Climate of the free troposphere and mountain peaks

Brönnimann, Stefan (2019). Climate of the free troposphere and mountain peaks (In Press). In: Oxford Research Encyclopedia of Climate Science. Oxford University Press

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The free troposphere is the location of important weather and climate processes. Here, horizontal and vertical transport of energy, mass and momentum take place and it holds greenhouse gases, water vapour and clouds. The free troposphere therefore plays an important role in global climate feedback processes. Mountains provide important ecosystem services for a large lowland population. Mountain ecosystems might react particularly strongly to climatic changes. This is because mountains intersect important environmental and geoecological boundaries such as the snow line and the tree line. In a changing climate, these boundaries may shift. Climate change thus affects mountain glaciers, water resources and mountain ecosystems.
Climates of mountains and of the free troposphere have attracted scientists of the enlightenment and have been studied scientifically at least since the 18 th century. High-altitude observatories were installed in the late 19 th century, and upper-air measurements were started soon afterwards. However, even in the early 21 st century, the climate observing systems do not well cover mountain regions and specifically mountain peaks. The temperature of the free troposphere is dominated by horizontal and vertical transport of sensible and latent heat, condensation and release of latent heat, and radiation to space. Mountain peaks sometimes reach into the free troposphere, but at the same time also share characteristics of surface climate. They are strongly influenced by radiative processes of the surrounding surface, while during the day they are often within the atmospheric boundary layer. With respect to climate change, temperature trends are amplified in the tropical upper-troposphere relative to the surface due to latent heat release, while in the Arctic the surface warms faster than the free atmosphere due to strong inversions and due to feedback processes operating at the surface. Mountain peaks may see both types of amplification. Several processes have been suggested to cause an elevation dependent warming, the most important of which arguably is the snow-albedo feedback. Elevation dependent warming is also seen in model studies and in observations, although detecting this signal in observations turns out rather difficult outside the tropics due to high variability and sometimes low data quality. The observed climatic changes are expected to continue into the future.

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