Highlight articles 

Understanding the specific dynamical processes leading to extreme floods is an important but challenging task. Ruff and Pfahl used an innovative approach that allowed them to go beyond single case studies. They used operational ensemble forecasts from the ECMWF during the period 2003-2019 and focused on five major river catchments in Central Europe. Comparing extreme events (with a return period of 100 years) with more moderate events revealed important differences between the catchments. For some catchments the main factors that distinguish 100-year events were the intensity of the upper-level cutoff and surface cyclone, whereas in other catchments the main factor was an increased low-tropospheric moisture supply. The original results clearly illustrate that regional variability is substantial and no single atmospheric process can be claimed responsible for the distinction between extreme and moderate flood events.

Numerical weather prediction depends essentially on high-quality upper-air wind observations to constrain the initial conditions. This study by Martin et al. investigates the impact of spaceborne Doppler wind lidar measurements from the Aeolus mission on forecast quality of the operational global forecasting system with ICON at Deutscher Wetterdienst. An observing system experiment shows an overall beneficial impact, and the authors go one important step further and present illustrative examples how events with strong forecast quality improvements can be related to specific dynamical phenomena such as ENSO and the extratropical transition of tropical cyclones.

This paper analyses the extensive scientific debate around the potential influence of Arctic warming and sea ice loss on recent cooling trends over the Eurasian continent. It provides a novel, holistic perspective on this debate that goes beyond a simple yes or no answer to the question whether a causal link exists between these two phenomena. As such, it has the potential to bring together seemingly diverging portrayals existing in the literature.

The paper by Tim Woollings et al. provides first a theoretical summary of the role of Rossby wave dynamics for polar variability, regarding the large-scale polar flow conceptually as a superposition of geostrophic turbulence and Rossby wave propagation, emphasising the key role of isolated vortices at high latitudes. In a second part, the paper addresses how comparatively weak Rossby waves can be triggered from high latitudes in the presence of a relative vorticity gradient as a direct response to sea ice loss, which often manifests as a heat low. The paper provides a well written and much needed foundation for an active area of research.

Analysing and understanding potential trends in the atmospheric circulation over the last decades is an important aspect of atmospheric dynamics. In this article, Adrian Simmons performed a very thorough global trend analysis, with a focus on near-tropopause winds, based on ERA5 reanalyses. The paper provides an excellent overview and emphasises important regional and seasonal differences. It is shown that despite Arctic amplification, the upper-level westerlies mainly strengthened at northern middle latitudes. Focusing on jet-stream wind maxima, it is found that they increased over the North Atlantic. These results and the detailed global trend overview will be useful for the community to further investigate the underlying physical processes that led to these trends in the last decades.