Formation of superimposed ice in Arctic : study from observations and modeling

The formation of superimposed ice on Arctic sea ice during the melt season is studied using in situ observations from ice mass balance instruments (SIMBA/IAOOS) and a one-dimensional thermodynamic model of snow and sea ice (LIM1D). Ice mass balance instruments measure vertical profiles of temperature and a proxy for thermal conductivity over a vertical extent of 5 m through the atmosphere, snow, ice and ocean at regular time intervals.

In this study, we analyse four time series from instruments deployed near the North Pole in April and drifting towards Svalbard, all of which clearly show periods of superimposed ice formation. The dataset covers three melt seasons (2013, 2015, and 2017). Air–snow, snow–ice, and ice–ocean interfaces are retrieved from the temperature and conductivity profiles with an accuracy of about 2 cm, but this uncertainty can become substantially larger during the melt period. Snowmelt periods are identified, and the subsequent formation of superimposed ice at the sea ice surface is estimated.

Numerical simulations with the LIM1D model are performed to complement the analysis of the observed time series. The sensitivity of the results to the atmospheric forcing is also analysed. Summer temperature profiles strongly depend on the longwave radiative flux. Simulations forced with ERA5 exhibit an earlier onset of snowmelt than observed, whereas experiments using ERA-Interim radiation lead to a substantially improved agreement with the observations. In contrast, variations in snow albedo and snow density have a limited impact during the pre-melt period.

The formation of superimposed ice is successfully reproduced by the LIM1D model. The role of precipitation and surface runoff in the days preceding superimposed ice formation, as well as the changes in sea ice suggested by the observations, are confirmed. Lastly, the deployment of the instruments modifies the surrounding environment and may influence local observations. This should be taken into account when interpreting ice mass balance measurements.