Publication Abstracts

Tornow et al. 2021

Tornow, F., A.S. Ackerman, and A.M. Fridlind, 2021: Preconditioning of overcast-to-broken cloud transitions by riming in marine cold air outbreaks. Atmos. Chem. Phys., 21, no. 15, 12049-12067, doi:10.5194/acp-21-12049-2021.

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. We use MERRA-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 > 25 g/m2), and only simulations that allow for aerosol depletion result in sustained breakups, as observed. Using a range of diagnostic ice nucleating particle concentrations, Ninp, 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) 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 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. 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 hotspots with in situ imaging probes that allow both for characterization of ice particles and abundance of supercooled droplets.

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BibTeX Citation

  author={Tornow, F. and Ackerman, A. S. and Fridlind, A. M.},
  title={Preconditioning of overcast-to-broken cloud transitions by riming in marine cold air outbreaks},
  journal={Atmos. Chem. Phys.},

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RIS Citation

ID  - to04200y
AU  - Tornow, F.
AU  - Ackerman, A. S.
AU  - Fridlind, A. M.
PY  - 2021
TI  - Preconditioning of overcast-to-broken cloud transitions by riming in marine cold air outbreaks
JA  - Atmos. Chem. Phys.
VL  - 21
IS  - 15
SP  - 12049
EP  - 12067
DO  - 10.5194/acp-21-12049-2021
ER  -

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