Anomalous convective flows carve pinnacles and scallops in melting ice

TL;DR

This study investigates how natural convective flows influence the surface shapes of melting ice in cold water, revealing temperature-dependent morphologies linked to water's density anomaly and flow instabilities.

Contribution

The paper combines experiments and phase-field simulations to connect water's density anomaly with macro-scale ice surface morphologies during melting.

Findings

Sharp pinnacles form at low temperatures

Scalloped waves occur at intermediate temperatures

Upward pinnacles develop at higher temperatures

Abstract

We report on the shape dynamics of ice suspended in cold fresh water and subject to the natural convective flows generated during melting. Experiments reveal shape motifs for increasing far-field temperature: Sharp pinnacles directed downward at low temperatures, scalloped waves for intermediate temperatures between 5 and $7^\circ$C, and upward pointing pinnacles at higher temperatures. Phase-field simulations reproduce these morphologies, which are closely linked to the anomalous density-temperature profile of liquid water. Boundary layer flows yield pinnacles that sharpen with accelerating growth of tip curvature while scallops emerge from a Kelvin-Helmholtz-like instability caused by counterflowing currents that roll up to form vortex arrays. By linking the molecular-scale effects underlying water's density anomaly to the macro-scale flows that imprint the surface, these results show that the morphology of melted ice is a sensitive indicator of ambient temperature.