Preconditioning of overcast-to-broken cloud transitions by riming in marine cold air outbreaks
<p>Marine cold air outbreaks (CAOs) commonly form overcast cloud decks that transition into broken cloud fields downwind, dramatically altering the local radiation budget. In this study, we investigate the impact of frozen hydrometeors on these transitions. We focus on a CAO case in the NW Atl...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/12049/2021/acp-21-12049-2021.pdf |
| Summary: | <p>Marine cold air outbreaks (CAOs) commonly form overcast cloud decks
that transition into broken cloud fields downwind, dramatically
altering the local radiation budget. In this study, we investigate
the impact of frozen hydrometeors on these transitions. We
focus on a CAO case in the NW Atlantic, the location of the multi-year
flight campaign ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment). We use MERRA-2 (Modern-Era Retrospective analysis for Research and Applications, version 2) reanalysis fields to drive large eddy simulations with mixed-phase two-moment microphysics in a
Lagrangian framework. We find that transitions are triggered by
substantial rain (rainwater paths <span class="inline-formula">>25</span> <span class="inline-formula">g m<sup>−2</sup></span>), and only
simulations that allow for aerosol depletion result in sustained
breakups, as observed. Using a range of diagnostic ice nucleating particle
concentrations, <span class="inline-formula"><i>N</i><sub>inp</sub></span>, we find that increasing ice progressively
accelerates transitions, thus abbreviating the overcast state. Ice
particles affect the cloud-topped boundary layer evolution, primarily
through riming-related processes prior to substantial rain, leading to
(1) a reduction in cloud liquid water, (2) early consumption of
cloud condensation nuclei, and (3) early and light precipitation
cooling and moistening below cloud. We refer to these three effects
collectively as “preconditioning by riming”. Greater boundary layer aerosol concentrations available as cloud condensation nuclei (CCN) delay the onset of substantial rain. However, cloud breakup and low CCN concentration final
stages are found to be inevitable in this case, due, primarily, to liquid water path buildup. An ice-modulated cloud transition speed suggests the possibility of a negative cloud–climate feedback. To address prevailing uncertainties in the model representation of mixed-phase processes, the magnitude of ice formation and riming impacts and, thereby, the strength of an associated negative cloud–climate feedback process, requires further
observational evaluation by targeting riming hot spots with in situ imaging probes that allow for both the characterization of ice particles and abundance of supercooled droplets.</p> |
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| ISSN: | 1680-7316 1680-7324 |