Cascading effect of meteorological forcing on extreme precipitation events: Role of atmospheric rivers in southeastern US

Abstract

The past few decades have witnessed an intensification of extreme precipitation (EP) events triggered by atmospheric rivers (ARs), leading to massive flooding, highlighting the importance of studying the physical mechanisms associated with these types of events. This study aims to investigate the spatiotemporal evolution of ARs and related extreme precipitation (AR-EP) events, and the cascading effect of the synoptic-scale meteorological patterns on underlying processes in the Coastal States of the Southeastern United States (SUES) during the 1979–2019 period. The seasonal frequency of EP events associated with ARs suggests that more frequent AR-EP events occur during the colder months (November to April). In contrast, the AR-EP events are less frequent but more severe in the warmer months (May to October). A total of 12–15 AR-EP events, with severity exceeding the 99th percentile precipitation threshold, were observed during the 3-month overlapping seasons between November and April from 1979 to 2019 that affected Georgia, Florida, Alabama, and South Carolina. On the other hand, the average precipitation magnitude of the AR-EP events is relatively higher (55–90 mm/day) in the warmer months (May to October). To explore the cascading nature of relevant meteorological forcing on the physical processes that favor such events, we performed an event-centered composite analysis based on the top 100 severe AR-EP events observed during the extended cold and warm season, separately. It was observed that during the progression of the AR-EP events, the anomalies associated with composite mean sea level pressure (MSLP) and 850mb geopotential height (Z850) make a transition from the trough to ridge formation along with a south-eastward extension of Bermuda High in the cold season. The spatiotemporal evolution of these meteorological variables is found to have a cascading effect on the mode of moisture transport indicated by integrated vapor transport (IVT) and moisture availability shown by total column water vapor associated with the major AR-EP events. The warm season IVT field gets stronger 2-days before the AR-EP event occurrences indicating a continuous increase in moisture influx into the Gulf and Atlantic Coastal Plains. Similar strengthening of IVT is noted over the Gulf Coastal Plains during the cold season. A cascading effect is also noted for the moisture availability indicated by a significant increase in total column water vapor (TCWV) over the Gulf of Mexico 2 days before the events. Overall, the cold season AR-EPs are driven by relatively stronger dynamical systems indicated by greater IVT intensity. In contrast, the warm season AR-EPs are associated with a weaker IVT field, higher atmospheric instability, and more moist conditions.