Lateral facet growth of ice and snow – Part 1: Observations and applications to secondary habits

Abstract

Abstract. Often overlooked in studies of ice growth is how the crystal facets increase in area, that is, grow laterally. This paper reports on observations and applications of such lateral facet growth for vapor-grown ice in air. Using a new crystal-growth chamber, we observed air pockets forming at crystal corners when a sublimated crystal is regrown. This observation indicates that the lateral spreading of a face can, under some conditions, extend as a thin overhang over the adjoining region. We argue that this extension is driven by a flux of surface-mobile molecules across the face to the lateral-growth front. Following the pioneering work on this topic by Akira Yamashita, we call this flux “adjoining surface transport” (AST) and the extension overgrowth “protruding growth”. Further experiments revealed other types of pockets that are difficult to explain without invoking AST and protruding growth. We develop a simple model for lateral facet growth on a tabular crystal in air, finding that AST is required to explain observations of facet spreading. Applying the AST concept to observed ice and snow crystals, we argue that AST promotes facet spreading, causes protruding growth, and alters layer nucleation rates. In particular, depending on the conditions, combinations of lateral- and normal-growth processes can help explain presently inexplicable secondary features and habits such as air pockets, small circular centers in dendrites, hollow structure, multiple-capped columns, scrolls, sheath clusters, and trigonals. For dendrites and sheaths, AST may increase their maximum dimensions and round their tips. Although these applications presently lack quantitative detail, the overall body of evidence here demonstrates that any complete model of ice growth from the vapor should include such lateral-growth processes.