Wind tunnel experiments to quantify the effect of aeolian snow transport on the surface snow microstructure

Abstract

Abstract. The evolution of the surface snow microstructure under the influence of wind is hardly understood but crucial for polar and alpine snowpacks. Available statistical models are solely parameterized from field data where conditions are difficult to control. Controlled experiments which exemplify the physical processes underlying the evolution of density or specific surface area (SSA) of surface snow under wind are virtually non-existing. As a remedy, we conducted experiments in a cold laboratory using a ring-shaped wind tunnel with an infinite fetch to systematically investigate wind-induced microstructure modifications under controlled atmospheric, flow and snow conditions. Airborne snow particles are characterized by high-speed imaging, while deposited snow is characterized by density and SSA measurements. We used a single snow type (dendritic fresh snow), cover wind speeds from 3 ms−1 to 7 ms−1 (for fixed temperature) and vary temperatures from -24 °C to -2 °C (for fixed wind speed). The measured airborne impact trajectories confirm the consistency of our coefficient of restitution with large scale saltation, rendering the setup suitable to realistically study interactions between airborne and deposited snow. Our measured densification rates in the deposit as a function of wind speed show clear deviations from existing statistical models, but can be re-parameterized through our data. The most drastic changes in densification and SSA rates of deposited snow are observed close to the melting point. This study, as a first of its kind, exemplifies a rich non-linear interplay between airborne and deposited snow particles which is discussed in view of a multitude of involved processes, i.e. airborne metamorphism, cohesion, particle separation and fragmentation.

Identifying airborne snow metamorphism with stable water isotopes

“…After settling, the PPP define energy balance-relevant characteristics of the resulting snowpack such as albedo (Domine et al, 2006), emissivity (Hori et al, 2013), density and specific surface area (SSA) (Proksch et al, 2015;Schleef et al, 2014b). Previous studies (Walter et al, 2023;Sommer et al, 2017) have shown that wind-driven aeolian transport of snow significantly affects the microstructure of the ultimately deposited surface snow, i.e. density increase and SSA decrease are intensified under the influence of wind.…”

“…Mechanisms that control the modification of the PPP and thus the resulting snowpack characteristics can be grouped in mechanical processes such as particle fragmentation, abrasion (Comola et al, 2017;Gromke et al, 2014;Clifton et al, 2006) and aggregation (Lo and Passarelli, 1982) or in thermodynamic (hereafter metamorphic) processes, such as sublimation at snow particle surfaces (Schmidt, 1982;Thorpe and Mason, 1966) and vapour deposition (resublimation) on the suspended snow particle (Sigmund et al, submitted;Yamaguchi et al, 2019;Sharma et al, 2018). In this context, Walter et al, (2023) first introduced the term "airborne snow metamorphism" which summarises the multiple cycles of sublimation and vapour deposition on the suspended snow particle resulting in modifications of the snow particle size and shape during particle transport by wind in analogy to isothermal and temperature-gradient metamorphism inside a stationary snowpack (Schleef et al, 2014b;Pinzer and Schneebeli, 2009b;Colbeck, 1982). However, the relative importance of the individual processes involved and their combined effect on the snow microstructure is still unknown due to missing observations.…”

“…However, the relative importance of the individual processes involved and their combined effect on the snow microstructure is still unknown due to missing observations. Snowpack or climate models that include wind-blown snow modules thus rely on field-based empirical parameterizations when incorporating wind-blown snow effects on snowpack characteristics, because the physical process understanding is limited (Walter et al, 2023).…”

“…Thus, in-situ observations are limited to point measurements within the snow cloud (Nishimura et al, 2014;Nishimura and Nemoto, 2005) or pre-and post-event monitoring of the snowpack characteristics (Walter et al, 2023). Field measurements are the foundation for wind-blown snow parameterizations that are currently implemented in snowpack and climate models (Vionnet et al, 2012;Liston et al, 2007;Lehning et al, 2002).…”

“…Field measurements are the foundation for wind-blown snow parameterizations that are currently implemented in snowpack and climate models (Vionnet et al, 2012;Liston et al, 2007;Lehning et al, 2002). As an approach to circumvent the difficulties of multiple unconstrained environmental variables during in-situ field observations, controlled laboratory experiments using straight wind tunnels (Nishimura et al, 2014;Wever et al, 2009) or ring-shaped wind tunnels with infinite fetch (Walter et al, 2023;Sommer et al, 2018b) have been used successfully to systematically validate model parameterizations. Additionally, shadowgraphy imaging based on high-speed camera recordings (Gromke et al, 2014) has been applied in wind tunnel experiments to investigate the vertical distribution of snow particle size and number.…”