B07: Influence of sea ice leads or polynyas on Arctic cloud properties

The wintertime Arctic sea ice area is characterised by different degrees of fracturing. Openings in the sea ice start as narrow cracks which can develop to larger leads reaching hundreds of meters to kilometres width. Mostly near the coast and along landfast ice, extended areas of open water (polynyas) can develop. Leads and polynyas result in a substantial heat and moisture flux from the relatively warm ocean to the cold atmosphere. They thus alter the atmospheric boundary layer structure, cloud cover, and the surface energy budget and also affect atmosphere-ocean chemical exchanges. In a warming Arctic characterised by a thinner and more mobile sea ice cover, the distribution of leads or polynyas are expected to change, yet their interactions with the atmosphere are not fully understood. Here, we want to quantify the influence of sea ice leads or polynyas on cloud properties such as cloud fraction, –altitude, –thickness, –phase, and their radiative effects in the winter/early spring.

We will make use of extensive long-term remote-sensing observations from the one-year long MOSAiC drifting observatory with the RV Polarstern and the coastal station Utqiaġvik (formerly known as Barrow) of the US-Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program at the North Shore of Alaska to extend the focus area of (AC)³ to the Central and Western Arctic. Synergistic observations of ground-based vertically-pointing Doppler cloud radar, depolarization lidar, microwave radiometer (MWR), and radiosondes will be used to develop climatologies of cloud macro- and microphysical properties including an estimation of the thermodynamic coupling of clouds to the surface. A special focus will be on the determination of cloud thermodynamic phase in the entire vertical column beyond lidar extinction using an artifical neural network (ANN). The cloud radiative effect (CRE) will be estimated via broadband surface sensor radiation observations in combination with clear-sky 1D radiative transfer calculations.
Leads or polynyas will be identified from daily maps of satellite-derived sea ice concentration and lead fraction products at various spatial resolutions (250 m – 5 km). To quantify the influence of leads or polynyas on clouds, the micro- and macrophysical properties of surface-coupled clouds during onshore winds (or winds towards the RV Polarstern observatory during MOSAiC) in the presence of leads or polynyas will be compared to clouds observed during onshore winds in closed sea-ice conditions.

Specifically, we want to study the questions:

• How is cloud cover changed in the presence of leads or polynyas (Q1)?
• How are macrophysical and microphysical cloud properties influenced by leads or polynyas (Q2)?
• Are there differences in Q1 and Q2 for different locations (Central Arctic vs Western Arctic)?