Extratropical Cyclones, Atmospheric Rivers, and the Hydrometeorology of Extreme Rainfall for the US East of the Rocky Mountains

Abstract

Extratropical cyclones and their associated atmospheric rivers (ARs) are important agents of extreme rainfall and flooding across the Contiguous US. In this dissertation an atmospheric water balance framework is used to examine the hydroclimatology of flooding from extratropical cyclones, focusing on the Lower Mississippi River and the eastern United States. The upper tail of atmospheric water balance components is characterized through Generalized Extreme Value (GEV) analyses of precipitable water and vertically-integrated water vapor flux (IVT) fields derived from North American Regional Reanalysis (NARR). Analyses point to precipitable water fields with bounded distributions, but unbounded, thick-tailed distributions for IVT. Exceptionally strong extratropical cyclones have a strong impact on IVT extremes. The 40-year record derived from NARR fields suggest significant increasing trends in precipitable water and IVT, especially for the development region of Lower Mississippi River floods. Significant trends in IVT are found for averaging time periods from 1 - 14 days. An AR detection algorithm, based on an IVTthreshold of 500 kg/s/m, is used with 20th Century Reanalysis fields from 1950 - 2015. Analyses reveal pronounced seasonality in U.S. ARs east of the Rocky Mountains, with elevated frequency from November to May and minimum frequency during the summer months. The winter mode of AR occurrence - a southwest-to-northeast arc extending from the Gulf of Mexico to the Ohio River basin - plays a central role in the climatology of Lower Mississippi River flooding. Significant increasing trends are found in AR annual count, duration, size, and impact area in the Lower Mississippi. Through Cox regression model analyses, we find that seasonal- to decadal-scale climate variability contributes to the temporal clustering in the AR occurrence process, which is central to Lower Mississippi flooding. We examine the role of winter season extratropical cyclones as flood agents in the mid-Atlantic region of the U.S. through analyses on a powerful February 2016 event. We find that supercells played a central role in the distribution of extreme rainfall and flooding. Climatological analyses on ARs provide insight into winter-season flooding in the eastern U.S.