Preprints
https://doi.org/10.5194/acp-2022-36
https://doi.org/10.5194/acp-2022-36
 
25 Jan 2022
25 Jan 2022
Status: this preprint is currently under review for the journal ACP.

Arctic mixed-phase clouds sometimes dissipate due to insufficient aerosol: evidence from observations and idealized simulations

Lucas J. Sterzinger1, Joseph Sedlar2,3, Heather Guy4,5, Ryan R. Neely III4,5, and Adele L. Igel1 Lucas J. Sterzinger et al.
  • 1University of California, Davis, Davis, California
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado
  • 3NOAA/Global Monitoring Laboratory, Boulder, Colorado
  • 4National Centre for Atmospheric Science, Leeds, U.K.
  • 5School of Earth and Environment, University of Leeds, Leeds, U.K.

Abstract. Mixed-phase clouds are ubiquitous in the Arctic. These clouds can persist for days and dissipate in a matter of hours. It is sometimes unknown what causes this sudden dissipation, but aerosol-cloud interactions may be involved. Arctic aerosol concentrations can be low enough to affect cloud formation and structure, and it has been hypothesized that, in some instances, concentrations can drop below some critical value needed to maintain a cloud.

We use observations from a Department of Energy ARM site on the north slope of Alaska at Oliktok Point (OLI), the ASCOS field campaign in the high Arctic Ocean, and the ICECAPS-ACE project at the NSF Summit Station in Greenland (SMT) to identify one case per site where Arctic boundary-layer clouds dissipated coincidentally with a decrease in surface aerosol concentrations. These cases are used to initialize idealized large eddy simulations (LES) in which aerosol concentrations are held constant until, at a specified time, all aerosols are removed instantaneously – effectively creating an extreme case of aerosol-limited dissipation which represents the fastest a cloud could possibly dissipate via this process. These LES simulations are compared against the observed data to determine whether cases could, potentially, be dissipating due to insufficient aerosol. The OLI case’s observed liquid water path (LWP) dissipated faster than its simulation, indicating that other processes are likely the primary driver of the dissipation. The ASCOS and SMT observed LWP dissipated at similar rates to their respective simulations, suggesting that aerosol-limited dissipation may be occurring in these instances.

We also find that the microphysical response to this extreme aerosol forcing depends greatly on the specific case being simulated. Cases with drizzling liquid layers are simulated to dissipate by accelerating precipitation when aerosol is removed while the case with a non-drizzling liquid layer dissipates quickly, possibly glaciating via the Wegener-Bergeron-Findeisen (WBF) process. The non-drizzling case is also more sensitive to INP concentrations than the drizzling cases. Overall, the simulations suggest that aerosol-limited cloud dissipation in the Arctic is plausible and that there are at least two microphysical pathways by which aerosol-limited dissipation can occur.

Lucas J. Sterzinger et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Lucas J. Sterzinger et al.

Data sets

Reproducible Plotting Code Sterzinger, Lucas https://doi.org/10.5281/zenodo.5851414

RAMS Horizontally-averaged model output Sterzinger, Lucas https://doi.org/10.5281/zenodo.5851364

Model code and software

RAMS Model code used for this study RAMS Development Team; Igel, Adele, and Sterzinger, Lucas https://github.com/lsterzinger/arctic-rams-6.1.22

Lucas J. Sterzinger et al.

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Short summary
Aerosol particles are required for cloud droplets to form. The Arctic atmosphere often has much fewer aerosol than at lower latitudes. In this study, we investigate whether aerosol concentrations can drop so low as to no longer support a cloud. We use observations to initialize idealized model simulations to investigate a "worse case scenario" where all aerosol are removed from the environment instantaneously. We find that this mechanism is possible in two cases and unlikely in the third.
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