Extreme environmental phenomena such as the eruption of the Icelandic volcano Eyjafjallajökull, but also less dramatic effects such as turbulence and the formation of ice, have a
long-term effect on air traffic. At the Institute of Atmospheric Physics of the German Aerospace Center, researchers are developing methods and products which will provide reliable information on climate-related phenomena in air travel.
Developments in the economy and society directly affect local and global transport. In particular, new challenges continually develop out of the air transport system, with safety, efficiency and environmental compatibility being three core areas of research. Not only is air transport already disrupted by various weather situations, which in future will probably be influenced by climate change even more, it also contributes to this climate change itself.
Due to the very long development cycles peculiar to air transport, new and alternative transport concepts that take more account of these factors require a correspondingly longer lead time and must be analysed and assessed early on. On the other hand, we have tools and products today – for example in meteorology – that can be used in present air transport systems now. At the Institute of Atmospheric Physics of the German Aerospace Center, in Oberpfaffenhofen near Munich, methods, products and instruments are being developed which will provide both customised and standardised information on safety and climate-related phenomena in the air such as volcanic ash, weather conditions, areas in which icing-up occurs and climate-sensitive regions.
The eruption of the volcano Eyjafjallajökull in Iceland in 2010 clearly demonstrated the vulnerability of air transport systems. More than 100,000 flights had to be cancelled, which resulted in economic damage of about two billion euros in the air transport sector alone. Volcanic ash in the sky was a major hazard for aircraft. Flying into a cloud of ash can damage the engines and other aircraft components to the point where the engines fail completely. Closing airspace during the Eyjafjallajökull eruption was based on model forecasts of the spread of ash from the Volcanic Ash Advisory Centre (VAAC) in London, which nevertheless entailed major uncertainties. Operational measuring systems for detecting clouds of ash were not available. To support the VAAC forecasts in future volcanic eruptions and to better contain contaminated airspace, the Institute of Atmospheric Physics is developing a satellite data algorithm for detecting volcanic ash. This will make it possible to predict both the height and the concentration of the detected layer of ash and its dispersal to within several hours. The basis of this algorithm is formed by the radiation measurements in the infrared area of operational weather satellites with a temporal resolution of 15 minutes.
Even in times with no catastrophic events such as volcanic eruptions, weather phenomena such as turbulence, wind shear, lightening, heavy rain, hail, and super-cooled cloud droplets also influence air traffic by reducing passenger comfort or endangering the safety of a flight. For the pilot, information on disruptive weather phenomena is based mainly on the so-called Significant Meteorological Phenomena Weather Charts (SIGMET). It is given to the pilot before the flight during take-off preparations; however, these data do not reproduce the customised and reliable information required for a dedicated flight and are often outdated when the aircraft takes off. To satisfy today’s air-travel requirements from a meteorological point of view, the Institute of Atmospheric Physics is designing and developing an integrated system for observing and predicting disruptive weather phenomena called WxFUSION. WxFUSION is an expert system and combines data from observation, short-term (nowcast) tools and forecasting models in order to detect and pursue potentially dangerous or disruptive weather situations and to forecast them for the near and more distant future. The aim is to supply both ground operations and the crews in the air with consistent, real-time and client-oriented analyses of the relevant weather phenomena and their trends. In order to support an efficient and rapid process of the joint decision-making process on the part of the user, the weather information is transmitted and presented in such a way that it requires no interpretation by its user, but indicates clearly and unmistakeably the current and coming meteorological hazard.
A particularly dangerous weather phenomenon is clouds with icing conditions. At heights of less than eight kilometres, various parts of an aircraft can ice-up when under-cooled water drops freeze. These areas in deep clouds contain a large number of small (less than 100 micrometres in diameter) ice particles. This is why aircraft radar, which is sensitive to larger particles, registers only minor reflections in these dangerous areas when climbing. This was one of the causes of the tragic crash of the Air France airliner over the Atlantic in 2009. Together with many partners in the aeronautics sector, the Institute of Atmospheric Physics is taking part in a comprehensive European project investigating which meteorological conditions promote the formation of such dangerous icing in high-reaching clouds. Satellite and aeroplane-borne warning systems are also being developed and tested so that accidents like the Air France crash caused by icing-up at great heights can be avoided in future.
The IPA is also carrying out research into aspects of the climatic effects caused by air traffic due to so-called non-CO2 emissions. Besides carbon dioxide, vapour trails and ozone, which is formed from nitrogen oxide emissions, contribute considerably to the influence of the air traffic on climate. Vapour trails, for example, form under special conditions in an aircraft’s wake where the hot exhaust gases mix with the ambient air. When the atmosphere is sufficiently cool and moist, steam condenses on the particles. At the level of air travel, the resulting droplets freeze very quickly. When the ambient air is saturated with respect to ice, i.e. when it contains very large quantities of water vapour, the frozen ice particles continue to grow, the contrail can then exist over an extended period and even extensive ice-clouds can form – the so-called contrail cirrus clouds. These cirrus clouds contribute to the effect of climate on air traffic as, similar to natural cirrus clouds, they affect the radiation transfer in the atmosphere. They both reflect incident solar radiation (a cooling effect) and reduce thermal radiation from the earth (a warming effect). The net average effect is a warming effect. In the current report of the Intergouvernmental Panel on Climate Change (IPCC), the radiative forcing through vapour trails and vapour-trail cirrus clouds is given as 50 mW/m2, a value in the magnitude of radiative forcing from CO2 emissions from air traffic. An extensive flight experiment to investigate these effects in greater detail was conducted spring 2014. The aim of the measuring flights was to determine exactly the chronological development of microphysics and radiation properties of vapour-trail cirrus clouds.
From the point of view of traffic planning, many of the phenomena described can be treated in a similar manner: on various spatial and time scales they cause a change in the usable airspace itself and in the possible frequency of use through individual traffic participants – either for reasons of efficiency given at least the same level of safety, of passenger comfort, or to the benefit of climate-friendly flying. The idea consists of introducing a fifth dimension to describe the trajectory besides the usual parameters of geographical length, width, height and time. This concept, currently developed at the IPA, bears the name 5D MET Advisory, a five-dimensional meteorological consulting/advisory system for air traffic. The fifth dimension, disruption, is intended to mean meteorological situations and conditions which either disrupt air traffic directly, lead to particularly climate-sensitive zones along flight-routes or control other events. The first category includes storms, turbulence, wind-shear, icing and snowfalls. The second category includes such phenomena as cold regions with high levels of ice saturation in which long-lasting and hence climate-relevant vapour trails are formed. Finally, the third category includes the spread or distribution of volcanic ash or dust particles and the transport and disintegration of wake vorteces.
In this context, the very often different spatial and time scales of the disruptions, which may range from a few minutes within a wing-span (wake vortex, turbulence) to several days in large regions (volcanic ash), often constitute a challenge for standardisation and adaption to specific requirements. We regard it as one of our research tasks to confront these complex challenges and to contribute to safe and sustainable development in air traffic.
Prof. Dr. Markus Rapp passed the qualifying examination for appointment to a professorship at the University of Rostock in the field of atmospheric physics. He was a research assistant at the Leibniz Institute of Atmospheric Physics (IAP) from 2000 to 2007 and guest professor at the University of Stockholm in 2005. From 2008 to 2012, he took over the professorship of Experimental Atmospheric Physics at the University of Rostock and the headship of a research department at the IAP, before he became director of the DLR Institute of Atmospheric Physics at the German Aerospace Center and professor of Atmospheric Physics at the University of Munich.