Winter atmospheric boundary layer observations and numerical modelling over sea ice in the coastal zone of the Bothnian Bay (Baltic Sea).

Representation of atmospheric boundary layer (ABL) over sea ice is a challenge for numerical weather prediction (NWP) models. Particularly, when it comes to stable or near neutral ABL, where models tend to erroneously simulate 2-m temperature, 10 m wind speed and other variables strongly affected by atmosphere-surface coupling. Studies indicate that submesoscale processes, not resolved directly by the models, might be responsible for those inaccuracies. Development of new parameterizations that would improve models performance is hampered by the limited amount of field data from the sea ice covered areas. To increase our knowledge about relevant processes a Hailuoto Atmospheric Observations over Sea Ice (HAOS) campaign was carried out over the sea ice in the Bothnian Bay (Baltic Sea) on 27 February - 2 March 2020. Observations included 27 Unmanned Aerial Vehicles (UAV) flights collecting meteorological data (temperature, wind speed, air pressure) and 4 photogrammetry missions over a stretch of sea ice, along with continuous, shore based automatic weather station (AWS) and LIDAR wind measurements. Obtained dataset is used for the validation of several mesoscale models results including WRF, run operationally by the Interdisciplinary Centre for Mathematical and Computational Modelling at University of Warsaw, AROME-ARCTIC run by Norwegian Meteorological Institute and HIRLAM from Finnish Meteorological Institute. Furthermore, the WRF Single Column Model (SCM) is launched multiple times for the comparison of different planetary boundary layer (PBL) and microphysics schemes influence on the model capability to reproduce the observed conditions. Considering that throughout the campaign the weather was dominated by low winds, clear skies and low temperatures, our research provides a valuable insight into the model strengths and shortcomings in the numerical modeling of stable boundary layer, together with the evaluation of alternative ABL parameterizations.