Predictability of midlatitude Rossby-wave packets

Abstract

Rossby-wave packets (RWPs) along the midlatitude jet are a fundamental ingredient of extratropical dynamics. RWPs have been linked to enhanced atmospheric predictability and the occurrence of extreme weather events. We here investigate the predictability of Northern Hemispheric RWPs as physical entities, defined by enhanced values of the Rossby-wave envelope field. Using a catalogue of RWPs identified and tracked in reanalysis data, we analyze RWP predictability in a 19-year period of NOAA GEFSv12 reforecasts. Our analysis adopts the so-called distance and amplitude score (DAS), a verification metric that avoids both the double-penalty issue of field-based verification of coherent features and the complications of an object-based approach. Applied to the envelope field, forecast errors defined by this metric asymptote towards saturation, but do not completely reach saturation within the 10 days lead time available to this study. The growth rate of the median DAS is highest initially and decreases with lead time. This is a nontrivial result, because the underlying envelope field largely de-emphasizes phase information, but still exhibits very similar error-growth characteristics to fields that contain the full phase information. 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 that (i) high-amplitude RWPs exhibit higher predictability than low-amplitude RWPs up to 6 days lead time and (ii) there is a general pattern of relatively high predictability over Eurasia. Finally, predictability of the propagating stage is higher if forecasts are initialized after RWP onset than if initialized before onset. In this sense, RWP onset acts as a predictability barrier to the subsequent propagation stage.