When a low-pressure system called Bernd decides to park itself in the Central European part of the summer of 2021, the dangers associated with heavy rainfall events have been made more noticeable in the form of catastrophic floods resulting. Climate records show that extreme natural disasters such as droughts, but also heavy rains and hail, are likely to occur more frequently in this part of the world due to climate change. And their consequences can be even more devastating.
Hailstones, for example, can cause damage to plants, cars, and buildings and can be dangerous to humans and animals, too. It is therefore very important that weather models are able to accurately predict the probability and extent of any such weather. In this regard, numerical climate models should be based on defining well-constructed statistics of processes visible in the cloud.
3D Hail printer
The air tunnel stationed at Johannes Gutenberg University Mainz (JGU), the only one of its kind in the world, provides valuable information for this interactive experiment using new hailstones made by a 3D printer. “One thing we have learned so far is that the type of hailstones that determine their speed before impact,” explains Drs. MiklósSzakáll of the Institute of Atmospheric Physics (IPA) at JGU. Szakáll’s team has been able to show that hailstones develop less kinetic energy and thus have more destructive power than smooth hail.
Hail and graupel, which is a term used to describe small, soft snow pellets, are formed when water droplets form ice in the middle of storm clouds. This cold process is triggered by the chaos and complex physical processes in these clouds that can extend to higher altitudes. These glaciers dissolve as they pass through layers of warm air as they descend. The result is heavy, cold drops and these are often the cause of heavy rainfall. Assuming that the ice particles do not have time to fully thaw before they reach the ground, they come in the form of hail or graupel.
Exploration with natural and artificial hailstones
The conditions within the clouds determine the type, size, and weight of these ice droplets. “In our experiments with natural hailstones, we found that they melted to form raindrops about a few millimeters in diameter. Heavy hailstones could explode and during melting, we would create many small water droplets,” Szakáll said. From the recorded measurements, his team was able to extract the parameters they could use as key elements for simulating cloud and rain numbers on computer models.
A team of researchers in Mainz produced hailstones and graupel particles in ice-cold laboratory. Using real-time temperatures and humidity conditions, researchers are looking at how these falls fall or melt in a vertical air tunnel. In addition, they have used a 3D printer to create artificial hail and graupel pellets that are similar to their natural counterparts – even the density of material is consistent with that of ice. Use this to measure the free fall features of descent, features that are closely related to microphysical processes in extreme rain events.
Trying Strom clouds models
Hail pellets and graupels are hung loosely in dry air produced by the artificial air tunnel six feet [6 m] high. Their behavior was recorded using high-speed and infrared cameras and a specially developed holographic imaging system. and Director at the Max Planck Institute for Chemistry.
The experiment in Mainz was conducted under aegis of the HydroCOMET project funded by the German Research Foundation (DFG). The results were published in five peer-reviewed journals as well as a book donation. Experts reviewing HydroCOMET’s findings provided excellent reviews of laboratory tests conducted in Mainz and related publications. They particularly emphasize the important role played by the available infrastructure, namely, the dry air tunnel.
Source Journal Reference: Karoline Diehl, Florian Zanger, MiklósSzakáll, Andrew Heymsfield, Stephan Borrmann. Vertical wind tunnel experiments and a theoretical study on the microphysics of melting low-density graupel. Journal of the Atmospheric Sciences, 2021; DOI: 10.1175/JAS-D-21-0162.1
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