Arctic stratocumulus respond differently to aerosol perturbations over the open ocean and the ice sh

Figure 1 [Courtesy: Eirund et al (2019)]: Schematic illustrating the change in Arctic stratocumulus cloud properties over the open ocean (top row) as compared to a stratocumulus layer over an ice surface (bottom row).

This case study lead by Gesa Eirund (ETH Zurich) demonstrates that Arctic stratocumulus clouds over the open ocean surface respond quite differently to perturbations in aerosol concentrations. Both, the absolute change as well as the timescale of the change differ between simulated clouds over the open ocean and simulated clouds over the sea ice surface.

Arctic low-level clouds are frequently observed all year around. They occur over the ice as well as the ocean surface and warm the surface by trapping outgoing thermal radiation near the surface. Their cloud properties, radiative properties and sensitivities to external perturbations such as anthropogenic aerosol remain quite uncertain and are actively explored by researchers around the world.

The sensitivity of these clouds to perturbations in condensation nuclei (i.e. aerosol on which water can condense to form cloud droplets), and ice-nucleating particles (i.e. aerosol on which water freezes within surface irregularities) are explored within a numerical model constrained by cloud observations obtained during the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign in March 2013.

Figure 2 [Courtesy: Eirund et al (2019)]: Top row: The 20-hour evolution of the liquid water path (LWP), that is the vertically integrated liquid water content, and ice water path (IWP) for clouds over the open ocean is shown. The domain mean and standard deviation are depicted for different specified initial cloud condensation nuclei (CCN) concentrations. Bottom row: the same is shown for the cloud evolution over the ice sheet

Over the open ocean the liquid water path (LWP) increases rapidly within the simulations perturbed by cloud condensation nuclei (CCN). The increased flux of moisture into the clouds over the open ocean surface sustains the rapid activation and growth of the additional aerosol into more numerous cloud droplets, which initially increases the amount of liquid water retained inside the cloud. However, within 18 hours the initial increase in LWP is diminished as the amount of ice increases inside the perturbed clouds over a longer timescale and removes the liquid water.

Meanwhile, the cloud response in the liquid and ice water content over sea ice occurs over much longer timescales and is more subdued as compared to the response in perturbed open ocean clouds. It is plausible that the increased LWP would also relax back to its initial state over sea ice, but this process occurs over longer time scales than analysed here.

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