Predictability of Rossby wave packets: Climatology and dynamical characteristics

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Rossby waves are of fundamental importance for the dynamics of the Earth's atmosphere, and midlatitude weather systems are often embedded in distinct Rossby wave packets (RWPs). The significance of these wave packets for weather forecasting has long been recognized and special attention has been given to the role of RWPs as precursors to high-impact weather events. In general, it may be expected that RWPs, as large-scale flow features obeying balanced dynamics, exhibit a large degree of predictability, which may then be inherited by embedded, smaller-scale weather features. On the other hand, a number of recent studies have demonstrated that forecast errors and forecast uncertainty propagate and amplify within RWPs, severely affecting predictability. The important question therefore arises: Under which conditions do RWPs exhibit high predictability and under which conditions low predictability? The work proposed herein will significantly contribute to answer this question. We propose to investigate RWP predictability from a climatological perspective using a 30-year ensemble reforecast dataset. Our approach considers RWPs as distinct, physically meaningful entities that can be described by specific characteristics: amplitude, size, shape, and position, Predictability of these characteristics will be assessed from the ensemble data. Variations of predictability with season, geographical region, duration, and the phase of the life cycle of the RWP will be investigated. This phenomenological investigation will be complemented by a process-based investigation of the RWP dynamics. The goal is to contrast the dynamics of RWPs with high and low predictability, respectively, and thus to better understand significant variations in RWP predictability.