Characterizing airborne snow metamorphism using stable water isotope measurements in snow and vapor from ring wind tunnel experiments

Drifting-blowing snow events are frequent phenomena in alpine and polar regions with direct effects on the local mass and energy balance that are difficult to quantify. In addition to this immediate impact of the blowing snow cloud the aeolian transport modifies the snow’s physical properties with implications for snowpack characteristics after deposition such as albedo or density. Apart from the mechanical fragmentation of crystals, this phenomenon of airborne snow metamorphism is thought to be due to sublimation and vapor deposition at suspended particles. Direct insitu observations, however, are virtually impossible due to the micro-scale nature of the process and the absence of Lagrangian observation techniques in a blowing snow cloud. Thus, coupled climate and snowpack models use parameterizations based on empirical data from before and after blowing snow events to simulate evolving snowpacks in a changing climate. However, as airborne metamorphism reacts instantaneously to variable atmospheric conditions, such bracketing observations are not sufficient for high-resolution modeling. Since airborne metamorphism is unfolding on the micro-scale, variables that reflect this process at the macro-scale such as stable water isotopes are key for process understanding. Here we show that aeolian transport of snow involves airborne metamorphism by presenting the evolution of the isotopic composition of snow and water vapor, and the linked evolution of the snow’s physical properties during blowing snow experiments. Our observations encompass snow and meteorological variables from 18 ring wind tunnel experiments under varying [-20°C to -3°C] temperature conditions. The observed evolution of the snow and vapor isotope signal supports the theory of airborne snow metamorphism with periods of dominating sublimation or deposition, which are clearly visible in the secondorder parameter d-excess. The general decrease in the snow dexcess value varies in magnitude with a maximum recorded decrease of 6‰ in 70 minutes. Our findings encourage the use of stable water isotopes as a means to constrain airborne metamorphism and to develop adequate model parameterizations. Further, they question the validity of preserved source region information in the d-excess signal of snow in dry high altitude and polar regions.