The field experiments ALKOR 2000 (consisting of three cruises: ALKOR 4/2000, 6/2000, 10/2000) and ALKOR 2001 (4/2001, 6/2001, 10/2001) took place in the central Baltic Sea. The six cruises of the German Research Vessel Alkor with duration of about seven days each led to a point of the Baltic Sea which is most remote from the adjacent lands and additionally a grid point of regional climate model REMO. The ALKOR experiments as well as BASIS 1998 and BASIS 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. 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 Alkor

The field experiments ALKOR 2000 (consisting of three cruises: ALKOR 4/2000, 6/2000, 10/2000) and ALKOR 2001 (4/2001, 6/2001, 10/2001) took place in the central Baltic Sea. The six cruises of the German Research Vessel Alkor with duration of about seven days each led to a point of the Baltic Sea which is most remote from the adjacent lands and additionally a grid point of regional climate model REMO. The ALKOR experiments as well as BASIS 1998 and BASIS 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. 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 Alkor

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

Sea level pressure is a fundamental weather and climate element and the very basis of everyday weather maps. Daily sea level pressure distributions provide information on the influence of high and low pressure systems, air flow, weather activity, and, hence, synoptic conditions. Using sea level pressure distributions from the NCEP/NCAR Reanalysis 1 (Kalnay et al., 1996) and a simplified variant of the weather-typing scheme by Jenkinson and Collison (1977) atmospheric circulation over the North Sea has been classified as to pattern and intensity on a daily basis starting in 1948. A full account of the original weather-typing scheme can be found in Loewe et al. (2005), while the variant scheme has been detailed in Loewe et al. (2006). The analysis has been carried out on the original 16-point grid. Though formally valid at its central point (55°N, 5°E), results are representative of the North Sea region between 50°N-60°N and 0°E-10°E. The modified scheme allows for six weather types, namely four directional (NE=Northeast, SE, SW, NW) and two rotational types (C=cyclonic and A=anticyclonic). The strength of the atmospheric circulation is classified by way of a peak-over-threshold technique, employing re-calibrated thresholds for the gale index G* of 28.3, 36.6, and 44.6 hPa for gale (G), severe gale (SG), and very severe gale (VSG), respectively (Loewe et al., 2013). Technically, the set of weather-typing and gale-classification rules is implemented as a lean FORTRAN code (lwtnssim.f), internally known as "Simple Lamb weather-typing scheme for the North Sea v1". The processing run was done on a Linux server under Debian 10 (Buster). Both, weather types and gale days, form a catalogue of more than 70 annual calendars since 1948 that is presented and continuously updated to the present day at https://www.bsh.de/EN/DATA/Climate-and-Sea/Weather-and-Gales/weather-and-gales_node.html. This catalogue concisely documents synoptic conditions in the North Sea region. Possible benefits are manifold. Special events and episodes in regional-scale atmospheric circulation are easily looked up and traced. Beyond that, the dataset is well suited for frequency, trend, persistence, transition, and extreme-value statistics.