A New Lagrangian Perspective on Atmospheric Heat Extremes
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Abstract
Heat extremes are among the most dangerous weather-related hazards. However, a full understanding of how heat extremes form in the Earth’s atmosphere is still lacking.
In particular, the relative importance of the three key processes—horizontal advection, subsidence, and diabatic heating—is a subject of ongoing debate. This thesis provides a new quantitative assessment on the relative importance of these processes employing the Lagrangian framework. A key aspect of this new assessment is the consideration of a Lagrangian climatology of the processes. The assessment reveals that horizontal advection can be seen as the main contributor to heat extremes across most of the globe.
In the first part of the thesis, a new method for extracting Lagrangian information about the atmospheric flow is developed, which is later used to quantify the key processes
of heat extreme formation. The method is based on the advection of passive tracer fields in combination with a relaxation term. As a result, the method provides accumulated
Lagrangian information, such as the recent diabatic heating experienced by air parcels along their pathways, at each point on an Eulerian grid at any time step. The method
can be regarded as an alternative tool to calculating trajectories, tailored to gaining accumulated Lagrangian information efficiently.
In the second part of the thesis, the tracer method is then used to quantitatively assess the roles of the processes involved in heat extreme formation from a Lagrangian perspective.
At each grid point and time step, the method provides a decomposition of temperature anomalies into the aforementioned processes. Two different decomposition approaches are
contrasted: one that has been established in previous studies and which is based on the absolute contributions of the respective terms; and one that is introduced in this thesis
and that considers the contributions in terms of anomalies, defined as deviations from their corresponding climatological means. The new decomposition is based on the understanding
that a particularly large contribution from a given term may be irrelevant if it typically occurs as part of the climatology. By removing the atmosphere’s climatological behaviour—
which is of limited use in explaining anomalies—the new approach arguably provides a more meaningful framework for understanding anomalous temperatures. The analysis of two
recent heatwaves in the extratropics reveals that the new decomposition offers a markedly different perspective on the relative importance of the processes compared to previous
assessments of these cases. In particular, anomalous horizontal advection—specifically the absence of cold-air advection—rather than anomalous subsidence or diabatic heating, is
found to be the key contributor to heat extremes in the studied regions.
In the final part of the thesis, the region-specific findings are extended to encompass the entire global domain. Most importantly, the analysis reveals that the contribution from anomalous horizontal advection dominates the formation of near-surface heat extremes across the entire midlatitude region.
The presented Lagrangian diagnostic, together with the newly developed tracer method, could be readily applied to climate model simulations, providing a powerful tool to deepen
our understanding of the processes driving heat extremes in a future climate, where they are likely to pose an even greater threat to society than today.