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C8 - Stratospheric influence on predictability of persistent weather patterns

Project leaders: Junior-Prof. Dr. Hella Garny, Prof. Dr. Thomas Birner, Prof. Dr. Joaquim Pinto

Other researchers: Selina Kiefer (PhD student), Sheena Löffel (PhD student), Jonas Späth (PhD student), Lisa-Ann Kautz (member)


Recent literature has provided evidence that stratospheric extreme events during Northern winter and spring may enhance the predictability of large-scale tropospheric circulation patterns such as the North Atlantic oscillation (NAO) and Greenland/European blocking. These circulation patterns are of high societal importance as they may lead to weather extremes in mid-latitudes (e.g., the recent March 2018 blocking event and associated cold spell over much of continental Europe).

In this project, we will leverage recent computing capabilities to provide new insights into how stratospheric events influence predictability of such persistent tropospheric weather patterns. Specifically, we will use large ensembles of specifically designed perturbation re-forecast experiments to study the downward stratosphere-to-troposphere (S-T) coupling and its impact on tropospheric predictability. Furthermore, the role of the tropopause communication layer during such downward coupling and more generally for predictability will be addressed. We will focus on events corresponding to anomalous weakening or strengthening of the stratospheric polar vortex during Northern winter and spring. Tropospheric predictability will be assessed in the extended to sub-seasonal forecast ranges (time scales of 10-30 days) by focusing on i) intense and persistent blocking events and their link to cold spells, and ii) extended zonal weather patterns with embedded storm series that lead to wet and windy conditions.

The project consists of three work packages (WPs).

WP1 deals with the statistical evaluation of ensemble forecasts in the sub-seasonal-to-seasonal database (S2S). The ensembles will be clustered with respect to their stratospheric and tropospheric evolution, quantifying the links between them.

In WP2 and WP3 we will conduct and evaluate experiments with the (global) ICON model, including idealized baroclinic eddy life-cycle experiments as well as full-blown global ensemble simulations for observed events. We will first study the sensitivity of the evolution of the coupled stratosphere-troposphere system to specific perturbations of the stratospheric flow, considering selected situations with persistent tropospheric weather patterns (WP2). Moreover, we will consider tropospheric perturbations to focus on the role of tropospheric variability in either masking or modifying the stratospherically-forced signal.

In WP3, the ensemble simulations from WP2 will be extended to a large number of stratospheric events (∼100 events) to gain more robust statistics and enhance detectability of the downward link between persistent tropospheric weather patterns and preceding stratospheric events.

The DLR contribution will deal with the overall question on predictability of the downward propagation of stratospheric circulation anomalies as a function of their specific characteristics. LMU will study the role of the tropopause communication layer in the downward coupling as well as carry main responsibility for linking the various aspects of the work plan and partners, while KIT will focus on the ultimate coupling of stratospherically modified large scale tropospheric flow patterns to the occurrence of extremes over Europe.