The field experiment ACSYS 1998 took place in the Greenland Sea west of Spitsbergen from 10 to 25 March 1998. It was planned and organized by scientists of the Meteorological Institute of the University of Hamburg within the national research project ACSYS (Arctic Climate System Study) which was funded by the German Bundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie (BMBF). The national ACSYS project is part of the international ACSYS research program which is a sub-program of the World Climate Research Program (WCRP). The objective of the ACSYS 1989 field experiment was the investigation of the atmospheric boundary layer in case of on-ice air flow in wintertime. Cyclones, reaching the Arctic sea ice from the Greenland Sea or Barents Sea are the strongest synoptic-scale weather signals in the Arctic region. They transport warm, moist and cloudy air from the open water to the Arctic shield. Especially in wintertime when the temperature contrast between the ocean and the ice surface is large the effects in the boundary layer over the ice are also significant. The research aircraft Falcon performed six flight missions from 11 to 21 March measuring meteorological parameters and turbulent fluxes.

The field experiment BASIS 1998 took place in the Gulf of Bothnia in the Baltic Sea in a boundary zone between the open sea and the ice-covered sea from 16 February to 7 March, 1998. BASIS 1998 as well as the field experiments BASIS 2001 and ALKOR 2000 and 2001 are part of the research compound BALTIMOS (BALTic sea Integrated MOdel System). BALTIMOS in turn is part of the Baltic Sea Experiment (BALTEX). The overall objective of all eight field experiments (ALKOR and BASIS) was to collect a comprehensive data set suited to validate the coupled model system BALTIMOS for the Baltic Sea region. The observations mainly focus on: - the atmospheric boundary layer structure and processes and the air-sea-ice interaction over areas with inhomogeneous sea ice cover - the atmospheric boundary layer structure over open water under different synoptic conditions such as cold-air advection, warm-air advection or frontal passages. Observations during BASIS 2001 were made at three land stations and the Finnish research vessel Aranda. All stations performed radiosonde measurements. The German research aircraft Falcon had six flight missions with measurements of meteorological parameters and turbulent fluxes. In addition to the published datasets several other measurements were performed during the experiment. Corresonding datasets will be published in the near future and are available on request. Details about all used platforms and sensors and all performed measurements are listed in the fieldreport. The following datasets are available on request: meteorological stations Kokkola, Umea, Merikarvia, ground data at RV Aranda

Products of liquid water path (LWP), rain water path (RWP) and integrated water vapor (IWV, also called precipitable water vapor (PWV)) are retrieved from microwave radiometer observations with auxiliary measurements from backscatter lidar and cloud radar. The nadir measurements were taken by the German High Altitude and Long range research aircraft (HALO) during the Next generation Advanced Remote sensing for VALidation campaign South (NARVAL-South) in December 2013. Products are provided over tropical Atlantic east of Barbados. This experiment provides column integrated quantities as seen from satellite perspective but with higher spatially resolution (about 1 km footprint) than available from microwave satellites.

Products of liquid water path (LWP), rain water path (RWP) and integrated water vapor (IWV, also called precipitable water vapor (PWV)) are retrieved from microwave radiometer observations with auxiliary measurements from backscatter lidar and cloud radar. The nadir measurements were taken by the German High Altitude and Long range research aircraft (HALO) during the Next generation Advanced Remote sensing for VALidation campaign 2 (NARVAL2) in August 2016. Products are provided over tropical Atlantic east of Barbados. This experiment provides column integrated quantities as seen from satellite perspective but with higher spatially resolution (about 1 km footprint) than available from microwave satellites.

From 26 March to 22 April 2003, Germany and Finland carried out a joint expedition (ABSIS - Arctic Atmospheric Boundary Layer and Sea Ice Interaction Study) to test and improve the description of the ocean-ice-atmosphere interaction processes. ABSIS was part of the German ACSYS (Arctic Climate System Study) contribution supported by the Ministry of Education and Research and was connected with the RV Polarstern Winter Arctic Polynia Study expedition. The main objectives of ABSIS were to collect data sets to quantify and study: (a) the thermodynamic and dynamic interaction processes at the ice-atmosphere interface and (b) the vertical structure of the atmospheric boundary layer, particularly the Arctic inversion. The latter is a nearly permanent feature over the sea ice. The Arctic inversion is particularly low and strong in winter and effects the exchange between the boundary layer and the free atmosphere as well as the interaction processes at the ice surface. To achieve the above-mentioned objectives various platforms were applied and placed within a 200 km2 area north of Spitsbergen during the expedition period 26 March to 22 April 2003: RV Polarstern and RV Aranda, a research aircraft (Falcon-20 of the German Aerospace Center), 11 automatic ice buoys, and one ice camp station close to RV Polarstern (8-18 April 2003 only). In addition to the published datasets several other measurements were performed during the experiment. Corresonding datasets will be published in the near future and are available on request. Details about all used platforms and sensors and all performed measurements are listed in the fieldreport. The following datasets are available on request: ground data at RV Aranda, ground data at RV Polarstern, ice station near RV Polarstern

The drift buoys experiment FRAMZY 2007 consisted of the deployment and tracking of an array of 29 drifting autonomous buoys (16 ice, 13 water) in the Fram Strait region. The buoys were deployed in March 2007 and sampled data until end of April 2007. The aim of the experiment was to study the Atmosphere-Ocean interaction and the Atmosphere-Ice-Ocean interaction, especially the impact of cyclones on the energy budget of sea ice and ocean surface. FRAMZY 2007 was the third one in a series of five field experiments (1999,2002,2008,2009) carried out in the frame of the Collaborative Research Centre 512 (Cyclones and the North Atlantic Climate System) funded by the German Science Foundation.

The field campaign LOFZY 2005 (LOFoten ZYklonen, engl.: Cyclones) was carried out in the frame of Collaborative Research Centre 512, which deals with low-pressure systems (cyclones) and the climate system of the North Atlantic. Cyclones are of special interest due to their influence on the interaction between atmosphere and ocean. Cyclone activity in the northern part of the Atlantic Ocean is notably high and is of particular importance for the entire Atlantic Ocean. An area of maximum precipitation exists in front of the Norwegian Lofoten islands. One aim of the LOFZY field campaign was to clarify the role cyclones play in the interaction of ocean and atmosphere. In order to obtain a comprehensive dataset of cyclone activity and ocean-atmosphere interaction a field experiment was carried out in the Lofoten region during March and April 2005. Employed platforms were the Irish research vessel RV Celtic Explorer which conducted a meteorological (radiosondes, standard parameters, observations) and an oceanographic (CTD) program. The German research aircraft Falcon accomplished eight flight missions (between 4-21 March) to observe synoptic conditions with high spatial and temporal resolution. In addition 23 autonomous marine buoys were deployed in advance of the campaign in the observed area to measure drift, air-temperature and -pressure and water-temperature. In addition to the published datasets several other measurements were performed during the experiment. Corresonding datasets will be published in the near future and are available on request. Details about all used platforms and sensors and all performed measurements are listed in the fieldreport. The following datasets are available on request: ground data at RV Celtic Explorer

The field experiment BASIS 2001 took place in the Gulf of Bothnia of the Baltic Sea. The Finnish Research Vessel Aranda kept a fixed position in the pack ice and was a base for standard meteorological measurements and radiosonde ascents. Surface measurements were also taken at four land stations (Marjaniemi, Oulu, Kuivaniemi, Haparanda). The research aircraft DO-128 of the University Braunschweig performed 10 flight missions. The BASIS experiments as well as ALKOR 2000 and 2001 are part of the research compound BALTIMOS (BALTic sea Integrated MOdel System). BALTIMOS in turn is part of the Baltic Sea Experiment (BALTEX). The overall objective of all eight field experiments (BASIS and ALKOR) was to collect a comprehensive data set suited to validate the coupled model system BALTIMOS for the Baltic Sea region. The observations mainly focus on: - the atmospheric boundary layer structure and processes and the air-sea-ice interaction over areas with inhomogeneous sea ice cover - the atmospheric boundary layer structure over open water under different synoptic conditions such as cold-air advection, warm-air advection or frontal passages. In addition to the published datasets several other measurements were performed during the experiment. Corresonding datasets will be published in the near future and are available on request. Details about all used platforms and sensors and all performed measurements are listed in the fieldreport. The following datasets are available on request: meteorological station Marjaniemi and Oulu airport, ground data at RV Aranda, 10 flights of research aircraft DO-128

The field experiment FRAMZY (in German: Framstraßen-Zyklonen; in English: Fram Strait Cyclones) 2002 took place in the Fram Strait region between Greenland and Spitsbergen and between 76-83¿N during the period 25 February to 25 March 2002. It was the second field experiment (following FRAMZY 1999) on cyclones in the Fram Strait and their impact on sea ice. The objectives of FRAMZY 2002 were to sample a data set in order to understand the processes of cyclone generation and sea ice forcing by the cyclones and to estimate the quality of atmospheric models in analysing and forecasting the cyclones and the quality of sea ice models in simulating the cyclone impacts on the sea ice. FRAMZY 2002 also aims to clarify the role of Fram Strait cyclones in the large interannual variations of the North-to-South sea ice transport through the Fram Strait. To reach the objectives measurements were taken simultaneously in the atmosphere and of the sea ice and covered a wide range of scales from the synoptic (cyclone) to the turbulent scale (turbulent fluxes at the air-ice interface). Measurements were taken in-situ by 14 autonomous ARGOS ice buoys, the Finnish Research Vessel Aranda and the German Research Aircraft Falcon and were supplemented by satellite data from NOAA-AVHRR, RADARSAT and DMSP-SSM/I. FRAMZY 2002 was the second one in a series of five field experiments (1999,2002,2007,2008) carried out in the frame of the Collaborative Research Centre 512 (Cyclones and the North Atlantic Climate System) funded by the German Science Foundation. In addition to the published datasets several other measurements were performed during the experiment. Corresonding datasets will be published in the near future and are available on request. Details about all used platforms and sensors and all performed measurements are listed in the fieldreport. The following datasets are available on request: ground data at RV Aranda

The Fram Strait Cyclone Experiment, FRAMZY 1999, took place in the Fram Strait and Greenland Sea region during April 1999. Using aircraft, ice buoys, ship and satellite measurements a data set was compiled to investigate the properties of Fram Strait cyclones, their cyclogenetic conditions on the large- and meso-scale, and their local effects on sea ice drift and sea ice distribution and, thus, on the freshwater flow through the Fram Strait. The data were used for validation of cyclone simulations with coupled mesoscale models of the atmosphere-ice-ocean system. FRAMZY 1999 was the first one in a series of five field experiments (2002,2007,2008,2009) carried out in the frame of the Collaborative Research Centre 512 (Cyclones and the North Atlantic Climate System) funded by the German Science Foundation. In addition to the published datasets several other measurements were performed during the experiment. Corresonding datasets will be published in the near future and are available on request. Details about all used platforms and sensors and all performed measurements are listed in the fieldreport. The following datasets are available on request: ground data at RV Valdivia