Sensitivity of Arctic Boundary Layer Mixed-Phase Clouds to Surface Forcing and Aerosol Perturbations

Arctic boundary layer clouds play a key role in the radiative balance of the Arctic region. In summer, the reflection of the incoming radiation dominates, while during the rest of the year the absorption and emission of longwave radiation prevails, causing a warming effect at the surface. The radiative properties of these clouds are strongly linked to the relative abundance of both phases (liquid and ice), which in turn is governed by a multitude of processes operating in conjunction across a wide range of spatial scales. The large-scale dynamical forcing, surface processes as well as the ambient aerosol concentration all impact mixed-phase cloud (MPC) amount and phase partitioning. Up to now, the persistence as well as the formation of Arctic MPCs remain largely unclear and their representation in models of all complexities poses a considerable challenge.

In this study we focus on the relative importance of different background aerosol concentrations on MPC formation and persistence in the Arctic. Motivated by current Arctic sea ice retreat, we additionally contrast the simulated MPC over open ocean versus sea ice surfaces. To address this, we perform high resolution COSMO-LES simulations on 20x20 km² domains based on the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign carried out in March 2013 in the European Arctic. In our setup we successfully simulate a realistic MPC as compared to observations.

We find that changes in surface conditions impact cloud dynamics considerably: MPCs over sea ice are rather homogeneous. In contrast, MPCs over the ocean display organized structures of shallow convection with high spatial variability and increased precipitation formation in the updraft cores.

To additionally investigate the effect of aerosol loadings on Arctic MPC properties, we performed several sensitivity studies, resembling pollution advected from the mid latitudes (known as Arctic haze in spring). Our simulations suggest that aerosols significantly affect the liquid and ice phase of MPCs. Smaller aerosol loadings are needed over sea ice than over the ocean to shift the cloud properties beyond the background state. To further address the robustness of our results, we apply different perturbations across a +-2K temperature range.