Artificial hail for more accurate weather forecasts

Date:
March 25, 2022
Source:
Johannes Gutenberg Universitaet Mainz
Summary:
A vertical wind tunnel has supplied important data to facilitate
the prediction of heavy rain, hail, and graupel precipitation.



FULL STORY ==========================================================================
When the low-pressure system dubbed Bernd decided to park itself over
part of Central Europe in the summer of 2021, the hazards associated
with excessive rainfall events were made dramatically apparent in the
form of the resultant catastrophic flooding. Weather records show that
extreme natural occurrences such as drought, but also heavy rainfall
and hail storms, are likely to occur even more frequently in this part
of the world as a result of climate change.

And their consequences could become even more devastating. Hailstones,
for example, can cause damage to crops, vehicles, and buildings and they
can be dangerous for exposed humans and animals, too. It is thus all
the more important that weather models are capable of most accurately predicting the possibility and extent of any such precipitation. For
this, the numerical weather models must be based on precisely formulated mathematical interpretations of the physical processes in clouds.


==========================================================================
The vertical wind tunnel at Johannes Gutenberg University Mainz (JGU),
which is the only one of its kind in the world, is providing essential information in this connection through new experiments that are being
conducted using artificial hailstones made by a 3D printer. "One thing we
have learned so far is that it is the form of hailstones that determines
their velocity prior to impact," explained Dr. Miklo's Szaka'll of the Institute of Atmospheric Physics (IPA) at JGU. Szaka'll's team has been
able to demonstrate that lobed hailstones develop less kinetic energy
and thus less destructive potential than hail with a smooth surface.

Hail and graupel, which is the term used to describe precipitated
small, soft ice pellets, are formed when water droplets freeze within
storm clouds. This freezing process is promoted by turbulences and
complex physical processes in these clouds that can extend to very high altitudes. These ice particles melt if they pass through warmer air layers
on the way down. The result is large, cold raindrops and these are often
the culprits behind extreme rainfall precipitation. Assuming that the
ice particles do not have time to melt completely before reaching the
ground, they arrive in the form of hail or graupel.

Experiments with natural and artificial hailstones The conditions
in the interior of clouds determine the characteristic form, size,
and mass of these frozen droplets. "In our experiments with natural
hailstones, we have seen that they melt to form raindrops that can be
several millimeters in diameter. Large hailstones can also burst during
the melting process, forming numerous small water droplets," Szaka'll
added. From the recorded measurements, his team was able to extrapolate parameters that they could use as the main elements for the numerical simulation of clouds and precipitation in computer models.

The research team in Mainz produced hailstones and graupel particles from frozen water in the lab. Employing realistic temperature and humidity conditions, the researchers looked closely at how these fell or melted in
the vertical wind tunnel. In addition, they used a 3D printer to create artificial hail and graupel pellets modeled on their natural counterparts
-- even the material density corresponded with that of ice. They used
these to measure the free fall properties of the descending objects,
factors that are particularly relevant to the microphysical processes
in extreme precipitation events.

The hail and graupel pellets were suspended freely in an artificially
produced vertical air stream 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 insights into microphysical aspects of precipitation we
have obtained through these experiments to 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 at the Max Planck
Institute for Chemistry. "This becomes particularly significant in view
of the probable increase in extreme weather events, such as drought and torrential rainfall, that will occur even in our part of the world due
to climate change," emphasized Borrmann.

The experiments in Mainz were undertaken 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 reviewing the HydroCOMET findings provided very positive assessments of the lab experiments performed in Mainz and the associated publications. They particularly stressed the important role played by
the available infrastructure, i.e., the vertical wind tunnel.


========================================================================== Story Source: Materials provided by
Johannes_Gutenberg_Universitaet_Mainz. Note: Content may be edited for
style and length.


========================================================================== Related Multimedia:
* Experimental_set-up_and_graupel_pellet ========================================================================== Journal Reference:
1. Karoline Diehl, Florian Zanger, Miklo's Szaka'll, 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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220325144653.htm

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