Snow metamorphism under alternating temperature gradients: Morphology and recrystallization in surface snow

The composition of the lower atmospheric boundary layer can be strongly influenced by snow photochemistry. Surface snow, where air‐snow interactions take place, is viewed as slowly changing quasi‐isothermal snow, implied by the observation of mainly rounded snow crystals. The role of snow for photochemistry is therefore expected to be purely geometric. However, large temperature gradients are often observed in these layers, which would imply high recrystallization and faceting. In controlled laboratory experiments we showed that temperature gradients on the order of 100 K m−1 do not lead necessarily to faceting if their sign changes with a daily cycle. The shape of snow crystals does not reflect necessarily the metamorphic process. With time‐lapse X‐ray tomography, recrystallization rates as high as 60% of the total ice mass were observed during 12 hours. The high recrystallization rates in apparently isothermal snow not only contradict the current understanding of snow metamorphism, they also might influence chemical air‐snow interactions.