Predictability of midlatitude Rossby wave packets and their underlying dynamics

Rossby Wave Packets (RWPs) are linked to extreme weather events 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 skill 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. Object-based methods tend to be highly sensitive to the choice of thresholds used to identify the objects and conceptual issues arise when assigning forecast to analysis objects. We assess RWP predictability using NOAA GEFSV12 ensemble reforecasts for RWPs that have been previously tracked in ERA5 data, due to the higher available temporal resolution. The ERA5 dataset was also used to quantify individual dynamical contributions to the amplitude evolution of single RWPs by partitioning the PV-tendency equation.

A prominent result is that RWP predictability depends on the stage of the RWP lifecycle: The propagation stage exhibits higher predictability than the decay or genesis stage. A small seasonal dependence is found, with summer being the least predicable season. No significant dependence is found on size and amplitude of RWPs or their geographical location. The presentation will further discuss differences in predictability that arise due to differences in the dynamical proccesses that contribute to the evolution of RWPs.