The GIGAWATT project is taking a closer look at a particular kind of current known as atmospheric gravity waves. These often originate in the lower atmospheric layers and spread to an altitude of over 100 kilometres, similar to waves in the ocean with the exception that they travel not only horizontally but also vertically. These gravity waves break in the area between the edge of the atmosphere and space, causing chaotic turbulence.

This process is of particular interest to researchers as the fate of the gravity waves has an impact on our climate. “To an extent, the waves set the course for the large earth-spanning flow systems,” explains Michael Bittner, professor for atmospheric remote sensing at the University of Augsburg. “These include, for example, the high-altitude wind systems that control the exchange of air between the Earth’s poles. In climate models, the effects of gravity waves have only been modelled very imprecisely up until now.”

One reason for this is that the spread and refraction of waves is physically well understood. However, the systems of equations that describe this process are so complex that not even the fastest supercomputer can solve them. Researchers therefore use approximations called parameterisations to model the development of the waves. “But these models must be fed with the most accurate initial data possible in order to deliver a realistic result,” explains Bittner’s colleague, Dr Patrick Hannawald.

Radar systems and optical cameras in the Alps

This means that we have to know where the waves are and how they are behaving in order to be able to extrapolate their future course of development. The more accurate the information, the better the result. However, until now it was very difficult to detect the waves in the area between the edge of the atmosphere and space. This is where the new project comes in: the participating research groups would like to make the gravity waves more accurately visible using a variety of different methods. “Together with our project partners, we will install radar systems and optical cameras in the German and Swiss Alps,” says Hannawald. “This will allow us to carry out topographical measurements.”

With this approach, the wave fronts can be visualised in their three-dimensional expansion. Researchers are using a phenomenon known as airglow: molecules in the upper atmosphere are excited by high-energy radiation from the sun, causing them to faintly glow permanently. From space, this can be seen with the naked eye, and with the cameras this should be possible from the ground. Areas with higher air pressure – the crests of the waves – glow particularly strongly. “From the intensity patterns, the course of the waves can be deduced,” explains Hannawald.
Characteristic structures are often visible in the photos, similar to ripples on a sandy beach at low tide. When the waves break, they leave behind a kind of spray trail in the photos. These results are not only relevant for climate research but also for other areas of research. “At the moment, the number of satellites in low orbit around the Earth are growing exponentially,” says Bittner. “A new area of industry is being established there. The satellites only have a limited life expectancy and at some point they start flying lower and lower and eventually crash.”
When via this process they finally plunge into the upper atmospheric layers with a speed of several kilometres per second, they slow down considerably. “It is in these areas, the areas in which the gravity waves break, that the satellites are strongly shaken like a car on a slope with potholes,” explains Bittner. This is one of the reasons why the trajectory of high-tech flying objects has not been so easy to predict.

The results from the GIGWATT project could also enable progress here. Perhaps one day it will be possible to the control how satellites crash at the end of their lives so that the debris lands in the sea and not over inhabited areas.

The German Research Foundation (DFG) and the Swiss National Science Foundation (SNSF) are funding the GIGAWATT project for three years with a total of €1.2 million.