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Discussion papers
https://doi.org/10.5194/acp-2018-1076
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1076
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 01 Nov 2018

Research article | 01 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Relaxation times of Arctic mixed-phase clouds to short-term aerosol perturbations under different surface forcings

Gesa K. Eirund1, Anna Possner2, and Ulrike Lohmann1 Gesa K. Eirund et al.
  • 1Institute for Atmospheric and Climate Sciences, ETH Zurich, Zurich, Switzerland
  • 2Department of Global Ecology, Carnegie Institution for Science, Stanford, California, USA

Abstract. The formation and persistence of low lying mixed-phase clouds (MPCs) in the Arctic depends on a multitude of processes, such as surface conditions, the environmental state, air mass advection and the ambient aerosol concentration. In this study, we focus on the relative importance of different aerosol perturbations (cloud condensation nuclei and ice nucleating particles; CCN and INP, respectively) on MPC properties in the central Arctic. To address this topic, we performed high resolution large eddy simulations (LES) using the COSMO model and designed a case study for the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign in March 2013. Motivated by ongoing sea ice retreat, we additionally contrast the simulated MPC that formed over an open ocean surface and a sea ice surface. We find that surface conditions highly impact cloud dynamics: over sea ice, a rather homogeneous, optically thin, mixed-phase stratus cloud forms. In contrast, the MPC over the open ocean has a stratocumulus-like cloud structure. With cumuli feeding moisture into the stratus layer, the cloud features a higher liquid (LWC) and ice water content (IWC) and has a lifted cloud base compared to the cloud over sea ice.

Furthermore, we analyzed the aerosol impact on these two dynamically different regimes. Perturbations in the INP concentration increase the IWC and decrease the LWC consistently in both regimes. The cloud microphysical response to potential CCN perturbations occurs faster in the stratocumulus regime over the ocean, where the increased moisture flux favors rapid cloud droplet formation and growth, leading to an increase in LWC following the aerosol injection. In addition, the IWC increases through increased immersion freezing and subsequent growth by deposition. Over sea ice, the maximum response is delayed by a factor of 2.5 compared to open ocean surface. However, independent of the cloud regime and aerosol perturbation, the cloud regains its original state after at most 12h for an aerosol perturbation of 1000cm−3. Cloud microphysical and macrophysical peoperties relax to their unperturbed range, and any aerosol perturbation is efficiently buffered. A substantial fraction of the aerosol is transported out of the boundary layer into the capping inversion, where the supersaturation is insufficient for aerosol activation. Our results are robust across different temperature ranges and insensitive to the aerosol injection period. Based on these results we postulate an efficient aerosol processing and transport mechanism that appears to inhibit any long-term aerosol impact on Arctic MPC properties.

Gesa K. Eirund et al.
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Low-level mixed-phase cloud (MPC) properties can be highly affected by the ambient aerosol concentration, especially in pristine environments as the Arctic. However, by employing high resolution model simulations we find that the effect of short-term aerosol perturbations on Arctic MPCs is efficiently attenuated with time. A substantial fraction of the aerosol is transported out of the boundary layer, which appears to inhibit any long-term impact of time-limited pollution events on Arctic MPCs.
Low-level mixed-phase cloud (MPC) properties can be highly affected by the ambient aerosol...
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