And now it’s time for the weather

© Gernot Walter

Turbulence researcher Eberhard Bodenschatz fires lasers at clouds to help him solve the mystery of why it rains. Meanwhile Jean-Pierre Wolf, an expert in nonlinear optics, is hoping to use his lasers to make his own weather.

GGermany’s highest-altitude building is packed to the rafters with high-tech equipment. The Schneefernerhaus was originally set up as a hotel for tourists on the Zugspitze at 2,656 meters above sea level. But in the 1990s, the last hotel guests left and the scientists arrived, transforming the building into an environmental research center. Now an ultrashort pulse laser from TRUMPF has made its way up the Zugspitze, too. Eberhard Bodenschatz, professor of experimental physics at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, purchased the device for his lab in the Alps: “We’re hoping to use the laser to reveal the dynamics of water droplets and ice particles in clouds so we can see exactly what’s going on.”

Capturing patches of turbulence on video

Eberhard Bodenschatz

Photo: Max Planck Institute for Dynamics and Self-Organization

Eberhard Bodenschatz is Professor of experimental physics at the Max Planck Institut for Dynamics und Self-Organization in Göttingen. For his lab on the Zugspitze he purchased an ultrashort pulse laser from TRUMPF.

The laser light cable runs from the laser through the laboratory ceiling to a flat roof where a climatized, waterproof container the size of a beer crate is positioned on a seven-meter-long rail. The crate contains four high-speed cameras. The researchers in the lab, and the measuring device on the roof, wait patiently for the weather to deteriorate—and they start collecting data the moment a cloud moves across the roof.

The optics system expands the laser light to a diameter of five centimeters and the light pulses are projected at a vertical angle toward the camera box onto the cloud particles. The forward scattering causes the particles to light up with every pulse of light, and the cameras record this in the form of stereoscopic images taken at a distance of around 60 centimeters from the lens. In order to create a three-dimensional video of the cloud particles, the high-tech crate keeps pace with the cloud’s average speed as it travels along the rails (# 1).

This enables the cameras to track individual cloud particles, snapping about 15,000 images a second. These images are then instantly coverted into 3D pictures by the computer. It takes just one second to complete the measurements. Then the sled returns to its starting position and the entire process starts over. This is the method the researchers use to film small patches of turbulence—each a few cubic centimeters in size—in order to discover what happens to tiny cloud particles in that single second of time. “Each measurement tracks somewhere between 300 and 1,000 individual cloud particles, each of which is no bigger than a few micrometers. We’ve been doing that in our wind tunnel for a while, but this summer we’ll be using our new laser to perform the first measurements on real clouds on the Zugspitze,” says Bodenschatz.

Poetry of the Clouds: The Project of Prof. Eberhard Bodenschatz on the Zugspitze on Video.

Laser flashes

To make the motion of the swirling droplets visible, Bodenschatz and his team need plenty of light. “That’s why we need the ultrashort pulse laser. It delivers up to 50,000 flashes a second with 40 millijoules per pulse. Those parameters give us enough photons in the measurement zone to make the swirling droplets visible,” Bodenschatz explains. He notes that the laser pulses have very little effect on the droplets themselves: “At most they get a little warmer, but that doesn’t affect how they move.”

I want us to finally get to the bottom of turbulent mixtures, and clouds are the perfect test environment.

Prof. Eberhard Bodenschatz, Max-Planck-Institut for Dynamics and Self-Organization

Bodenschatz is currently developing another experiment designed to investigate droplet distribution in clouds on a larger scale. His idea is to hang a high-speed camera from a long rope and fly it directly into a cloud using a balloon-kite hybrid known as a Helikite. “We fan out laser light from below, firing high-intensity, green pulses around 100 meters into the cloud. That makes the turbulence visible—and with the TRUMPF laser we finally have a beam source that is up to the task.” (# 2)

Three unsolved puzzles

However fascinating he finds the weather, Eberhard Bodenschatz really has his sights set on a broader context. He has devoted virtually his entire career to investigating turbulent fluxes. “We already have a few equations for this field of physics, but the process is so complex that the parts we understand are just the tip of the iceberg. And when it comes to inertial (i. e. heavy) particles in turbulence, we don’t have any equations at all! So, the only solution is to collect data on particles in the turbulence and subject it to statistical analysis. Clouds are perfectly suited to studying these complex processes because they occur in nature and consist of just four ingredients: water vapor, ice, airborne particles known as aerosols, and, of course, air.

“My goal is to understand how collisions and evaporation occur in turbulent mixtures.” Bodenschatz hopes this will allow him to draw conclu-sions on other mixing processes such as those that take place in ocean currents, technical sprays, and even combustion engines.

Using lasers for cloud measurements and weather Manipulation: All These experiments are already up and running in the lab. But in recent years Researchers have made the leap to real weather.

The combustion process in a diesel engine, for example, mixes together thousands of individual components. “Understanding clouds eventually improves one’s understanding of combustion, too.” And clouds harbor yet another secret. Rain only falls when numerous tiny droplets suspended in the air combine to form one large raindrop. This raises the fundamental question of how large droplets are formed from smaller ones.

“And that’s something physics can’t answer at the moment. The droplets are heavier than air and spin off at an angle as they whirl around. We understand the process pretty well on an individual level, but unfortunately we don’t understand the ‘avenues’ of turbulence, and those are changing all the time,” says Bodenschatz. “The droplets live in the wildest rollercoaster imaginable. What we’re interested in now is how many collisions take place and how this causes larger droplets to form which then fall toward the ground, swallowing up numerous tiny droplets on their way.”

This question of how small particles combine to form larger ones in turbulence is also relevant to the formation of planets. “Look at our current models and you would be forced to conclude that space shouldn’t contain any clumps of matter with a diameter larger than about one meter, since they burst apart when they collide. To have the kind of gravitational pull required to cluster matter together you would need clumps measuring at least 100 meters across. “Obviously we know that planets exist. But how is that even possible? We’re hoping that our experiments on particles in turbulence will give us some clue as to how our world was formed in the first place.”

It won’t be long before climate change becomes so serious that we’ll be actively intervening in weather processes.

Prof. Eberhard Bodenschatz, Max Planck Institute for Dynamics and Self-Organization

An improved understanding of clouds will also have some more immediate, practical benefits, such as improving the accuracy of predicting when and where it will rain or snow. “Wet clouds produce rain up to ten times sooner than current models predict. And I believe that has something to do with turbulence.” For climate researchers, cloud formation is one of the most important issues of all. A huge community of scientists is currently striving to create better climate models—and Bodenschatz’s work forms part of these efforts, too. “But soon things will develop even further: it won’t be long before climate change becomes so serious that we’ll be actively intervening in weather processes.”

Stretching things out

Jean-Pierre Wolf

Photo: Prof. Jean Pierre Wolf

Jean-Pierre Wolf is Professor at the University of Geneva and is an expert in nonlinear optics. He wants to use laser beams to influence cloud formation and control lightning.

Jean-Pierre Wolf has already reached precisely this stage at the University of Geneva. He wants to use laser beams to influence cloud formation and control lightning. The Swiss expert in nonlinear optics came up with the idea while investigating how air reacts when you expose it to a high-power laser beam. His results showed that the air ionizes. This is due to a phenomenon known as Kerr-induced self-focusing: the field strength of a high-intensity laser beam affects the refractive index of the air in such a way that the air itself acts as a kind of focusing lens for the laser beam.

This creates high intensities that ionize the air. Electrons are released, defocusing the laser beam and causing the entire process to start over. The beam remains stable, continuously focusing itself. “With the right beam source, we can stretch the focus out to a length of one hundred meters,” says Wolf. “Ionization produces plasma channels known as filaments—and that’s what we can use to affect the weather.”

Using lasers to trigger lightning from clouds

Wolf is currently working on three applications for his filaments, the first of which is to trigger lightning in clouds. “The filaments trigger discharges and the lightning follows the channel. So, in addition to triggering lightning, we can also cause it to discharge in a certain direction,” says Wolf. This gives Wolf two possible ways of eliminating the risks posed by thunderclouds. He can either trigger lightning within a cloud that never actually reaches the ground—discharging the cloud until it becomes calmer (# 3)—or he can use the filament to guide the lightning to a standard lightning conductor on the ground (# 3a). “There is huge demand for improved protection. The costs associated with thunderstorms and lightning strikes run to five billion dollars a year in the US alone, primarily due to disrupted air traffic and damage to aircraft and power lines.”

Wolf would like to see stationary laser systems installed around airports and power plants, which could discharge approaching storm clouds before they pose any danger. “I think we’ll be ready to do that in five years’ time. It works perfectly under laboratory conditions and we’ve already run successful tests outdoors, too.” The key to success is choosing the right beam source. “You need a femtosecond laser with a peak pulse power of one terawatt and a high, stable repetition rate of more than one kilohertz. TRUMPF Scientific Lasers is currently developing a laser for me with those specifications.” The collaboration stems from some trial measurements for lightning conduction carried out at the TRUMPF laboratory in Munich.

The man who wants to control the weather:  Prof. Jean-Pierre Wolf explains on the Video, how he want´s manipulate the weather using the laser.

Delaying rainfall

The laser-generated filaments can also be used to affect the weather in other ways. Wolf converts vapor into small water droplets that hover in the air — in other words, he creates clouds. For water vapor to condense in the air, you need surfaces on which the phase transition can take place, in this case aerosols such as dust or sand. “Ionization with the high-power laser enables us to make the existing aerosols more hydrophilic. They attract more moisture, forming droplets where there were none before,” says Wolf (# 4). “What we can’t do is make the cloud rain. Once we’ve created it, it simply lives out its natural lifespan.”

If we use lasers in the right way, we could eliminate the damage caused by lightning strikes in the future.

Prof. Jean-Pierre Wolf, University of Geneva

Where Wolf has succeeded, however, is in preventing wet clouds from raining for a certain period of time under labo-ratory conditions. To do this, he draws on the principle investigated by Eberhard Bodenschatz, which states that small droplets do not fall. By turning the aerosols contained in the cloud into genuine moisture magnets, Wolf ensures that the water spreads itself over multiple surfaces. “The droplets split up and are not big enough to fall to the ground. In the future, this could enable us to prevent wet clouds from raining until they are over dry areas. This could help us combat both droughts and flooding,” Wolf explains, setting out his vision of how the technology could be used. (# 5)

Drilling holes in clouds

Wolf also hopes to use his filaments to improve communication between satellites and ground stations, which is often obstructed by clouds and fog. In the course of his experiments, Wolf hit upon a further phenomenon of ionization. The jump in temperature of the air molecules triggers a shock that produces a sudden sound wave.

“We can use this acoustic explosion to push aside droplets in mist and clouds, basically using the long laser focus to drill a channel through the clouds.” This requires the individual shocks to take place at very short intervals. “And that’s why this application is all about having a very high repetition rate”. The hole in the clouds could be used to transfer information between the Earth and space without anything getting in the way—and the data transfer could also take place via laser!

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