The prevalence of precipitation from polar supercooled clouds
<p>Supercooled clouds substantially impact polar surface energy budgets, but large-scale models often underestimate their occurrence, which motivates accurately establishing metrics of basic processes. An analysis of long-term measurements at Utqiaġvik, Alaska, and McMurdo Station, Antarctica,...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/3949/2021/acp-21-3949-2021.pdf |
| Summary: | <p>Supercooled clouds substantially impact polar surface
energy budgets, but large-scale models often underestimate their occurrence,
which motivates accurately establishing metrics of basic processes. An
analysis of long-term measurements at Utqiaġvik, Alaska, and McMurdo
Station, Antarctica, combines lidar-validated use of soundings to identify
supercooled cloud layers and colocated ground-based profiling radar
measurements to quantify cloud base precipitation. We find that more than
85 % (75 %) of sampled supercooled layers are precipitating over the
Arctic (Antarctic) site, with more than 75 % (50 %) precipitating
continuously to the surface. Such high frequencies can be reconciled with
substantially lesser spaceborne estimates by considering differences in
radar hydrometeor detection sensitivity. While ice precipitation into
supercooled clouds from aloft is common, we also find that the great
majority of supercooled cloud layers without ice falling into them are
themselves continuously generating precipitation. Such sustained primary ice
formation is consistent with continuous activation of immersion-mode ice-nucleating particles (INPs), suggesting that supercooled cloud formation is
a principal gateway to ice formation at temperatures greater than
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mo>-</mo><mn mathvariant="normal">38</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="52fcdae65a2e7a1244d5b38bb82db9dd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-3949-2021-ie00001.svg" width="32pt" height="10pt" src="acp-21-3949-2021-ie00001.png"/></svg:svg></span></span> <span class="inline-formula"><sup>∘</sup></span>C over polar regions. The prevalence of
weak precipitation fluxes is also consistent with supercooled cloud
longevity and with well-observed and widely simulated case studies. An
analysis of colocated microwave radiometer retrievals suggests that weak
precipitation fluxes can be nonetheless consequential to moisture budgets
for supercooled clouds owing to small liquid water paths. The results here
also demonstrate that the observed abundance of mixed-phase clouds can vary
substantially with instrument sensitivity and methodology. Finally, we
suggest that these ground-based precipitation rate statistics offer valuable
guidance for improving the representation of polar cloud processes in
large-scale models.</p> |
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| ISSN: | 1680-7316 1680-7324 |