PIs: Andreas Macke, Roel Neggers

The interaction between sea ice cover and solar heating of the polar ocean (surface albedo effect) is considered as one of the most remarkable examples for a positive feedback mechanism in the climate system of the Earth. The recent dramatic reduction in Arctic sea ice in the late boreal summer strongly demands a more detailed analysis of the strength and sign of this mechanism. Clouds and aerosol particles play a significant but quantitatively uncertain role in this feedback. Therefore, it is planned to synchronize cloud observations with aerosol measurements onboard the Research Vessel (RV) Polarstern and at an ice station to identify and quantify aerosol direct and indirect  effects and to relate the state of the atmosphere to its radiative fluxes at the surface. We propose a detailed characterization of Arctic aerosol and clouds (local and regional).  Optical and microphysical properties of aerosols are separately inferred for the Atmospheric Boundary Layer (ABL) and the free troposphere (long–range transport regime) by means of state–of–the–art multiwavelength Raman/polarization lidar and sun/sky photometry. Long–term studies at Svalbard, Alomar, and Sodankyla, and with the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite lidar will be included in the characterization of Arctic aerosol conditions.

Cloud properties are taken from a combination of lidar and radar observations, solar and microwave radiometer, and sky imager measurements. In addition, the spatio–temporal variability of the surface radiative fluxes will be obtained from a large set of  pyranometer/pyrgeometer. Radiation closure studies will be performed to constrain the remotely sensed aerosol and cloud properties to the in–situ measurements of the surface radiative fluxes.

The observational tools available at TROPOS which are used within the framework of the long–term OCEANET project are excellently suited to monitor the role of clouds and aerosols in the overall surface energy budget onboard the RV Polarstern in the Arctic. All observations in A01 are synchronized to in–situ profiling of aerosol and cloud properties (A02), to the characterization of the turbulent ABL (A03), to top–of–atmosphere satellite observations (B01), to in–situ aircraft observations (B03), to in–situ cloud and ice condensation nuclei measurements (B04), to the investigation of latitudinal aerosol and cloud variability (B06), as well as to modelling of aerosol–cloud interactions (D02).

Modelling in A01 will consist of large–eddy simulations (LES) of low–level Arctic clouds that collocate with the ship–track of the RV Polarstern vessel. These simulations will be prepared and generated in E03, after the field campaign has taken place. A state–of–the–art double–moment microphysics scheme will be used that is able to represent mixed–phase Arctic clouds in the LES. Data will be sampled from these LES realizations to supplement the observational data record with information on the horizontal variability of the cloudy and thermodynamic state.

Specific research questions in addressing this scientific hypothesis are:

• Can we establish a significant relation between observed aerosol and cloud properties on the one hand, and surface radiative fluxes on the other?
• Do aerosol particles and clouds amplify or dampen surface warming?
• Are high–resolution models able to reproduce observed cloud properties?

• Ground site in Cluster A, microphysics closure in Cluster B and D
• Radiation budget in Cluster C, process understanding in E and A-D