Preprints
https://doi.org/10.5194/acp-2021-82
https://doi.org/10.5194/acp-2021-82

  08 Feb 2021

08 Feb 2021

Review status: this preprint is currently under review for the journal ACP.

Preconditioning of overcast-to-broken cloud transitions by riming in marine cold air outbreaks

Florian Tornow1,2, Andrew S. Ackerman2, and Ann M. Fridlind2 Florian Tornow et al.
  • 1Center for Climate Systems Research, Earth Institute, Columbia University, New York, USA
  • 2NASA Goddard Institute for Space Sciences, NY, USA

Abstract. 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 m−2) 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-aerosol concentration final stages are found to be inevitable in this case owing 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.

Florian Tornow et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-82', Anonymous Referee #1, 07 Mar 2021
  • RC2: 'Comment on acp-2021-82', Anonymous Referee #2, 08 Apr 2021

Florian Tornow et al.

Florian Tornow et al.

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Short summary
Cold air outbreaks affect the local energy budget by forming bright boundary layer clouds that, once raining, evolve into dimmer, broken cloud fields that are depleted of condensation nuclei – an evolution consistent with closed-to-open cell transitions. We find that cloud ice accelerates this evolution, primarily via riming prior to rain onset, which (1) reduces liquid water, (2) reduces condensation nuclei, and (3) leads to early precipitation cooling and moistening below cloud.
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