When the low pressure system dubbed Bernd decided to lay over parts of central Europe in the summer of 2021, the dangers associated with excessive rainfall were dramatically highlighted in the form of the resulting catastrophic flooding. Weather records show that extreme natural events such as drought, but also heavy rains and hailstorms, are likely to occur even more frequently in this part of the world due to climate change. And their consequences could become even more devastating.
Hailstones, for example, can damage crops, vehicles and buildings and they can also be dangerous to exposed humans and animals. It is therefore all the more important that weather models are able to predict with the greatest accuracy the possibility and extent of such precipitation. For this, numerical weather models must be based on precisely formulated mathematical interpretations of physical processes in clouds.
The vertical wind tunnel at the Johannes Gutenberg University Mainz (JGU), unique in the world, provides essential information in this regard thanks to new experiments carried out using artificial hailstones produced by a 3D printer. “One thing we have learned so far is that it is the shape of hailstones that determines their speed before impact,” explained Dr Miklós Szakáll from the Institute of Atmospheric Physics (IPA) in JGU. Szakáll’s team was able to demonstrate that lobed hailstones develop less kinetic energy and therefore less destructive potential than smooth-surfaced hailstone.
Hail and graupel, which is the term used to describe precipitated small pellets of soft ice, form when water droplets freeze in thunderclouds. This freezing process is aided by turbulence and complex physical processes in these clouds which can extend to very high altitudes. These ice particles melt if they pass through warmer layers of air on their way down. The result is large cold raindrops and these are often the culprits of extreme precipitation. Assuming that the ice particles do not have time to melt completely before reaching the ground, they arrive as hail or graupel.
Experiments with natural and artificial hailstones
Conditions inside clouds determine the characteristic shape, size, and mass of these frozen droplets. “In our experiments with natural hailstones, we have seen that they melt to form raindrops up to several millimeters in diameter. Large hailstones can also burst during the melting process, forming many small water droplets,” Szakáll added. From the recorded measurements, his team was able to extrapolate parameters that they could use as primary inputs for numerical simulation of clouds and precipitation in computer models.
The Mainz research team produced hailstones and graupel particles from frozen water in the laboratory. Using realistic temperature and humidity conditions, the researchers closely examined how these fell or melted in the vertical wind tunnel. Additionally, they used a 3D printer to create artificial hail and graupel pellets modeled after their natural counterparts – even the density of the material matched that of ice. They used them to measure the free-fall properties of descending objects, factors particularly relevant to microphysical processes during extreme precipitation events.
The hail and graupel pellets hung freely in an artificially produced vertical air current in the six meter high wind tunnel. Their behavior was recorded using high-speed and infrared cameras and a specially developed holographic imaging system.
“If we apply the knowledge about the microphysical aspects of precipitation that we have obtained through these experiments to the models used for the analysis of storm clouds, we can better anticipate what they will do,” explained Professor Stephan Borrmann of the IPA and director of the Max Planck. Institute of Chemistry. “This becomes particularly important given the likely increase in extreme weather events, such as drought and torrential rains, which will occur even in our part of the world due to climate change,” Borrmann stressed.
The experiments in Mainz were carried out under the aegis of the HydroCOMET project sponsored by the German Research Foundation (DFG). The results have been published in five peer-reviewed journals and as a book contribution.
The experts who reviewed the results of HydroCOMET provided very positive assessments of the laboratory experiments carried out in Mainz and the associated publications. They particularly underlined the important role played by the available infrastructure, ie the vertical wind tunnel.