Break the ice by melting and freezing

At the 73rd Annual Meeting of the American Physical Society’s Division of Fluid Dynamics, researchers shared new ideas about melting icebergs and the formation of lake ice.

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

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

At the 73rd Annual Meeting of the Division of Fluid Dynamics at the American Physical Society, Hester presented the results of his group’s experiments to understand how melting alters the changing boundary of a shrinking iceberg – and how these alterations in turn affect melting.

The dynamics of melting icebergs are missing from most climate models, Cendese said. Including them could help with prediction: Icebergs pump fresh water from ice caps to the oceans, thereby stimulating communities of living organisms. Icebergs are the main source of freshwater in the Greenland fjords – and a major contributor to the loss of freshwater in Antarctica. Icebergs play a vital role in climate, Cenedese said, and should not be overlooked in models. The physics of melting ice is well understood and some models simulate it with precision, she said. Others don’t. “But what you can’t do in these simulations is change the shape of the ice.”

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

Hester conducted his experiments by submerging a block of dyed ice in a channel with a controlled flow of water passing, and watching it melt. He and his colleagues have found that the side facing the current melts faster than the sides parallel to the flow. By combining experimental and numerical approaches, Hester and his collaborators mapped the relative influences of factors such as the relative speed of water and the aspect ratio, or the aspect ratio of one side. Not surprisingly, they found that the background had the slowest melting speed.

Cenedese said that Hester’s project brings together collaborators from various disciplines and countries and that diverse collaboration is needed for such an interdisciplinary project. “Working in isolation is not so productive in this case.”

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

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

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

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 system behaviors,” Sun said. “This has often been overlooked in previous studies.”

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

“He made a fair prediction of the ice cover thickness and icing weather,” Sun said.

Since the formation and melting of ice play such a critical role in climate, he said, a better understanding of the fluid dynamics behind the process could help researchers identify and study precisely the markers of a warming world. “The time it takes for ice to form and melt, for example, could potentially provide an indicator of climate change.”