Mechanisms and Characteristics of Landfalling Atmospheric Rivers Affecting Southern California

Author(s): Harris, Sarah May | Advisor(s): Carvalho, Leila MV | Abstract: Atmospheric rivers (ARs) are filamentary channels of water vapor flux that play a vital role in the meridional transport of heat and moisture to midlatitudes. These features travel horizontally through the low atmosphere and over the open oceans. When they interact with orographic barriers such as coastal mountain ranges they are often responsible for high-intensity storms. In Southern California (SCA), an area that receives most of its annual precipitation from relatively few storms per season, ARs are important components of the region’s hydrological cycle. They provide significant proportions of annual precipitation totals within only a handful of storms and are responsible for many high-intensity rainfall events leading to hazards such as flooding. There is abundant AR research that analyzes the events affecting North America’s west coast. Yet, few examine ARs that landfall in SCA and instead focus on latitudes farther north. This research analyzes the AR events that make landfall in SCA, about their dynamics, characteristics, and lifecycles on the day of and in the days before landfall. It is the goal of this research to increase our understanding of these events to enhance AR modeling and forecasts for SCA. This may improve general preparedness, mitigate against hazards, and aid with water resources management.In this dissertation, we create an algorithm to identify ARs within reanalysis fields to determine the historical events affecting the west coast of North America (30degN-55degN) during the last four decades. We categorize these events according to landfall areas and create composites of regional events using additional reanalysis fields to establish atmospheric characteristics on the day of and the days before landfall. We aim to determine the defining characteristics of SCA ARs. We find that all ARs landfalling in western North America have landfall day characteristics consistent with baroclinic wave trains in the position and organization of moisture, temperature, and geopotential (500mb) heights. In the days before landfall, we determine that the position, phase, and amplitude of the wave train are important drivers behind SCA ARs which we see in the development of 500mb trough-ridge couplets in the western Pacific and subsequent changes to the 200mb jet core (g60ms^-11). We also investigate the relationship between these ARs and modes of variability, the El Nino Southern Oscillation, the Madden-Julian Oscillation and the Pacific North American Teleconnection Pattern. We find that there are no strong relationships between these modes with AR landfall locations indicating that while these modes may increase AR frequency for North America’s west coast, they are not the drivers of specific AR landfall locations. We also complete a backward trajectories model of identified west coast ARs to determine the hourly movement and lifecycles of air parcels from these events at various atmospheric levels. We run the model from the hour of landfall and hourly for the 72 hours before landfall. For all ARs, we find that variables indicative of orographic uplift occur in the hours (0-15 hours) right before landfall. However, we find that SCA ARs are slower and warmer, giving them the potential to hold more moisture. Significantly higher specific humidity values confirm these observations. A case study analysis of a particularly strong SCA AR is also introduced. We find commonalities between this AR and the average AR characteristics but also differences in along-trajectory variables of temperature and specific humidity values as well as temporal characteristics that highlight why this event was so extreme.Collectively these analyses show us fundamental differences between SCA ARs and ARs landfalling farther north along North America’s west coast. These characteristics need to be accounted for to improve event modeling and/or forecasting.