Intense mid-latitude cyclones are one of the main weather hazards in Europe. They are associated with strong winds and heavy precipitation which can lead to wind damage and flooding. It is therefore important to understand how mid-latitude storms will respond to climate change, in order to predict future weather risks and to guide climate change adaptation strategies. It is well-known that the large-scale environment in which mid-latitude cyclones develop will change under global warming. Also, more latent heating is expected. The impact of these changes at the cyclone scale, however, remains uncertain. Therefore, this project investigates cyclone-scale features like strong winds and fronts, with a focus on windstorms in the Atlantic-European sector.

Overall, CyclEx consists of two parts. In the first part extreme cyclones are analyzed in global climate models (GCMs), in the second part they are analyzed in idealized simulations. These two parts enable us to bridge the gap between low-resolution climate models and small-scale processes that are better resolved in high-resolution simulation. In general, we aim to better understand (1) the cyclone dynamics under global climate change, (2) the relative impact of environmental and diabatic processes on the cyclone scale and (3) the consistency, or differences, of cyclone changes in high- versus low-resolution simulations.

In the first part of CyclEx, cyclone statistics is applied to recent and future climate conditions (A6-Task 1). Cyclone tracks, numbers and characteristics (e.g. peak intensity, intensification rate) are computed with the ZYKPAK tool (Pinto et al., 2005). Additionally, on the large-scale, environmental features related to the cyclone development are quantified. These include jet stream location and intensity, temperature gradients and Rossby wave breaking. On the cyclone-scale, the impact of diabatic processes, such as latent heating and radiation, are determined by means of the pressure tendency equation (PTE) (A6-Task 2). In this first part we aim to study cyclone statistics under global warming, their model spread, and the impact of both environmental and diabatic processes on extrme cyclones in the future.

In the second part, we conduct idealized simulations with the German numerical weather prediction model ICON (A6-Task 3). Again, the PTE diagnostics is applied to quantify diabatic processes. The initial environment is varied based on recent and future climate conditions derived from GCMs, and the horizontal resolution will be varied from 2.5 km to 80 km. We contrast the representation and impact of small-scale diabatic processes on different resolutions and compare it to the low-resolution GCM data from part 1. This enables us to study to what extent the relatively low resolution of current GCMs leads to systematic biases in extreme cyclone projections. Furthermore, the high-resolution ICON simulations are analyzed in terms of fronts, wind and precipitation. In this second part we aim to answer how the model resolution and setup affects extreme cyclones and associated diabatic processes, and if this leads to systematic biases in low-resolution GCMs.

CyclEx contributes to the goals of Module A by looking into the recurring generating processes associated with windstorms (WP1), by studying the representation of extreme events in models given large-scale conditions (WP2), and by quantifying the impacts of climate change on windstorms (WP3). Furthermore, within ClimXtreme, CyclEx will cooperate with other sub-projects by delivering wind- and cyclone track data to sub-projects in module A and C, and by using cyclone statistics tools developed in module B.