When the low-pressure system known as Bernd decided to park itself in parts of Central Europe in the summer of 2021, the hazards associated with excessive rainfall events in the form of the resulting catastrophic flooding became apparent. Weather records show that extreme natural events such as droughts, torrential rains and hail are likely to occur more frequently in this part of the world due to climate change. Their consequences could become even more devastating. For example, hail can cause damage to crops, vehicles and buildings, and can also be dangerous to exposed humans and animals. Therefore, it is more important that weather models can most accurately predict the likelihood and extent of any such precipitation. To do this, numerical weather models must be based on precise mathematical interpretations of the physical processes in the cloud.
The vertical wind tunnel at Johannes Gutenberg University Mainz (JGU), the only one in the world, provides important information in this regard through new experiments using artificial hailstones made with 3D printers. Dr Miklós Szakáll from JGU’s Institute of Atmospheric Physics (IPA) explained: “One of the things we’ve learned so far is that the form of hailstones determines their velocity before impact.” Szakáll’s team has been able to demonstrate that with Leaf-shaped hail produces less kinetic energy than smooth-surfaced hail and therefore has less destructive potential.
Hail and graupel are terms used to describe the precipitated small, soft ice particles that form when water droplets freeze in storm clouds. This freezing process is facilitated by turbulence and complex physical processes in these clouds, which can extend to very high altitudes. If these ice particles pass through a layer of warm air on their way down, they will melt. The result is large, cold raindrops, which are often the culprits of extreme rainfall precipitation. Assuming the ice particles don’t have time to melt completely before reaching the ground, they arrive as hail or graupel.
Experiments with natural and artificial hail
Conditions inside the cloud determine the characteristic form, size and mass of these frozen droplets. “In our experiments with natural hailstones, we have seen them melt to form raindrops that can be up to a few millimeters in diameter. Large hailstones can also break up during the melting process, forming many small water droplets,” Szakáll added. From the recorded measurements, his team was able to deduce parameters that could be used as the main elements of numerical simulations of clouds and precipitation in computer models.
The Mainz research team created hail and graupel from frozen water in the laboratory. Using realistic temperature and humidity conditions, the researchers took a closer look at how they fell or melted in the vertical wind tunnel. In addition, they used 3D printers to create artificial hail and graupel that mimic their natural counterparts—even the density of the material corresponds to that of ice. They used these to measure the free-fall properties of descending objects, factors that are particularly relevant to microphysical processes in extreme precipitation events.
In the six-meter-high wind tunnel, hail and graupel grains are freely suspended in an artificially generated vertical airflow. Their behavior was recorded using high-speed and infrared cameras and a specially developed holographic imaging system.
“If we apply the insights from the microphysics of precipitation gained through these experiments to the models used to analyze storm clouds, we can better predict what they will do,” said Professors Stephen Borman and Max Planck of IPA. The director of the Institute of Chemistry explained. “This is especially important given that extreme weather events such as droughts and torrential rains are likely to increase, even in our region with climate change,” stressed Borrmann.
The experiments in Mainz were carried out with the support of the HydroCOMET project sponsored by the German Research Foundation (DFG). The findings have been published in five peer-reviewed journals and as a book contribution.
Experts reviewing the results of the HydroCOMET survey provided a very positive assessment of the laboratory experiments conducted in Mainz and related publications. They particularly highlighted the important role played by existing infrastructure, namely vertical wind tunnels.