17.06.2019: FB1-Seminar

Gesa Eirund, Institute for Atmospheric and Climate Science, Department of Environmental Systems Science, ETH Zürich, Zürich (Switzerland): "Into the cold: processes constraining Arctic low clouds"


11:00 Uhr, Hörsaal, Düsternbrooker Weg 20


Arctic boundary layer mixed-phase clouds (MPCs), comprising both, ice and supercooled liquid water, occur frequently during all seasons at high latitudes. They 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. Even though MPCs are microphysically unstable, they tend to persist for many days at a time. Up to now, this persistence as well as their formation remain largely unclear and the representation of low-lying MPCs in models of all complexities still poses a considerable challenge.
Arctic MPC persistence and phase partitioning are governed by a multitude of processes operating in conjunction across a wide range of spatial scales; such as the large-scale dynamical forcing, surface processes, boundary layer turbulence as well as the ambient aerosol concentration.
Here, we investigate these processes in high-resolution large eddy simulations (LES), and designed a case study for the Aerosol-Cloud Coupling and Climate Interactions in the Arctic (ACCACIA) campaign carried out in March 2013 in the European Arctic. We show that surface conditions significantly impact the cloud structure, which feeds back onto the response to aerosol perturbations within clouds over sea ice and open ocean surfaces. Over the open ocean we simulate a similar response to aerosol perturbations as observed in the subtropics, which finally leads us to investigating the dynamics and cloud field organization of MPCs in the Arctic under the framework of known mechanisms from the subtropics.



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