Anomalous natural convective sculpting of melting ice

We study the shape dynamics of ice melting in cold, initially quiescent fresh water, subject to the natural convective flows generated during melting. Experiments reveal three shape motifs associated with increasing far-field water temperature: sharp pinnacles pointed downward, scalloped waves, and sharp pinnacles pointed upward. Phase-field simulations reproduce these morphologies, which are closely tied to the anomalous density-temperature profile of liquid water. Analysis shows pinnacles sharpen with accelerating growth of tip curvature, while scallops emerge from a Kelvin-Helmholtz-like instability caused by counter currents that roll up to form arrays of wall-bound vortices.