During MiKlip significant skill has been established for probabilistic forecasts of cyclone and windstorm frequencies in the North Atlantic. Positive prediction skill over the climatological forecast was estimated, as well as over historical simulations (Kruschke et al., 2015). STOC aims at a better understanding of mechanisms leading to this skill. The ocean variability is the basis for decadal prediction and it is known that an increase in the meridional overturning circulation induces baroclinicity in the North Atlantic region which favors European windstorm development (Nissen et al., 2014). This project investigates the relationship between the ocean state and European windstorms in the MiKlip decadal prediction system.
Relevant oceanic variables and the associated time scales for windstorm development are identified. These results are the basis for the model validation of oceanic processes leading to European winter windstorm development (In cooperation with project VALOCEAN). STOC analyses how far the representation of the identified oceanic variables influences the forecast skill for winter windstorms and extratropical cyclones. This helps understanding and communicating skill found in the operational forecast system, and to explore possibilities to identify oceanic states leading to improved skill.
The aim of STOC is the analysis of interactions between the oceanic state and winter windstorm activity on decadal time scales. STOC contributes to process-oriented studies investigating decadal storm forecast skill. It is aimed to define relevant oceanic parameters and states in the North Atlantic, which influence winter windstorm development on a decadal scale. These processes will be identified for observations and for the MiKlip model system. Analysis of modes of co-variability will be implemented as plug-ins for the central evaluation system. Results guide a stratified verification of winter windstorm forecast and thus STOC contributes to a better understanding and improvement of the MiKlip operational system. Three project goals can be formulated:
2. Atmosphere-ocean interaction in the Earth system model MPI-ESM
We investigate ocean-storm-interaction in the MPI-ESM using the pre-industrial control run. The relationship between the increase in meridional overturning circulation and European storm frequency is explored with respect to decadal time scales. Modes of co-variability of oceanic parameters and winter windstorms are detected for different time scales, and compared to the observed relationships.
3. Forecast skill of windstorms analyzing decadal hindcasts
Previous findings are synthesized to see how far the defined oceanic processes lead to decadal predictability for European windstorms; together with the project VALOCEAN the presence of the relevant oceanic processes in the initialized decadal hindcasts is investigated. Based on a framework developed in the project ECO stratified verification for European winter windstorms along relevant oceanic parameters will be carried out.
The variability of European wind storms has been investigated for different frequency bands. For Eastern Europe an analysis was done for both bands, 3-5 years and 6-15 years. Storm frequencies have been correlated with North Atlantic (NA) sea surface temperatures (SST). It was found that storm occurrence for both frequency bands is connected with enhanced meridional temperature gradients in the NA.
Kruschke, T., Rust, H., Kadow, C., Müller, W., Pohlmann, H., Leckebusch, G., and Ulbrich, U., 2015: Probabilistic evaluation of northern hemisphere winter storm frequencies in the miklip decadal prediction system. Meteorol. Z., under revision.
Nissen, K. M, U. Ulbrich, G. C. Leckebusch, and I. Kuhnel, 2014: Decadal windstorm activity in the North Atlantic-European sector and its relationship to meridional overturning circulation in an ensemble of simulations with a coupled climate model, Clim. Dyn, 43, 1545-1555, doi: 10.1007/s00283-013-1975-6
Freie Universität Berlin, Institute for Meteorology
Prof. Dr. U. Ulbrich
Prof. Dr. H. Rust
Dipl.-Math. I. Höschel