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Abstract
The jet stream is a high-altitude circumpolar band of westerly winds and acts as a guide to
large-scale weather systems, making it a crucial feature of the global atmospheric circulation.
While the subtropical jet stream is in agreement with the thermal wind balance, the eddy-driven
jet is closely connected to the storm tracks in the major ocean basins. The evolution of the North
Atlantic jet stream in a warming climate is an important research field in atmospheric dynamics
and climate science. The jet features "jet streaks", regions of enhanced wind speeds that can
exceed 100ms-1 in extreme cases. Jet streaks affect air travel times and can cause clear-air
turbulence. Jet streaks also influence surface weather. Second, the ageostrophic flow associated
with jet streaks can facilitate extreme weather events such as heavy rain and cold spells,
indicating that accurate representation of upper-level jet streaks is crucial for reliable weather
forecasts. While the influence of jet streaks on surface weather extremes is a longstanding area
of research, the understanding of jet streak dynamics and specifically the impact of diabatic
processes on their evolution needs further research.
Classical approaches to study jet streak dynamics include the four-quadrant model based on
quasi-geostrophic theory, and the potential vorticity (PV) gradient perspective that views jet
streaks as upper-level fronts. Both are grounded in dry dynamics. Recent research highlights
the importance of diabatic processes, which can intensify jet streaks, and shows that the
misrepresentation of diabatic processes can impair forecast accuracy. This prompts the need
for a new framework to investigate jet streak dynamics that includes the influence of diabatic
processes. In a first step, we expand upon the well-established relationship between the wind
field and PV gradients at upper levels to develop the Lagrangian PV gradient framework. We
show that a broad jet and slowly varying stability are necessary to establish a quantitative link
between wind speed and PV gradients. We then consider the enhanced waveguidability in
regions of high PV gradients and round out the first part by developing a method to separate
diabatic and adiabatic contributions to material PV-frontogenesis.
In a next step, we employ the Lagrangian PV gradient framework in case studies of two jet
streaks in 1km resolution COSMO simulations of the NAWDEX period in September 2016 with
online calculated air parcel trajectories. We analyze a nearly stationary zonal jet streak using
3 h trajectories. Changes in the PV gradient, primarily driven by adiabatic processes, align
with expected patterns of confluence and diffluence in the jet streak. The 3 h trajectories yield
negligible diabatic contributions to Lagrangian PV gradient change in this case. The second case
study investigates a jet streak that intensifies in alignment with a strong warm conveyor belt
rising to upper levels along the surface cold front of the mature extratropical cyclone Vladiana.
Convection within the warm conveyor belt causes mesoscale PV-change dipoles at upper levels
in this case, which cause pronounced positive diabatic PV gradient change along the jet axis,
broadening and intensifying the jet. The study highlights the advantages of high-resolution
modeling for representing the intensification of jet streaks.
Next, we conduct a climatological analysis of North Atlantic jet streak intensification, focusing
on the role of diabatic processes and extreme events in different dynamic environments in
reanalysis data. Some key findings are that the maximum PV gradient displays a near-linear
relationship to maximum wind speed, and strong jet streaks tend to live longer than weak
ones. Jet streaks typically propagate from southwest to northeast and are most common in
storm track region of the North Atlantic. Categorising jet streaks based on an established
jet regime definition of the eddy-driven jet shows that extreme jet streak events occur more
frequently when the eddy-driven jet is in a central regime with strong baroclinic eddy growth.
This suggests a heightened interaction between lower and upper levels for extreme jet streaks.
Lastly, we examine how adiabatic and diabatic processes influence the intensification of
zonally and anticyclonically oriented jet streaks. The dynamics surrounding anticyclonically
oriented jet streaks, which develop upstream of the Rossby wave ridge crest with strong
anticyclonic wave breaking, are too variable to identify a consistent intensification process prior
to peak intensity. This is different for zonally oriented jet streaks, which show more consistent
behaviour across cases up to one day before peak intensity. North of the left jet streak exit, a
strong cyclone intensifies and is accompanied by warm sector precipitation below the left jet exit.
A small but intensifying cyclone produces mesoscale precipitation beneath the right jet entrance.
Deformation and shear predominantly determine PV gradient evolution in jet streak air parcels
throughout the last day of intensification. However, diabatic processes remain significant and
the contributions of individual diabatic processes evolve. Radiation and turbulence initially
intensify and narrow the jet. Shortly before peak jet streak intensity, moist diabatic processes
gain prominence, intensify the jet, and enhance anticyclonic wavebreaking — a pattern that
is also observed in anticyclonically oriented jet streaks at their peak intensity. Moist diabatic
processes are more prominent in extreme jet streak intensification, and we propose a positive
feedback mechanism between the enhanced diabatic contributions to intensification and strong
ageostrophic flow in extreme jet streaks.
In summary, this thesis introduces novel methods for investigating jet streak dynamics,
with a focus on the interaction between diabatic and adiabatic processes. It highlights the
importance of considering diabatic processes for advancing our understanding of jet streak
evolution. However, our work is only a first step on the way to a comprehensive framework to
study jet streak dynamics that natively includes the role of diabatic processes. We hope that it
will inspire future research to work further towards this goal. Mehr anzeigen
Publikationsstatus
publishedExterne Links
Printexemplar via ETH-Bibliothek suchen
Thema
Atmospheric dynamics; JET STREAM (METEOROLOGY); diabatic processes; Potential vorticity diagnostics; PV gradientOrganisationseinheit
09705 - Schemm, Sebastian / Schemm, Sebastian
Anmerkungen
This research was funded by the European Research Council under the Horizon
2020 program (grant no. 848698).ETH Bibliographie
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