The undervalued role of turbulence in cloud droplet formation

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Laboratory experiments using a cloud chamber reveal how small-scale air turbulence influences the process of cloud droplet formation. Results recently published in the Journal of Geophysical Research: Atmospheres.

Clouds form when volumes of air are cooled sufficiently large to condense the water vapor they contain. Indeed, as cold air can contain less water vapor than hot air, the drop in temperature ends up inducing saturation. As the air can no longer take charge of all the vapor it initially carried, the excess quantity condenses into droplets of liquid water or ice crystals. This is when the water becomes visible and our eyes perceive clouds.

Most of this cooling occurs in connection with the upward movements found in low pressure areas, for example. As this rise is associated with a drop in pressure, the temperature decreases accordingly. But the factors mentioned so far are not sufficient to lead to the formation of clouds as we know them. The presence of small dust, around which the water will be able to condense, is also necessary. Of multiple and heterogeneous origins, they populate the atmosphere from the poles to the equator for the first 20 kilometers of altitude. We speak more commonly ofaerosols.

Droplets produced in a fog chamber illuminated by a green laser beam (left). Note the turbulence, the effects of which have been isolated by the researchers. A study that would be almost impossible to carry out in situ (right). Credits: Abu Sayeed Md Shawon.

The importance of turbulence in the condensation process

Generally, theoretical studies and laboratory experiments did not take into account the turbulence within the air volumes involved in cloud formation. Also, the physicists made the assumption that they were well described by a single value of humidity and temperature. However, recent measurements have shown that the turbulence produced significant heterogeneity of parameters within a given volume. In fact, the saturation threshold may be modified by 0.3% or even more.

In this context, researchers from theMichigan Technological University (MTU, United States) have recently studied its influence on cloud formation. To do this, they used a cloud chamber to replicate the processes at a scale for laboratory study. Indeed, such an analysis would be almost impossible to carry out in field conditions (say in situ). Initially moist and devoid of aerosols, the chamber was subjected to differential heating to produce turbulence. Finally, it was loaded with sodium chloride particles to allow condensation. Note that the device of MTU is, with that of Leipzig (Germany), the only one to allow this type of experimentation.

By isolating the effect of turbulence, scientists found that it caused fluctuations large enough to determine if, locally, the air crossed or not the saturation threshold. “These fluctuations can increase the fraction of aerosols that become cloud droplets and, in general, mimic the effect of heterogeneity related to the size and chemical composition of these aerosols”, indicates the paper in its summary. These results have notable implications for transition regimes. For example between the cloudy and non-cloudy phase or for systems passing from an oceanic to terrestrial zone.

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