C4 - Predictability of European heat waves
A heat wave is a meteorological phenomenon with great relevance for both natural ecosystems and societies. The projected global warming as well as the increasing fraction of people living in big cities exacerbate the issue. While heat waves appear to be an inescapable weather hazard in future summers, reliable forecasts are crucial to mitigating their most severe impacts.
During Phase 1 we investigated the processes associated with heat waves over Europe, focusing on the interplay between upper-tropospheric dynamics versus processes that operate close to and within the planetary boundary layer. To this end we developed, refined, and applied several methods to diagnose these processes in both a Eulerian and a Lagrangian framework. It was shown that localized Rossby wave packets play an important role for the occurrence of heat waves — considerably more important than circumglobal Rossby waves — and this implies potential for their predictability. The analysis of several heat wave episodes revealed that adiabatic warming in subsiding air played a much more dominant role than horizontal warm air advection. In addition, in some cases warming of air parcels within the boundary layer turned out to be important.
During Phase 2 this project will apply the tools and results from Phase 1 to study in detail the practical predictability of European heat waves. Our key hypothesis is that predictability is limited by different processes at different lead times. At lead times longer than one week, tropical waves, soil moisture, and sea surface temperature anomalies are considered essential. Between 7 and 3 days, Rossby wave characteristics, including the phase and amplitude of trough/ridge constellations become more important, while closer to the event, "stagnant air vessels", the strength of subsidence, and boundary layer heating need to be considered. Using inventories of past heat waves from Phase 1, one focus will be on their Lagrangian characterization involving the computation of backward trajectories, Lagrangian tracers, and Lagrangian coherent structures. We also plan to perform model simulations with a special focus on exploring the role of slowly varying boundary conditions for weather-scale processes and their predictability. We will make extensive use of existing (multi-model) ensemble forecasts at lead times from synoptic to subseasonal. An important tool will be ensemble sensitivity analysis, which will be applied, in particular, to the diagnostics developed during Phase 1. The selection of cases will be based on "interesting" behavior of the ensemble spread such as a strong sensitivity to the date of initialization or to key fields such as soil moisture. Backward trajectories from forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) are currently being archived at KIT, and this will allow for a novel, in-depth analysis and physical explanation of the skill of individual ensemble members. In this context, we will explore the severe 2018 European heat wave. Finally, "predictability jumps" at various lead times will be investigated. For lead times up to a week, point forecasts of 2-meter temperature will be considered and near-discontinuous behavior in the predictability will be related to upstream synoptic developments, characteristics of Rossby wave packets, Rossby wave breaking, and Lagrangian characteristics of the involved air masses. We will also extend our investigations towards longer-term subseasonal forecasts, where suitable aggregations in terms of the forecast regions and periods need to be defined and predictability jumps will be related to tropical and extratropical planetary wave activity. All analyses will be carried through in the framework of automated application to re-analyses and large forecast ensembles.