Air pockets and secondary habits in ice from lateral-type growth

Abstract

Often overlooked in studies of ice growth is how the crystal faces grow laterally. This paper explores the implications of such lateral-type growth and how it may explain air pockets and other secondary features of vapor-grown ice in air. For example, using a new crystal-growth chamber, we observed air pockets forming at crystal corners when a sublimated crystal is regrown. This and other observations support the idea that the lateral spreading of a face, and its (in some cases) extension as a 10 thin overhang over the adjoining region, is driven by a flux of surface-mobile admolecules across the face to the lateral-growth front. Inspired by recent work on this topic by Prof. A. Yamashita of Osaka Kyoiku University, we call this flux "adjoining surface transport" (AST) and the extension overgrowth "protruding growth", then apply the concepts to observed ice and snow crystals, including some from a cloud chamber and others from our experiments. We also suggest that such lateral-type growth may explain other air pockets, droxtal centers in dendrites, hollow terracing and banding, multiple-capped columns, scrolls, 15 trigonals, and sheath clusters. For dendrites and sheaths, AST may increase their maximum dimensions and round their tips.

Confronting the Challenge of Modeling Cloud and Precipitation Microphysics

“…Though some progress has been made on modeling single crystals, major uncertainties exist for the growth of ice with more complex shapes and at low temperatures (below −20°C). It has long been known that ice crystal habits can become complex, with "peculiar" or "irregular" forms appearing especially at low temperatures in surface (e.g., Kikuchi, 1969) and airborne (Lawson et al, 2019;Nousiainen et al, 2011;Stoelinga et al, 2007) in situ observations, and in laboratory experiments (Bailey & Hallett, 2002;Magono, 1970;Nelson & Swanson, 2019). The mass growth rates of these sorts of faceted crystals have not been measured; in fact, even the primary surface growth mechanism of atmospheric ice crystals is not presently known (Nelson, 2005).…”

Estimating Surface Attachment Kinetic and Growth Transition Influences on Vapor-Grown Ice Crystals

“…In the classical model of faceted growth, water molecules must then adsorb onto the crystal, though some molecules may be reflected from the surface. The fraction of molecules that adsorb onto the surface is often called a ''sticking'' efficiency b s and is thought to be near unity (Nelson 2001). Adsorbed molecules (ad-molecules) then migrate across the surface until they encounter an attachment site, which is provided by ledges in the crystal surface produced by dislocations in the crystal lattice or by the nucleation of two-dimensional islands on the surface.…”

“…One approach focuses on the growth rates of crystals with fully formed facets. Faceted growth theory is well-established (see Beckmann and Lacmann 1982;Sei and Gonda 1989;Nelson and Knight 1998;Libbrecht 2003) and there is substantial evidence that facets grow primarily by two mechanisms: spiral dislocations and ledge nucleation (Nelson and Knight 1998). These mechanisms produce variable deposition coefficients that depend on the supersaturation immediately above the crystal surface (surface supersaturation s surf ).…”

“…In contrast, ledge nucleation has a strong supersaturation dependence (Fig. 1) because s surf must exceed a characteristic supersaturation s char , so that ledges will form on the surface (Nelson and Knight 1998). While classical theoretical expressions for dislocation (Burton et al 1951) and ledge nucleation (Frank 1974) growth exist, most of the surface parameters required in those theories are unknown or cannot presently be measured.…”

“…While classical theoretical expressions for dislocation (Burton et al 1951) and ledge nucleation (Frank 1974) growth exist, most of the surface parameters required in those theories are unknown or cannot presently be measured. However, a convenient parameterization was formulated by Nelson and Baker (1996),…”