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C4 - Coupling of planetary-scale Rossby wave trains to local extremes in heat waves over Europe

Principal investigators: Prof. Dr. Volkmar Wirth, Prof. Dr. Andreas H. Fink

Other researchers: Georgios Fragkoulidis (PhD), Philipp Zschenderlein (PhD)

Former researcher: Pila Bossmann (PhD)

This project aims at improving the basic understanding of heat waves over Europe. Such heat waves have a significant impact on society as well as on natural ecosystems. They can be expected to become more severe in future decades owing to the projected global warming. The strength and duration of a heat wave is hard to predict with state-of-the-art weather forecast systems, presumably because of the interaction between multiple scales and processes involved.

This project focuses on the downscale coupling between the planetary-scale flow in the upper troposphere and synoptic and mesoscale processes which eventually lead to local hot weather over a prolonged period. It is hypothesized that upper-tropospheric quasi-stationary Rossby wave packets play an important role, but that a true heat wave requires further processes acting on smaller scales. An important goal is to find out which of these scales and processes limit the predictability of heat waves.

The statistics of past heat waves will be investigated and characterized using reanalysis data. Based on this work, individual cases of heat waves will be selected and studied in detail. Tools will be developed and applied in order to study the upper tropospheric waveguide and associated wave trains with significant ridging over a localized area for an extended period. The main idea here is to focus on regional wave packets rather than global-scale waves; correspondingly, this project goes beyond the traditional Fourier analysis and uses wavelet analysis instead. In addition, the upper tropospheric Rossby waveguide will be analyzed using established ideas from linear wave theory, but generalizing these ideas to zonally non-uniform references states. This will allow one to evaluate a previously suggested "quasi-resonance hypothesis" in a more focused and, arguably, more relevant framework.

This set of diagnostic tools will be complemented by another set of tools that will quantify the relevant smaller-scale processes like warm air advection, subsidence, irradiation and cloudiness/rainfall as well as soil moisture. One particular focus will be on the evolution of the heat that builds up in the deep boundary layer during several diurnal cycles. Here, we will distinguish between periods with and without synoptic-scale warm air advection. Related to the strength of the boundary layer inversion and to the moisture distribution, the role of cloudiness and shallow thunderstorm lows in augmenting/reducing the heat will be studied. This will be done in case studies using atmospheric reanalyses, surface and upper-air data as well as assessments of radiation budget terms from satellites.

This project will also investigate the predictability of heat waves based on ensembles from atmospheric reforecast data sets; this activity will be started during phase 1, but it will become more dominant during later phases, when the diagnostic methods developed during phase 1 have reached a mature stage.