Macroscopic water vapor diffusion is not enhanced in snow

Kévin Fourteau,Florent Domine,Pascal Hagenmuller

doi:10.5194/tc-15-389-2021

Abstract

Abstract. Water vapor transport in dry snowpacks plays a significant role for snow metamorphism and the mass and energy balance of snowpacks. The molecular diffusion of water vapor in the interstitial pores is usually considered to be the main or only transport mechanism, and current detailed snow physics models therefore rely on the knowledge of the effective diffusion coefficient of water vapor in snow. Numerous previous studies have concluded that water vapor diffusion in snow is enhanced relative to that in air. Various field observations also indicate that for vapor transport in snow to be explained by diffusion alone, the effective diffusion coefficient should be larger than that in air. Here we show using theory and numerical simulations of idealized and measured snow microstructures that, although sublimation and deposition of water vapor onto snow crystal surfaces do enhance microscopic diffusion in the pore space, this effect is more than countered by the restriction of diffusion space due to ice. The interaction of water vapor with the ice results in water vapor diffusing more than inert molecules in snow but still less than in free air, regardless of the value of the sticking coefficient of water molecules on ice. Our results imply that processes other than diffusion play a predominant role in water vapor transport in dry snowpacks.

Highlights

When a snowpack is submitted to a temperature gradient, macroscopic water vapor transfer occurs from the warmer to the colder parts of the snowpack in a process sometimes referred to as layer-to-layer vapor flux
We have shown that most of the previous theoretical studies reporting enhanced macroscopic vapor flux compared to free air used faulty computations of the macroscopic vapor flux, which resulted in systematic overestimation
This work investigated the macroscopic vapor fluxes that arise in snowpacks due to large-scale vapor gradients

Summary

Introduction

When a snowpack is submitted to a temperature gradient, macroscopic water vapor transfer occurs from the warmer to the colder parts of the snowpack in a process sometimes referred to as layer-to-layer vapor flux. This redistribution of mass plays a significant role in the evolution of the snowpack and its physical properties. The physics at play in the pores is generally agreed upon, even though questions about the precise kinetics of the sublimation and deposition of water molecules onto ice surfaces in snow remain open (Legagneux and Domine, 2005; Pinzer et al, 2012; Calonne et al, 2014; Krol and Löwe, 2016). A proper understanding of vapor transport in snow at the macroscopic scale is a prerequisite for accurate snowpack physical modeling

Objectives

Findings

Discussion

Conclusion