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

Jon Nelson,Brian D Swanson

doi:10.5194/acp-19-15285-2019

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.

Highlights

Snow crystals, or ice crystals precipitated to the ground, are known for their wide variety, a notion perhaps first popularized via the photomicrographs of Bentley (1901)
The rate of the lateral growth via adjoining surface transport” (AST) in that case was much faster than normal growth, making the closing-up of hollows into center pockets a likely consequence of ASTdriven P growth
We argued that such features arose partly from lateral facet spreading and protruding growth, both phenomena driven largely by surface transport across the boundary of a face to the advancing edge, a process we termed adjoining surface transport or AST

Summary

Introduction

Ice crystals precipitated to the ground, are known for their wide variety, a notion perhaps first popularized via the photomicrographs of Bentley (1901). The most widely used model for the growth of crystal faces from the vapor is the “BCF” (Burton et al, 1951) model (see Woodruff, 2015, for updates and history) This model supposes that a given molecule in the vapor above a faceted surface strikes the crystal surface and becomes temporarily trapped in a mobile state until either desorbing back to the vapor or migrating along the surface and reaching a more strongly bound state at a step edge. As a source of step edges, BCF and later studies considered layer nucleation and defect-generated steps, most commonly spiral-step sources The former has been argued to be the main source for ice-crystal growth from the vapor under most atmospheric conditions

Methods

Findings

Discussion

Conclusion