INTERACT

Weather fronts are an important link between large scale and regional weather. Climate change affects the large-scale drivers of fronts and likely influences fronts themselves. As a consequence also the associated extreme events are expected to change. But despite their relevance for extreme events, research on fronts in a changing climate is only an emerging topic. The main aim of INTERACT is to understand the role of fronts in scale interactions that shape extreme events in a changing climate. In an international collaborative effort with colleagues from two Russian Institutions, INTERACT will address: 1. How are regional extreme events that are tied to frontal activity controlled by the internal variability and interaction of dynamical phenomena across scales? 2. How realistically are these phenomena, their interaction, and their influence on extreme events simulated by a hierarchy of climate models, included added value of high resolution? 3. How do these phenomena, their interaction and thus the related extreme events change in a warming climate? To answer these research questions, INTERACT will - conduct global climate model sensitivity simulations; - conduct simulations of selected target regions with very-high resolution regional climate models; - simulations of individual extreme events under present and future climatic conditions; - diagnose cyclones and fronts in observations and a hierarchy of climate model simulations; - develop new diagnostic tools; - evaluate climate model performance based on process-understanding; - explore uncertainties in our future knowledge of extreme events related to fronts. Innovation INTERACT will improve our process understanding of extreme events in a changing climate, and thus increase the credibility of future simulations of weather extremes. INTERACT will contribute to the WCRP Grand Challenge on Extreme Events and to the understanding of future weather, a new frontier in climate research. INTERACT will generate new high resolution simulations, including the simulations of selected extreme events, in present and future climate. INTERACT will for the first time assess the added value of high-resolution global and regional climate models by comparing standard global climate models with high-resolution global climate models, standard regional climate models and very-high resolution regional climate models. INTERACT will develop new diagnostic tools by embedding frontal diagnostics into cyclone lifecyle diagnostics. These tools will help to improve our understanding of how cyclones and fronts interact to shape extreme weather. INTERACT will explore uncertainties about our knowledge of future climate change in innovative ways by using a hierarchy of climate models, and by developing physically consistent storylines.

Cold and warm fronts are a key element of mid-latitude cyclones and can cause severe damage, by both strong winds and heavy rainfall. Cold fronts can produce short convective rainfall events of very high intensities, causing flooding of small river catchments and urban flooding. Warm fronts can cause several days of rainfall and subsequent flooding of major rivers. Despite their importance, comparatively little research has been conducted on the climatic aspects of fronts: What is the relative role of synoptic (large-scale related to the cyclone) and meso-scale (within the front) processes in producing severe weather? How are these processes represented in climate models of different spatial resolutions? And how could these processes change in a warmer climate? The INTERACT project has addressed these questions. First, we analysed the characteristics of cold fronts and their connection to extreme short rainfall events in observational data. We find that humidity close to the fronts, convergence and the low level jet speed contribute most to the formation of extreme precipitation. Synoptic conditions favoring the formation of extreme events are also identified. We also conducted the first climatological study of front life cycles, based on tracking thousands of observed fronts and their parent cyclones. We analysed regional characteristics of fronts as well as their relationship to the North Atlantic Oscillation. The analysis of front characteristics and their relationship with extreme weather provided the foundation for a process-based evaluation of frontal dynamics in climate models. We found that state-of-the-art climate models at essentially all resolutions well represent synoptic-scale dynamics, while the representation of meso-scale dynamics depends strongly on the model resolution: it is much better resolved by regional climate models. The simulated relationships between these dynamical processes and severe weather depends strongly on the individual model. Future projections of frontal dynamics and the associated severe weather depend strongly on region, season and front type. For cold fronts, we found up to about 14% more intense hourly precipitation rates per degree of global warming, for warm fronts up to about 7% more intense 3-day precipitation per degree of global warming. During the summer season, fronts are, along with their parent cyclones, projected to move polewards such that the resulting rainfall will decrease over southern Europe.