From the moment snow grains start to fall, they undergo continuous changes in their physical and isotopic properties, driven by mechanical and thermodynamic processes alike. Besides the processes occurring within the snowpack, wind can also influence snow grains throughout their journey. The blowing snow particles are subjected to mechanical stresses, as well as to a recently described airborne snow metamorphism (ASM), involving concurrent sublimation and vapor deposition at the particle surface. This newly identified metamorphism is particularly relevant in polar regions, where wind can transport snow particles over long distances and for extended durations before deposition. As a result, ASM strongly alters the physical properties of snow particles and the resulting microstructure of the snow surface. In particular, it modifies the specific surface area (SSA), defined as the total ice-air interface area per unit mass of snow and which encompasses grain shape and size. SSA is a key parameter controlling how snow interacts with light, and therefore snow albedo, i.e. the fraction of sunlight it reflects. ASM also has implications for the surface energy and mass balance. Beyond its physical effects, ASM also modifies the snow stable water isotopic composition, i.e. the ratio of heavy to light water molecules, since any thermodynamic process involving phase changes drives isotopic fractionation, namely, the preferential partitioning of isotopes during phase changes. This signal is stored in snow, and subsequently in glaciers and ice sheets, where it is used as a proxy for past climate. ASM-induced changes to this signal must therefore be accounted for in ice core paleoclimate reconstructions. However, the dependence of ASM on wind speed, snow surface and air temperature, relative humidity and transport duration has not yet been quantified. Fully assessing the magnitude of these coupled thermodynamic and aerodynamic processes therefore requires experimental investigation under controlled laboratory conditions, in a facility providing long particle transport times, stable boundary conditions, and continuous recirculation.
We used a newly developed ring wind tunnel (RWT) installed in a cold laboratory to study airborne snow metamorphism under well-controlled transport and thermal conditions. The obround closed-circuit geometry enables sustained particle transport over extended durations while maintaining constant atmospheric boundary conditions, thus closely mimicking natural aeolian transport. The facility features enhanced thermal stabilization through a revised turbine integration and improved wall thermal coupling with the cold laboratory environment. An active snow surface temperature control allows independent regulation of air and surface temperatures. A dedicated measurement section monitors atmospheric parameters such as wind speed, air temperature, relative humidity, and snow temperature. A small quantity of air is also continuously drawn from the tunnel interior through a tube inlet into a cavity ring-down spectrometer to analyse its stable water isotopic composition. The isotopic composition of regularly collected snow samples is analysed in parallel, providing a proxy for the underlying phase change processes. In combination with atmospheric parameter monitoring and isotopic analysis, micro-computed tomography (micro-CT) is performed on regularly collected snow samples to characterize changes in microstructure.
Here, we report the first results of our experiments confirming the occurrence of ASM in the newly developed ring wind tunnel facility, evidenced by concurrent changes in stable water isotopic composition and snow microstructure. We will discuss how the intensity of this metamorphism varies with transport duration, wind speed, air and snow temperatures, and relative humidity. This work provides an initial controlled quantification of ASM and constitutes a step towards improved representation of this process in snow and climate models. It also lays the groundwork for future extensive parametrizations, to be complemented by planned field measurements at Neumayer III research station in Antarctica.