From left: Erik Thomson, Jan Pettersson and Mattias Hallquist.

Solving the Mystery of Clouds

Most of us occasionally glance at the sky to decide whether to bring an umbrella. When a group of researchers at the University of Gothenburg look to the sky, they have more than that in mind – our future climate.

Clouds may seem uncomplicated to the layperson. Water evaporates and forms clouds as it cools down at high altitudes. Every so often, the water returns to the ground in the form of rain or snow. If you have ever tried to forecast weather, you know how difficult it is. Even professional meteorologists struggle, as we all know. Clouds are unpredictable, and we still don’t understand all the processes that take place inside them.

Several research teams at the University of Gothenburg are involved in cloud research. Jan Pettersson, professor of atmospheric science, and his colleagues are studying how ice particles are formed in the atmosphere. Cloud droplets freeze when exposed to aerosol particles in the air. The particles can be either naturally occurring or the result of emissions from human activities.
‘The way aerosol particles affect clouds is one of the most uncertain factors in modern climate models. The clouds cool the ground and generate necessary precipitation, and a better understanding of how they work would enable us to better predict the future.’

IMPROVED climate models would also help politicians make better climate-related decisions. But the necessary calculations are cumbersome.
‘Not even the best computers can simulate everything that goes on in a turbulent raincloud, since the difference in scale is too large – from turbulence-causing convection on a kilometre scale to turbulent winds measured in millimetres throwing small water drops around. It’s just too complex and we are forced to simplify things,’ says Bernhard Mehlig, professor in complex systems.

Professor Bernhard Mehlig studies turbulent aerosols with his colleagues at the Department of Physics. He is also a supervisor and lecturer in the Master’s programme in Complex Systems at the University of Gothenburg and Chalmers, where students study these types of issues in-depth.

Professor Bernhard Mehlig studies turbulent aerosols with his colleagues at the Department of Physics. He is also a supervisor and lecturer in the Master’s programme in Complex Systems at the University of Gothenburg and Chalmers, where students study these types of issues in-depth.

He and his colleagues are studying how small particles move in turbulent flows, such as inside a cloud. That the airflow inside a cloud can be turbulent is old news to anybody who has ever boarded an aircraft. Mehlig says that, according to an old theory from the early 1900s, stirring can speed up a reaction. Does the turbulence speed up the formation of rain by stirring the droplets and making them collide with each other?
‘When the drops have become so big they start falling down, they crash into each other and grow even larger. But we still don’t know how the small droplets grow. Turbulence speeds up the collisions, but we lack a model to describe how often they collide.’

Nor can the scientists explain the fast course of a monsoon rain. Is it due to other factors than the turbulence-caused collision of drops? The final goal is to parameterise the microscopic processes, or find formulas describing them in a manageable way.
‘We need to understand what happens at the microscopic level, so we’re studying the dynamics of water drops in just 1 cm3 of the cloud,’ says Mehlig.

Aerosols affect not only clouds but also the air we breathe. The air in Gothenburg is no exception, with a large share of the emissions coming from motor vehicles. Air quality is an important issue, especially in large cities where the traffic is the most problematic, or where the geographic conditions make the air remain in the area instead of getting diluted.
‘Bad air quality is associated with significant health risks. Air pollution causes a large number of early deaths every year around the world,’ says Pettersson.

Atmospheric chemist Mattias Hallquist studies the link between vegetation and particle formation, which in turn is directly linked to the climate. Vegetation releases gases that turn into particles in the atmosphere.
‘The largest share of the small particles in the atmosphere stem from conversion of gases,’ he says.

If most of the particles come from natural sources, how dangerous are the human-made particles, such as those emitted from my car? Very dangerous, says Hallquist. Particles from traffic can contain more harmful substances. When it comes to climate impact, different types of particles have different effects.
‘Particles from combustion, for example from motor vehicles, contain a lot of light-absorbing soot, whereas particles from vegetation are mostly transparent and only reflect light. One complication, however, is that particles from different sources interact and form much more complex mixtures with various effects on the climate.’

Then there is the health aspect.
‘Particles from vehicle traffic occur in an environment where there are a lot of people. There are not as many people in the middle of a forest in northern Sweden,’ says Hallquist.

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