Sights on hailstones and heavy rain drops: In order to gain insights into the physical basis of extreme precipitation, German researchers let real and 3D-printed plastic hailstones whiz through a wind tunnel that is unique in the world. You have already gained information that can benefit predictive models.
Sometimes the weather is known to hit hard: hail or sleet showers as well as driving rain are pattering from the sky. These extreme forms of precipitation can cause serious damage to agriculture and, above all, hailstones endanger buildings and vehicles as well as people and animals. As is already apparent, these threats are expected to increase as a result of climate change. It is therefore important to study the processes involved in the formation of extreme forms of precipitation. In this way, risks can be better assessed and weather models can be developed in order to be able to predict the occurrence and extent as best as possible.
What happens fundamentally is well known: hail and sleet are formed when water droplets freeze in the storm clouds that reach very high into the atmosphere. Depending on the conditions in the cloud, the frozen particles acquire their characteristic shape, size and mass. If they then reach warmer layers when they fall, they can melt completely. This is how the large, cold raindrops that fall as driving rain are formed. On the other hand, if the time it takes for the ice particles to fall to the ground is not long enough to melt them completely, sleet or hailstones fall. However, there are uncertainties about the more precise processes and the cloud-physical circumstances.
Insights into the physics of precipitation
As Johannes Gutenberg University Mainz (JGU) is now reporting, Germany has a test facility that is unique in the world for researching hail and the like: the vertical wind tunnel at the Institute for Atmospheric Physics at JGU. In the six meter high facility, an air current is generated that corresponds to the headwind when hailstones and sleet fall. Various temperature and humidity conditions can be set as they can occur under real conditions. As the test objects move in the vertical airflow, the scientists can observe them closely with the help of high-speed and infrared cameras and a specially developed holographic image recording system, JGU reports.
For the experiments, the team of researchers from JGU and the Max Planck Institute for Chemistry in Mainz produced hailstones and sleet from ice in the laboratory. They were then able to examine exactly how these structures fall or melt in the facility. In addition, based on models from real versions, they produce artificial hail and sleet structures made of plastic, in which even the material density corresponds to the icy models. These structures could be used to study the flow properties of the falling objects and their destructive power.
Icy and artificial hailstones provide clues
JGU reports that the scientists have already gained new insights into the physics of precipitation through their experiments. “In our experiments with real hailstones, we were able to show how, when they melt, they turn into raindrops several millimeters in size. We were also able to see how large hailstones burst during the melting process, creating numerous small water droplets,” says Miklós Szakáll from JGU. The researchers have already been able to incorporate the recorded processes into numerical simulations of clouds and precipitation.
As far as the experiments with the artificial hailstones are concerned, there were indications of how the shape and surface properties influence flight and the potential for destruction, reports JGU. “For example, we have shown to what extent the shape of the hailstones is decisive for their speed before impact,” says Szakáll. It was also shown that nubbed hailstones have less kinetic energy and therefore less destructive power than unevenly shaped smooth versions.
The wind tunnel has thus provided important data for a better understanding of the physical processes involved in heavy rain, hail and sleet, writes JGU. Stephan Borrmann from the Max Planck Institute for Chemistry concludes: “If you apply our microphysical description of precipitation obtained from these experiments to models for calculating thunderclouds, you can better predict their consequences. This is particularly important in view of the expected increase in extreme events in our latitudes as a result of climate change,” says the scientist.