Gravity waves affect the climate and weather. For the first time ever, scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), together with colleagues from the Karlsruhe Institute of Technology (Karlsruher Institut für Technologie; KIT) and the Jülich Research Centre (Forschungszentrum Jülich), as well as other national and international partners, have succeeded in measuring almost the entire life cycle of atmospheric gravity waves. On board DLR's High Altitude LOng range (HALO) and Falcon research aircraft, atmospheric researchers flew on coordinated measurement flights from Kiruna in northern Sweden.
Gravity waves are gravity-driven fluctuations in air masses. They are initiated in the lower layers of the atmosphere when, for example, air masses flow over mountains. To examine this previously insufficiently explored weather phenomenon, scientists filmed the interaction of gravity waves with the 'airglow' region of the upper atmosphere – at a height of approximately 85 kilometres – using a special camera. This 'airglow' is caused by chemical reactions and physical processes occurring at this altitude range. In addition, the scientists use a number of instruments – laser devices for wind measurements; trace gas and aerosol detection systems; an imaging infrared spectrometer for remote sensing of the three-dimensional distribution of temperature and trace gases; and equipment for the measurement of the concentration of trace gases in the atmosphere. The results of these experiments will help to improve climate models and weather forecasts.
The missing pieces in climate models
Several studies have already been carried out on the phenomenon of gravity waves. However, these mainly addressed either the lower or upper layers of the atmosphere. The individual layers of the atmosphere are in a state of continuous exchange. Scientists at the DLR Institute of Atmospheric Physics and the German Remote Sensing Data Center, together with their partners in the GW-LYCLE (Gravity Wave Life Cycle) project, are examining the generation, propagation and the breakup of gravity waves as they travel through the atmosphere.
Gravity waves form in the lower atmosphere (troposphere) and transport energy and momentum to the higher layers (stratosphere and mesosphere, at an altitude of approximately 10 to 100 kilometres). There, they become unstable and dissipate, which influences the temperature, air circulation and long-term climate. To accurately describe the dynamic processes occurring between the layers of the atmosphere, gravity waves must be directly computed or their effects represented in simplified form in existing climate and weather models.
The problem is that gravity waves are comparatively small-scale phenomena and therefore extremely difficult to observe. "So far, no one has succeeded in fully understanding their properties and integrating them correctly into a climate or weather forecasting model," explains Markus Rapp, Director of the DLR Institute of Atmospheric Physics. "By combining proven measuring devices with the Fast Airglow IMager (FAIM) airglow camera recently developed at DLR, we have been successful in tracking down gravity waves from their excitation level in the lower atmosphere to the place where they dissipate in the upper atmosphere." A better understanding of the effects of gravity waves on atmosphere and weather will help to create more accurate models for better climate research and more precise medium-term weather forecasts.
Luminous tsunamis in the atmosphere
For this reason, the two research aircraft, HALO and Falcon, are flying coordinated measurement flights above the Arctic Circle in northern Sweden. HALO flies at 15 kilometres above sea level in the tropopause, the transition between the troposphere and the stratosphere. "HALO flies and measures in the interesting transition zone between atmospheric layers. Falcon, however, flies much lower and aligns its measuring instruments sometimes downwards and sometimes upwards to an altitude of up to 85 kilometres," explains Oliver Brieger, Head of DLR Flight Operations. As a result, the installed measuring instruments create an overall picture of the phenomenon of gravity waves that is otherwise almost impossible to fully observe. From the aircraft, the DLR scientists were one of the first groups worldwide to be able to observe airglow and the gravity waves dissipating in that region. "This has been the decisive advantage, that the propagation character of the waves has been captured much more effectively," stresses Michael Bittner, Head of the Atmosphere Department at the German Remote Sensing Data Center, who developed the camera with his staff. "The fact that we are observing a much larger area than in previous studies means we can see directly in which direction gravity waves propagate."
Project partners complement the measurements
In addition to the measurement flights, the scientists have also included data from project partners. The University of Stockholm contributed test results from their ground-based instruments. In Norway, the Leibniz Institute of Atmospheric Physics complemented the measurements of the DLR Institute of Atmospheric Physics, which also operates ground-based laser devices in Finland. In parallel with the FAIM airborne airglow camera, the German Remote Sensing Data Center operates another ground-based camera in Kiruna, and the Ground-based Infrared P-branch Spectrometer (GRIPS) at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in Norway. The GW-LCYCLE project is funded by the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung; BMBF). The HALO measurements were carried out within the framework of the POLSTRACC/GW-LCYCLE/SALSA measurement campaigns, coordinated by KIT, DLR, and the University of Frankfurt.
The HALO research aircraft is a collective initiative by German environment and climate research bodies. HALO is funded by contributions from the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung; BMBF), the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG), the Helmholtz Association, the Max Planck Society, the Leibniz Association, the Free State of Bavaria, the Karlsruhe Institute of Technology (KIT), the German Research Centre for Geosciences (Deutsches GeoForschungsZentrum; GFZ), the Jülich Research Centre (Forschungszentrum Jülich) and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).