesis that the free-tropospheric ice
nucleating particle (INP) reservoir over the Arctic is controlled by
continental aerosol. Two sensitivity studies, also using the cloud radar for
detection of ice particles and applying a modified coupling state detection,
both confirmed the findings, albeit with a lower magnitude. Possible
explanations for the observations are discussed by considering recent in situ
measurements of INP in the Arctic, of which much higher concentrations were
found in the surface-coupled atmosphere in close vicinity to the ice shore.</p> |
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| ISSN: | 1680-7316 1680-7324 |