Linkages between extreme precipitation events in the central and eastern United States and rossby wave breaking

Abstract

Linkages between extreme precipitation events (EPEs) in the central and eastern United States andsynoptic-scale Rossby wave breaking are investigated using 1979-2015 climatologies of EPEs and upper-levelpotential vorticity (PV) streamers. The investigation focuses on two domains over the central and easternUnited 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 toclimatology immediately upstream of each domain during widespread EPEs. Majorities of the widespreadEPEs in the central (∼79%) and eastern (∼56%) U.S. domains co-occur with a PV streamer positionedimmediately upstream. Odds ratios of EPEs for days when a PV streamer occurs upstream of each domainindicate a strong, statistically significant association between EPEs and Rossby wave breaking. The strengthof the EPE-Rossby wave breaking linkage, as measured by co-occurrence fractions and odds ratios, tends toincrease with increasing EPE precipitation volume, such that the strongest linkage exists for widespreadEPEs. Composite analyses reveal that Rossby wave breaking can result in widespread EPEs by establishing apersistent high-amplitude synoptic-scale wave pattern, within which strong poleward water vapor transportand 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 casesresulting in widespread EPEs exhibit a significantly higher-amplitude wave pattern that favors greaterpoleward transport of moist, conditionally unstable air and stronger ascent over the EPE region.