Predictability of midlatitude Rossby wave packets

Rossby Wave Packets (RWPs) are linked to extreme weather events, such as heavy precipitation and flooding, and exert a strong influence on the predictability of weather systems in the midlatitudes. Considering the whole wave packet, in the sense of the packet envelope, RWPs can be viewed as entities that describe variability of the atmosphere beyond the synoptic scale. We here examine the predictability of RWPs as such entities. As a verification metric we used the so-called Displacement and Amplitude Score (DAS) applied to the envelope field of the midlatitude flow. The DAS is based on a field deforming method and, as one of its major advantages, avoids the “double-penalty” verification problem without the need to identify single RWP objects. We assess RWP predictability using a 19-year period of NOAA GEFSV12 ensemble reforecasts for RWPs that have been previously tracked in reanalysis data.

Variations in RWP predictability are dominated by the stage of the RWP life cycle, with higher predictability found for the propagation stage than the onset and decay stages. In addition, RWP predictability exhibits a seasonal cycle, with higher predictability in winter than in summer. Controlling for seasonality and the stage of the life cycle, we find i) higher predictability for high-amplitude RWPs than low-amplitude RWPs and ii) a general pattern of higher predictability over Eurasia than over the ocean basins. Finally, predictability of the propagating stage is higher if forecasts are initialized after RWP onset than if initialized before onset. RWP onset thus acts as a partial predictability barrier to the subsequent propagation stage.