Break the ice when it melts and freezes

At the 73rd annual meeting of the American Physical Society’s fluid dynamics division, researchers shared new insights into iceberg melting and lake ice formation.

Eric Hester has spent the past three years hunting for icebergs. A graduate student in mathematics at the University of Sydney in Australia, Hester and researchers from the Woods Hole Oceanographic Institution in Massachusetts are studying how the shape of an iceberg shapes the way it melts.

“Ice deforms when it melts,” said physical oceanographer Claudia Cenedese, who worked with Hester on the project. “It makes these shapes very strange, especially at the bottom, like the way the wind shapes a mountain on a longer time scale.”

At the 73rd Annual Meeting of the American Physical Society’s Division of Fluid Dynamics, Hester presented the results of her group’s experiments aimed at understanding how melting alters the shifting boundary of a shrinking iceberg and how these alterations in turn affect the dissolution.

The dynamics of melting icebergs are absent from most climate models, Cendese said. Including them could help with the prediction: Icebergs pump fresh water from ice sheets into the oceans, stimulating communities of living organisms. Icebergs are the main source of freshwater in the Greenland fjords and contribute significantly to the loss of freshwater in Antarctica. Icebergs play a vital role in climate, Cenedese said, and shouldn’t be overlooked in models. The physics of melting ice are well understood and some models accurately simulate it, he said. Others don’t. “But what you can’t do in those simulations is change the shape of the ice.”

Icebergs form in a wide range of shapes and sizes, Hester said, and distinct thermodynamic processes affect different surfaces. The base, immersed in water, does not melt in the same way as the side. “And every face doesn’t melt evenly,” Cenedese added.

Hester conducted her experiments by dipping a block of colored ice into a channel with a controlled flow of water passing by and watching it melt. He and his colleagues found that the side facing a current melts faster than the sides that run parallel to the flow. Combining experimental and numerical approaches, Hester and his collaborators plotted the relative influences of factors such as the relative speed of water and proportions, or the proportion of height and width on one side. Unsurprisingly, they found that the bottom had the lowest melting rate.

Cenedese said Hester’s project brings together collaborators from a wide range of disciplines and countries and that diverse collaboration is needed for such an interdisciplinary project. “Working in isolation isn’t that productive in this case.”

Other studies discussed at the conference focused on ice formation rather than melting. During a session on particle-laden flows, engineer Jiarong Hong of the St. Anthony Falls Laboratory at the University of Minnesota, Minneapolis, discussed the results of experiments showing how turbulence affects both the speed and distribution of snow. as it falls and settles. The findings could also help scientists better understand precipitation, Hong said.

Another project, presented by physicist Chao Sun of Tsinghua University in China and his team during a session on convection and buoyancy-driven flows, focused on ice formation in lakes.

Working on a scholarship from the Natural Science Foundation of China with Ziqi Wang of Tsinghua University, Enrico Calzavarini of the University of Lille in France and Federico Toschi of the University of Technology in Eindhoven in the Netherlands, Sun showed how the formation of ice on a lake it is closely linked to the dynamics of the fluids of the underlying water.

A lake can have layers of water of different densities and temperatures. “Water density anomalies can induce elaborate fluid dynamics under a moving ice front and can drastically change the behavior of the system,” Sun said. “This has often been ignored in previous studies.”

Sun’s team combined physical experiments, numerical simulations, and theoretical models to study the connection between ice and convective (turbulent) flows. They identified four distinct regimes of different flow dynamics, each interacting with other layers and ice in its own way. Even with that complexity, however, the group developed an accurate theoretical model that could be used in future studies.

“It made a correct prediction of the thickness of the ice sheet and the time of ice formation,” Sun said.

Since ice formation and melting plays such a critical role in climate, he said, a better understanding of the fluid dynamics underlying the process could help researchers accurately identify and study markers of a warming world. “The time when ice forms and melts, for example, could potentially provide an indicator of climate change.”