Search for: All records

Abstract Linkages between extreme precipitation events (EPEs) in the central and eastern United States and synoptic-scale Rossby wave breaking are investigated using 1979–2015 climatologies of EPEs and upper-level potential vorticity (PV) streamers. The investigation focuses on two domains over the central and eastern United States, respectively, and emphasizes widespread EPEs, events exhibiting exceptionally large precipitation volumes. The relative frequency of PV streamers is found to be significantly enhanced relative to climatology immediately upstream of each domain during widespread EPEs. Majorities of the widespread EPEs in the central (~79%) and eastern (~56%) U.S. domains co-occur with a PV streamer positioned immediately upstream. Odds ratios of EPEs for days when a PV streamer occurs upstream of each domain indicate a strong, statistically significant association between EPEs and Rossby wave breaking. The strength of the EPE–Rossby wave breaking linkage, as measured by co-occurrence fractions and odds ratios, tends to increase with increasing EPE precipitation volume, such that the strongest linkage exists for widespread EPEs. Composite analyses reveal that Rossby wave breaking can result in widespread EPEs by establishing a persistent high-amplitude synoptic-scale wave pattern, within which strong poleward water vapor transport and ascent are forced over the EPE region immediately downstream of an elongated upper-level trough. Additional analyses demonstrate that, compared to corresponding null cases, Rossby wave breaking cases resulting in widespread EPEs exhibit a significantly higher-amplitude wave pattern that favors greater poleward transport of moist, conditionally unstable air and stronger ascent over the EPE region. 

Abstract This study examines climatological potential vorticity streamer (PVS) activity associated with Rossby wave breaking (RWB), which can impact TC activity in the subtropical North Atlantic (NATL) basin via moisture and wind anomalies. PVSs are identified along the 2-PVU (1 PVU = 10 −6 K kg −1 m 2 s −1 ) contour on the 350-K isentropic surface, using a unique identification technique that combines previous methods. In total, 21 149 individual PVS instances are identified from the ERA-Interim (ERAI) climatology during June–November over 1979–2015 with a peak in July–August. The total number of PVSs identified in this study is more than previous PVS climatologies for this region, since the new technique identifies a wider range of cases. Variations in PVS size and intensity prompt the development of a new PVS activity index (PVSI), which provides an integrated measure of PVS activity that can improve comparisons with TC activity. For instance, PVSI has a stronger negative correlation with seasonal TC activity ( r = −0.55) relative to PVS frequency, size, or intensity alone. PVSI in June–July is also positively correlated with PVSI in August–November ( r = 0.67), suggesting predictive capability. Compared to the ERAI and Japan Meteorological Agency 55-Year Reanalysis (JRA-55) climatology, there are more PVSs in the Climate Forecast System Reanalysis (CFSR) but these have weaker average intensity overall. While no long-term trend in PVSI is observed in the ERAI or JRA-55 climatologies, a negative trend is observed in CFSR, which could be related to differences in near tropopause static stability early in the climatological period (1979–86) between the CFSR and ERAI datasets. 

Cool-season extreme weather events (EWEs) (i.e., high-impact weather events that are societally disruptive, geographically widespread, exceptionally prolonged, and climatologically infrequent) are typically associated with strong extratropical cyclones (ECs). The opportunity to investigate the genesis locations, tracks, and frequencies of ECs leading to EWEs over central and eastern North America and compare them to those of ordinary ECs forming over and traversing the same region motivates this study. ECs leading to EWEs are separated from ordinary ECs according to the magnitude, areal extent, and duration of their 925-hPa standardized wind speed anomalies in the 0.5° NCEP CFSR dataset. This separation allows for the construction of an October–March 1979–2016 climatology of ECs leading to EWEs over central and eastern North America. The climatology of ECs leading to EWEs over central and eastern North America reveals that these ECs typically form in the lee of the Rocky Mountains, over the south-central United States, and along the east coast of North America at latitudes equatorward of the typical genesis locations of ordinary ECs. ECs leading to EWEs exhibit equatorward-shifted tracks relative to ordinary ECs, likely associated with an equatorward shift in the position of the subtropical or polar-front jet. ECs leading to EWEs form most frequently in November and March, when the seasonal alignment of baroclinic and diabatic forcings is maximized. Similar to ordinary ECs, the genesis locations, tracks, and frequencies of ECs leading to EWEs are partially determined by the states of the Pacific–North American pattern and North Atlantic Oscillation.