When snowflakes are blown by the wind, they are subjected to massive forces that crush the delicate crystals. But the wind also ensures that the water in the snow is alternately solid and gaseous, as experiments in the wind tunnel have surprisingly shown. Depending on the weather, the snow absorbs or releases water vapor from the air. This wind-induced metamorphosis of snow crystals and their interactions with the atmosphere have not yet been taken into account in climate models. However, they can have a massive impact on the local water balance. In particular, the models for the polar regions, where snow drifts are common, now need to be adjusted.
Depending on the ambient temperature, water has three states: It exists as solid ice or snow, liquid water or gaseous water vapor. But anyone who has ever been caught in a snowstorm knows that not all snow is the same. Sometimes the flakes are delicately soft, other times they are icy hard – especially when the snow is blown by the wind. Such snowdrifts are common in mountainous and polar regions. Huge masses of snow sometimes shift. But does the wind change the shape and condition of the snow crystals?
Massive interactions between snow and atmosphere
A team led by Sonja Wahl from the École polytechnique fédérale de Lausanne (EPFL) in Sion has now investigated this using experiments in a ring-shaped wind tunnel. The climate researchers used microcomputed tomography to analyze the size, shape and composition of the snow crystals. They also analyzed the proportions of various natural isotopes of hydrogen and oxygen in the water molecules. Because solid, liquid and gaseous water contain different proportions of the heavy and light hydrogen and oxygen isotopes, it is possible to draw conclusions about the respective state of the snowflakes. Phase transitions, for example from solid to gaseous, can also be recognized.
The experiments showed that when the wind blows snowflakes around, it mechanically reshapes the ice crystals by crumbling and grinding them. They become smaller and deformed. In addition, the water in the snow crystals often changes back and forth between solid and gaseous phases in the wind. Some of the sublimated water vapor returns to the atmosphere and the crystals shrink. This allows smaller flakes to dissolve completely. However, when there is particularly heavy snowfall, the opposite process can also occur: snow crystals then absorb the water vapor from the environment, solidify it and thereby grow over time. They then form a more rounded shape.
“The results of the wind tunnel studies showed that there is a temperature difference between particles and air, especially for larger particles, which favors the deposition of vapor and thus particle growth, while smaller particles are completely sublimated,” said the team. This change in the water aggregate states in the snow changes the local energy balance and also the air humidity and water balance in the area. “It is a previously unobserved process,” say the researchers.
Models of climate in polar regions could be wrong
This gas exchange between snowflakes and the atmosphere changes the isotope ratios in snow on Earth – especially in the polar regions, where the wind shifts and transforms huge amounts of snow. In addition to temperature, wind also influences the state in which we find water on Earth. This is also reflected in the snow compacted into ice in glaciers and deeper layers in the polar regions. Ice cores from polar regions can be used to look back at what the weather and climate was like on Earth thousands of years ago.
However, researchers have so far incorrectly only read the temperature at the time of snowfall from these drill cores and the isotopes they contain, but have not taken the influence of the wind into account. This data was then incorporated into models for forecasting further climate developments and could therefore be wrong, as Wahl and her colleagues explain. In order to correctly interpret the isotopes in the ice cores in the future and improve polar climate models, the influence of wind must also be taken into account, conclude Wahl and her colleagues. The researchers now want to investigate the newly discovered phenomenon in more detail and integrate it into climate models.
Source: Sonja Wahl (École polytechnique fédérale de Lausanne) et al.; The Cryosphere, doi: 10.5194/tc-18-4493-2024