Depending on the position of the Sun, the time of day and various other factors, the thin, high cirrus clouds over the Arctic have a predominantly warming effect. So far, however, almost no cirrus cloud measurements have been conducted at high latitudes, and climate models do not take these into account to a sufficient degree. In July 2021, the German research aircraft HALO is flying to Northern Europe and the Arctic, among other destinations. The aim is to gain a better understanding of how cirrus clouds contribute towards the particularly strong warming of this region. The 70-member research team is also looking at the effects of air traffic on Central Europe, where the skies are very congested. The researchers are investigating at what time of day contrail cirrus clouds have the weakest warming effect and whether their creation can be avoided under certain weather conditions. This could be of great value for climate-friendly flight planning. Nine atmospheric research institutes and universities are involved in the CIRRUS in High Latitudes (CIRRUS-HL) mission. The point of departure is the Oberpfaffenhofen site of the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).
"Cold, high ice clouds are modified by anthropogenic pollutants and air traffic. The exact role they play in the increased warming of the Arctic is as yet unresolved," says Christiane Voigt of the DLR Institute of Atmospheric Physics and the University of Mainz, who is the scientific coordinator for the mission. "Reducing the climate impact of air transport is an urgent research topic. Due to the relatively short lifespan of contrail cirrus clouds, reducing and preventing them is a promising approach in the endeavour to make air transport more climate friendly."
Approximately 25 HALO flights are planned as part of CIRRUS-HL. "The flight routes will be at altitudes of eight to 14 kilometres and will reach as far north as Spitzbergen and Greenland, but the aircraft will also fly over Central Europe, Spain, Scandinavia and Iceland," says Andreas Minikin of the DLR Flight Experiments facility, who is responsible for operations involving HALO, a modified Gulfstream G550. During the mission, the aircraft will carry extensive measurement instruments for the remote sensing of clouds and contrails. While flying through clouds and contrails, instruments on the wings will characterise ice particles and water droplets with a high level of precision. Other instruments will record atmospheric trace gases and aerosol particles. The research flights will be supplemented with satellite observations of the ice clouds and simulations using computer models.
Contrail cirrus clouds
Contrails and the resulting cirrus clouds have contributed more to the climate impact of air transport than the carbon dioxide emissions of all aircraft since aviation first began. Aircraft engines emit soot particles and other substances. These act as condensation nuclei for very small, supercooled water droplets, which freeze instantly to form ice crystals and become visible as contrails in the sky. The ice crystals in the contrails can survive for several hours in the cold and humid conditions present at altitudes of eight to 12 kilometres.
The researchers are using the current mission to measure how much heat radiating from Earth is retained in the atmosphere, and how much solar radiation the atmosphere reflects back into space. They want to use this information to determine at which time of day the cooling effect due to the reflection of solar radiation is at its greatest. They also want to understand more precisely which weather conditions are particularly conducive to contrails. Recent studies have shown that only a small number of flight routes are responsible for 80 percent of the climate forcing caused by contrails. The aim of the HALO measurements is to improve the prediction of these routes. This will help with the planning of climate-friendly flight routes in future that will either avoid the formation of contrails altogether or only permit them if their cooling effect predominates.
In the high latitudes of the Arctic, natural ice clouds have a net warming effect due to the low position of the Sun in the sky. Thermal radiation from Earth's surface is retained within the atmosphere by ice clouds as it they were acting like a warming blanket. Conducting measurements of ice clouds at high altitudes in the sparsely populated Arctic is challenging, so there is little experimental data from this region. "We are excited to see the numbers, sizes and shapes of the ice crystals that we will be measuring in these clouds," says Voigt. "The properties of the ice crystals have a clear effect on their radiative forcing." During the mission, the researchers will examine a wide range of more complex ice crystal shapes.
Small ice crystals, large climate impact
The previous mission, Mid-Latitude CIRRUS (ML-CIRRUS), showed that the ice crystals of natural cirrus clouds were on average over ten times as large as the ice crystals in cirrus contrails (two to 10 micrometres). The number of ice crystals in contrail cirrus is significantly higher, as is their climate impact compared to naturally forming cirrus clouds with the same water ice content. The researchers are now expecting to also find differences in the particle shapes between contrails and natural cirrus clouds.
CIRRUS-HL – a joint mission involving many research institutes
CIRRUS-HL is partly financed by the German Aerospace Center (DLR), the German Research Foundation (DFG) – within its Infrastructure Priority Programme for HALO (SPP 1294) – the universities of Leipzig, Mainz and Munich (LMU), the Max Planck Institute for Chemistry (MPI-C), Forschungszentrum Jülich (FZJ), Karlsruhe Institute of Technology (KIT) and the Institute for Tropospheric Research (TROPOS). Targeted weather forecasts are being developed by the Swiss Federal Institute of Technology (ETH) in Zurich. The results of the current CIRRUS-HL mission are expected in 2022.