Reflectances measured in the visible frequency range at three channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Earth Observation Satellite (EOS) TERRA were used to derive the melt pond fraction on Arctic sea ice using an artificial neural network. This analysis was done on reflectances gridded onto a polar-stereographic grid tangent to the Earths' surface at 70 deg N with 500 m grid resolution. The reflectances used originate from the 8-day composite reflectances provided via https://wist.echo.nasa.gov/api/ as product: "MODIS surface Reflectance 8-Day L3 Global 500m SIN Grid V005". After gridding and flagging for clouds and other disturbances the artificial neural network was applied, providing fractions of three surface classes: 1) melt ponds, 2) sea ice and snow, and 3) open water at 500 m grid resolution. This data has been interpolated onto a similar polar-stereographic grid but with 12.5 km grid resolution. The data set offered here comprises several data layers: the melt pond fraction, its standard deviation, the open water fraction, and the number of individual valid grid cells with 500 m grid resolution included in each 12.5 km grid cell. In addition, in three separate data layers melt pond fraction, its standard deviation, and the open water fraction are given with those grid cells (with 12.5 km grid resolution) flagged as invalid where less than 90 % of the native 500 m grid resolution data indicate clear sky conditions. Valid for all these layers is, that grid cells with an open water fraction larger than 85 % have been flagged as invalid as well. The data set offered here is version 02 of the melt pond data set. The main difference to version 01 is a bias correction carried out to remove a positive bias in the melt pond fraction and in the open water fraction.

Reflectances measured in the visible frequency range at three channels of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Earth Observation Satellite (EOS) TERRA were used to derive the melt pond fraction on Arctic sea ice using an artificial neural network. This analysis was done on reflectances gridded onto a polar-stereographic grid tangent to the Earths' surface at 70 deg N with 500 m grid resolution. The reflectances used originate from the 8-day composite reflectances provided via https://wist.echo.nasa.gov/api/ as product: "MODIS surface Reflectance 8-Day L3 Global 500m SIN Grid V005". After gridding and flagging for clouds and other disturbances the artificial neural network was applied, providing fractions of three surface classes: 1) melt ponds, 2) sea ice and snow, and 3) open water at 500 m grid resolution. This data has been interpolated onto a similar polar-stereographic grid but with 12.5 km grid resolution. The data set offered here comprises several data layers: the melt pond fraction, its standard deviation, the open water fraction, and the number of individual valid grid cells with 500 m grid resolution included in each 12.5 km grid cell. In addition, in three separate data layers melt pond fraction, its standard deviation, and the open water fraction are given only for those grid cells (with 12.5 km grid resolution) where more than 90 % of the native 500 m grid resolution data indicate clear sky conditions. Grid cells with an open water fraction larger than 85 % have been generally flagged as invalid. The data set is updated annually.

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 DAMOCLES 2008 (Hamburg Arctic Ocean Buoy Drift Experiment DAMOCLES 2008-2009) consisted of the deployment and tracking of 9 drifting autonomous ice buoys in the Arctic Ocean. Seven buoys were deployed in the Canadian sector of the Arctic Ocean in late April 2008. Two more buoys were deployed in the Beaufort Sea and in the Laptev Sea in September and October 2008. The platforms report position, atmospheric pressure, temperature and humidity, wind speed and ice temperature at 3-hourly time steps. The last two buoys additionally report wind direction. The aim of the experiment was to study the Atmosphere-Ice-Ocean interaction, especially the impact of cyclones on the formation and transport of sea ice. DAMOCLES 2008 and its predecessor DAMOCLES 2007 are a contribution to European integrated project DAMOCLES (Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies) which is funded by the European Union. DAMOCLES is a contribution to IPY 2007-2008 (International Polar Year).

The field experiment DAMOCLES 2007 (Hamburg Arctic Ocean Buoy Drift Experiment DAMOCLES 2007-2008) consisted of the deployment and tracking of an array of 16 drifting autonomous buoys in the Central Arctic Ocean. The buoys were deployed in a quadratic array with 400 kilometres side length in the Siberian sector of the Central Arctic Ocean in April 2007. While drifting towards Fram Strait the buoys delivered at approximately 1-hourly time intervalls position, sea level pressure and temperature for several months with the last buoy transmitting until January 2008. The aim of the experiment was to study the Atmosphere-Ice-Ocean interaction, especially the impact of cyclones on the formation and transport of sea ice. DAMOCLES 2007 and DAMOCLES 2008 are a contribution to European integrated project DAMOCLES (Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies) which is funded by the European Union. DAMOCLES is a contribution to IPY 2007-2008 (International Polar Year).

The expedition ARKTIS 1988 was initiated and directed by the Collaborative Research Centre 318 of the German Research Foundation entitled "Climatically relevant proceses in the system ocean-atmosphere-ice" and established at the University of Hamburg. The field experiment took place in the Fram Strait in the area straddling the ice margin west of Spitsbergen during the period from 4 to 26 May 1988. The experimental concept for the investigation of boundary layer modification and certain cloud structures in cases of off-ice and on-ice air flows was to maneuver one ship, the research icebreaker POLARSTERN, about 100 km into the ice and to operate with a second ship, the research vessel VALDIVIA, in the open water near the ice edge. Several aircraft operating from the airport at Longyearbyen on Spitsbergen were intended to measure the mean structures, variances and covariances at different distances from the ice edge. ARKTIS 1988 was followed by the two experiments ARKTIS 1991 and ARKTIS 1993.

The field experiment ARKTIS 1993 was the third one in a series of experiments in the Arctic region performed by a group of climate researchers from Hamburg. The campaign took place in the Greenland Sea west of Spitsbergen from 1 to 25 March 1993. The preceding experiments were ARKTIS 1988 (5 - 25 May 1988) in the same geographic region and ARKTIS 1991 (20 February - 13 March 1991) located between North Norway and Bear Island. The main objective of ARKTIS 1993 was the investigation of cold air outbreaks from the Arctic sea ice onto the open water, in this case the West Spitsbergen current. To get a concise picture of all stages of boundary layer modification in cold air outbreaks a wide variety of measurement platforms was employed. Three reseach vessels (Polarstern, Valdivia, Prof. Multanovsky) operated in the experimental area providing surface observations and radiosonde data. Aerological data was also collected at three land stations (Bear Island, Danmarkshavn, Ny Alesund) which intensified their operational radiosonde program. Most essential measurement were taken by the research aircraft Falcon and DO-128 which took profiles and cross sections within the air flow. Eleven flight missions were performed.

The field experiment ARKTIS 1991 was an expedition planned by meteorologists of the Collaborative Research Centre 318 entitled "Climatically relevant processes in the system ocean-atmosphere-ice" which is funded by the German Research Foundation and established at the University of Hamburg. The expedition took place in the Norwegian Sea between Northern Norway, Bear Island and Jan Mayen during the period 17 February until 15 March 1991. The main aim of the experiment was the investigation of cold air outbreaks from the surrounding Arctic ice sheets. During such weather episodes the air mass coming from the ice is rapidly modified over the water due to the contrasts in temperature, heat conduction, humidity and roughness between ice and water. This leads to the formation of a "new" boundary layer. Its depth, mean temperature and moisture increases with increasing distance from the ice edge mainly due to sensible and latent heat supply from the ocean. The investigations of cold air outbreaks and Arctic stratus by scientists of the Collaborative Research Centre 318 began already three years before with the field experiment ARKTIS 1988 which took place in the area west of Spitsbergen in May 1988. ARKTIS 1991 is a continuation of this work under winterly weather conditions. ARKTIS 1991 was followed by the experiment ARKTIS 1993. As in ARKTIS 1988 the research vessel Valdivia and the two research aircraft FALCON-20 of the DLR at Oberpfaffenhofen and DO-128 of the TU Braunschweig were at our disposal. Radiosonde measurements were performed on board of RV Valdivia and on Bear Island.

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 field drift buoys experiment FRAMZY 2009 consisted of the deployment and tracking of an array of 7 drifting autonomous buoys, of which one did not transmit values. The experiment aimed at the measurement of the sea ice drift in the Fram Strait and its relation to the atmospheric forcing, primarily to that by cyclones. The buoys were deployed in October 2009 in the Fram Strait region and sampled data until February 2010. FRAMZY 2009 was the last 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.