Project

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Understanding storm tracks and extratropical cyclone growth in a future climate

Applicant Schemm Sebastian
Number 204181
Funding scheme Project funding
Research institution Institut für Atmosphäre und Klima ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Meteorology
Start/End 01.03.2022 - 28.02.2025
Approved amount 379'632.00
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Keywords (6)

Atmospheric Dynamics; Climate Dynamics; Warmer climate; Extratropical cyclones; Storm tracks; Jet stream

Lay Summary (German)

Lead
Das Projekt untersucht im Detail den Energiekreislauf in aussertropischen Zyklonen in einem wärmeren Klima.
Lay summary

Tiefdruckgebiete beeinflussen massgeblich das regionalen Klima und das täglichen Wetter. Grundsätzlich ist ihr Energiekreislauf in Tiefdruckgebieten bereits gut verstanden, jedoch sind noch viele Fragen offen bezüglich dessen Veränderung in einem wärmeren Klima. Der Temperaturunterschied in der Atmosphäre wird in bodennähe abnehmen, in der Höhe zunehmen und es wird mehr Feuchte zur Verfügung stehen. Diese Veränderungen wirken gegensätzliche auf den Energiekreislauf  in Tiefdruckgebieten. Diese Projekt beschäftigt sich inbesondere damit ob die sich verändernde verfügbare potentielle Energie für das Wachstum von Tiefdruckgebieten in einer wärmeren Klima weniger effizienter genutzt werden kann und somit der zunehmenden Feuchte entgegen wirkt. Die Effizient hängt massgeblich von der Neigung der Tiefdruckgebiete ab und ihrer Position zum Jetstream welcher seine Position und Intensität ebenfalls ändern wird.

Direct link to Lay Summary Last update: 27.09.2021

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Abstract

Global warming is a scientific fact, and scientific attention has shifted towards changes on regional scales, where projected changes in floods, droughts or windstorms are largely driven by changes in the atmospheric circulation. A substantial part of the variability in atmospheric circulation is related to the growth and propagation of extratropical cyclones; however, changes that will occur in the growth and propagation of extratropical cyclones in a warmer climate are highly uncertain. This uncertainty complicates predictions about related changes in extreme weather. A major challenge is the prediction of changes in the intensity of extratropical cyclones. This question is particularly pressing because many weather extremes are controlled by the intensity of extratropical cyclones. In general, extratropical cyclones extract their kinetic energy from the equator-to-pole temperature gradient of the atmosphere via a process known as baroclinic conversion. The equator-to-pole temperature gradient is therefore regarded as a measure for the growth potential of extratropical cyclones. Global warming, however, starts to reduce the equator-to-pole temperature gradient due to a faster warming of polar latitudes in a process known as Arctic Amplification. Therefore, the Arctic Amplification has the potential to weaken extratropical cyclones because it reduces the potential energy available for cyclone growth. According to the recent literature, the reduced equator-to-pole temperature gradient will further weaken the jet stream, thereby leading the way to more stationary high-pressure systems and prolonged heat waves. However, observational evidence from current and historical climates questions the direct link between extratropical cyclone intensity and the equator-to-pole temperature gradient. For example, during midwinter over the North Pacific, the equator-to-pole temperature gradient and the baroclinicity are very large while extratropical cyclones are weak. Recent studies suggest that this counter intuitive behavior of extratropical cyclones is related to a reduction in the efficiency of the baroclinic conversion process. Although the baroclinic conversion process is the main and most fundamental energy source for extratropical cyclones, changes in the baroclinic conversion efficiency are not well understood. In addition, the increased water storage capacity of the atmosphere in a warmer climate will increase the contribution by latent heat release to storm growth while at the same time alter the thermal stratification and the mean baroclinicity of the atmosphere. On the scale of individual storms, studies have shown that latent heating amplifies the growth, while evaporation weakens it. On the scale of the entire storm tracks and integrated over a season, the net effect of diabatic processes could be an increase of the static stability and hence reduced growth potential. This project proposes a systematic assessment of several processes underlying changes in the growth of extratropical cyclones: the baroclinic contribution and its efficiency, the barotropic contribution and also the role of diabatic processes. The latter contains also air-sea interactions, the role of sea surface temperature fronts and ocean eddies in determining future trends in the intensity of storm tracks. In this study, a fully coupled 40-member CESM2.0 ensemble of present-day and future climate simulations with varying grid-spacing is analyzed using eddy energy diagnostic in combination with feature-based cyclone and trough-ridge identifications. This combination will allow for studying the eddy total energy budget from both, Eulerian and Lagrangian viewpoints. Because we must expect the influence of diabatic processes, including the ocean’s influence on the atmosphere, to hinge on the model grid spacing, shorter 10-yr climate simulations will be repeated at kilometer-scale resolution and with smoothed ocean surface temperatures. This is intendent to give an estimate of the extend by which the contributions of diabatic processes, which include the air-sea interactions by sensible and latent heat flux, add to the eddy available energy and to what extends this contribution depends on the model resolution.
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