Experimental determination of snow sublimation rate and stable-isotopic exchange

Abstract

ABSTRACT. Snow sublimation is a fundamental process that affects the snow crystal structure and is important for ice-core interpretation, remote sensing, snow hydrology and chemical processes in snow. Prior studies have shown that sublimation can change the isotopic content of the remaining snow; these studies have inferred sublimation rates using field data, and were unable to control many of the environmental parameters that determine sublimation rate (e.g. temperature, relative humidity, snow microstructure). We present sublimation rate measurements on snow samples in the laboratory, where we have controlled many of these parameters simultaneously. We use the same experimental apparatus to determine sublimation rate, investigate the isotopic effects of sublimation, and study the isotopic exchange between vapor and solid. Our results suggest that pore spaces in snow are almost always at saturation vapor pressure; undersaturation may be possible in large pore spaces or in regions of rapid interstitial airflow. We present a revised formulation for determining the mass-transfer coefficient for snow as a linear function of Reynolds number (h m ¼ 0.566Re + 0.075), estimate the fractionation coefficient for sublimating snow, and provide evidence for isotopic exchange between vapor and solid.

Experimental observation of transient <i>δ</i><sup>18</sup>O interaction between snow and advective airflow under various temperature gradient conditions

“…This process, compared to interstitial diffusion (Johnsen, 1977;Johnsen et al, 2000), will alter the isotope mean value. The field observations challenge the previous assumption that sublimation occurred layer-bylayer with no resulting isotopic fractionation (Dansgaard, 1964;Friedman et al, 1991;Town et al, 2008;Neumann and Waddington, 2004). It is assumed that the solid undergoing sublimation would not be unduly enriched in the heavier isotope species due to the preferential loss of lighter isotopic species to the vapour (Dansgaard, 1964;Friedman et al, 1991).…”

“…In a typical Antarctic and Greenland snow profile, strong interactions between the atmosphere and snow occur, especially in the first 2 m (Neumann and Waddington, 2004;Town et al, 2008), called the convective zone. In the convective zone, air can move relatively freely and therefore exchange occurs between snow and the atmospheric air.…”

“…In the convective zone, air can move relatively freely and therefore exchange occurs between snow and the atmospheric air. Air flowing into the snow reaches saturation vapour pressure nearly instantly through sublimation (Neumann et al, 2008;Ebner et al, 2015a). Models of the influence of the so-called "wind pumping" effect (Fisher et al, 1983;Neumann and Waddington, 2004), in which the interstitial water vapour is replaced by atmospheric air pushed through the upper metres of the snow pack by small-scale high and low pressure areas caused by irregular grooves or ridges formed on the snow surface (dunes and sastrugi), have assumed that the snow grains would equilibrate with the interstitial water vapour on timescales governed by ice self-diffusion.…”

The growth of sublimation crystals and surface hoar on the Antarctic plateau

“…Since surface hoar crystals form from atmospheric water vapor while sublimation crystals come from snow water vapor, the isotopic signature may be different (Neumann et al, 2008;Steen-Larsen et al, 2014), and this might be usable to quantify water vapor exchanges and therefore improve our understanding of the processes involved in the mass balance of the East Antarctic Ice Sheet.…”

Numerical experiments on isotopic diffusion in polar snow and firn using a multi-layer energy balance model

“…The equation of Calonne et al (2014) is based on the numerical analysis of 3D tomographic images of different types of snow. It relates normalized effective We assume that vapor is in general at saturation in the snow layers (Neumann et al, 2008;Neumann et al, 2009). The local 265…”