Microstructural characterization of depth hoar and ice‐crust layers using a micro‐CT, and hypothesis of ice‐crust formation under a thunderstorm

Abstract

AbstractThe microstructural features typical of depth hoar and ice‐crust layers in both blocks of snow and a firn core that were extracted at Summit, Greenland (72°35′ N, 38°25′ W) in June, 2017 have been characterized using x‐ray microcomputed tomography (micro‐CT). In the depth hoar, the density is much lower, and the porosity, pore sizes, and specific surface area (SSA) are greater than those in adjacent layers. In the ice‐crusts, the density and the particle size are greater, and the porosity, pore size, and SSA are less than those in adjacent layers. Note that the mean structure thickness in the depth hoar was greater than that in adjacent layers, but that increase was simply related to the one‐ or two‐dimension ice crystals, that is, needle‐like or plate‐like structures, being included in the measurements for depth hoar. Using related microstructural parameters derived from the micro‐CT data, we propose a model based on refreezing of pre‐melted water (PMW) droplets electrostatically‐transported by the electric field between thunderclouds and the ice sheet created by a thunderstorm that describes the processes of the ice‐crust formation (ICF). Whether the ice‐crust forms with depth hoar depends on both the kinetic energy from the PMW droplets and the latent heat liberated from the freezing of the PMW. This work is the first to build the relationship between the atmosphere and ice sheets by a thunderstorm. Finally, we provide an experimental geophysics‐based method through the ICF under laboratory conditions to learn more about the interaction between atmospheric electrodynamics and thermodynamics.