ºÚÁÏÍø

Title of the thesis: Numerical modeling of sea-ice ridging: Towards linking scales in ice dynamics

Thesis defender: Marek Muchow
Opponent: Dr. Devin O'Connor, Sandia National Laboratories, USA 
Custos: Prof. Arttu Polojärvi, Aalto University School of Engineering

Sea ice can get pushed together by wind and ocean currents, forming piles of broken ice called ridges. These ridges consist of broken ice pieces piled above and below the waterline, making the ice much thicker than in the surrounding ice cover. In turn, ridges influence ice motion by affecting air and ocean drag and by limiting large-scale ice strength. Most conventional continuum models simplify ridging by either increasing the mean ice thickness or adjusting the distribution of ice thickness, using approximations for small-scale processes. The research in this thesis contributes to understanding ridging by conducting detailed simulations of ridge formation. The main modelling method used is the discrete element method (DEM), which allows for an explicit description of ridge formation, including ice failure and ice accumulation within the ridge.

First, the three-dimensional DEM model A3D-DEM is used to simulate the formation of individual ridges. The simulations are validated using results from laboratory-scale experiments at the Aalto Ice and Wave Tank. The simulations show that the force required to form a ridge increases linearly with ice thickness, which is different to earlier two-dimensional simulations. This difference is likely because the ice doesn’t break all at once—it fails at different places and times along the ridge, a process called non-simultaneous failure. 

Then, another three-dimensional DEM model, HiDEM, is used to simulate ridging across a significantly larger sea-ice domain. These simulations lead to a deformed ice cover composed of multiple ridges of varying shapes, including triangular and trapezoidal ridges. The trapezoidal ridges are especially important because they increase the amount of thicker ice, creating a noticeable bump in the ice thickness distribution (ITD). Further, the ITD resulting from the DEM simulations is compared with ITDs from a continuum model and from two commonly used ridging functions used as sub-grid parametrizations within redistribution schemes. The results show that these traditional models produce a different thickness pattern than the more detailed simulations.

Overall, the suitability of DEM models for understanding larger-scale sea-ice dynamics is also demonstrated in this research. The results from the DEM models are set in the context of the current understanding of ridge properties and ridging processes and discussed with respect to their application in large-scale continuum models.

Key words: Sea-ice dynamics, Ridged ice, Ice mechanics, Numerical Modeling, Discrete element method 

Thesis available for public display 7 days prior to the defence at . 

Contact information: marek.muchow@aalto.fi