Scanning electron microscopy and molecular dynamics of surfaces of growing and ablating hexagonal ice crystals

We present the first clearly resolved observations of surfaces of growing and ablating hexagonal ice crystals using variable-pressure scanning electron microscopy. The ice surface develops trans-prismatic strands, separated from one another by distances of 5-10 mu m. The strands are present at a wide range of supersaturations, but are most pronounced at temperatures near the frost point. Pyramidal facets consistent with Miller-Bravais indices of 10 < span style='border-top: 1px solid #000; color: #000;'> 1 1, and possibly also 20 < span style='border-top: 1px solid #000; color: #000;'> 2 1, are associated with ice growth under these conditions. A molecular-dynamics model of a free-standing ice I-h nanocolumn containing 8400 water molecules does not develop trans-prismatic strands, suggesting these features originate at larger spatial or temporal scales. The possible relevance of these surface features to cirrus ice is discussed.