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A8 - Dynamics and predictability of blocked regimes in the Atlantic-European region

Project leaders: PD Dr. Michael Riemer, Dr. Christian Grams, Prof. Dr. Volkmar Wirth

Other researchers: Dr. Franziska Teubler (PostDoc), Christopher Polster (PhD student), Seraphine Hauser (PhD student)

Weather regimes govern an important part of the variability of the midlatitude circulation on the sub-seasonal time scale and on continent-size regions. Of particular importance are regimes that feature a blocking anticyclone located over a specific region. Such blocked regimes are frequently associated with high impact weather events and, due to their longevity, are a putative source of sub-seasonal predictability. However, the correct representation of blocked-regime life cycles presents a major challenge for current numerical weather prediction (NWP) models. In fact, the failure to predict the onset and decay of blocking constitute major forecast errors at the medium-range to sub-seasonal timescale.

This project will investigate the underlying dynamics and the predictability of the life cycles of blocked regimes. Both nonlinear Rossby wave dynamics and cloud diabatic processes have been demonstrated to play major roles during the life cycle, but our understanding of the interactions between both and the associated sensitivities in the overall evolution are still insufficient. We aim at contributing to a major improvement of our understanding by combining two complementary approaches that have emerged recently: first, a potential-vorticity (PV) budget perspective will here be adapted to the PV- anomaly pattern that constitutes the respective blocked regime. The PV-anomaly patterns will be identified for the four blocked regimes of a novel year-round definition of seven weather regimes in the Atlantic-European region. The PV framework captures nonlinear balanced dynamics as well as the impact of diabatic processes. Within this framework, we will apply and extend recent insight into the PV dynamics of upscale error growth to the specific situation of onset and decay of blocking. Second, diagnostics will be applied that focus on describing larger-scale Rossby wave characteristics (based on local finite amplitude wave activity) and the state of the Rossby wave guide upstream of the blocking (using a local refractive index analysis). Within this framework, we will in particular test a recent hypothesis that relates the sensitivity in the onset of blocking to threshold behavior with respect to the amplitude of the oncoming wave.