Atmospheric Rivers: Genesis, Representation, and Structure

The goal of this dissertation is to advance atmospheric river (AR) research in 3 distinct areas: (i) genesis of ARs, (ii) model representation of ARs, and (iii) the impact of AR core structure on landfalling precipitation.In Chapter 2, the range of synoptic patterns that north Pacific landfalling ARs form under are objectively identified using genesis day 500 hPa geopotential height anomalies in a self- organizing map (SOM). The SOM arranges the synoptic patterns to differentiate between two groups of climate modes - the first group with ENSO (El Nin ̃o Southern Oscillation), PDO (Pacific Decadal Oscillation), PNA (Pacific North American) and NP (North Pacific index) and the second group with AO (Arctic Oscillation), EPO (East Pacific Oscillation), and WPO (West Pacific Oscillation). These two groups have their positive and negative modes organized in opposite corners of the SOM. The ARs produced in each of the syn- optic patterns have distinct lifecycle characteristics (such as genesis and landfall location, duration, velocity, meridional/zonal movement) and precipitation impacts (magnitude and spatial distribution). The conditions that favor AR trajectories closer to the tropics tend to produce higher amounts of precipitation. The large-scale circulation associated with AR genesis shows a close relationship between the genesis location and the location and intensity of the upper level jet in the west/central pacific as well as anomalous, low level southwesterly winds in the east pacific. Chapter 3 focuses on evaluating The Energy Exascale Earth System Model (E3SM) version v1.0 for its ability to represent ARs, which play significant roles in water vapor transport and precipitation. The E3SM Project is an ongoing, state-of-the-science Earth system modeling, simulation, and prediction project developed by the U.S. Department of Energy (DOE). With an emphasis on supporting DOE’s energy mission, understanding and quantifying how well the model simulates water cycle processes is of particular importance. The characteristics and precipitation associated with global ARs in E3SM at standard resolution (1◦ x 1◦) are compared to the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA2). Global patterns of AR frequencies in E3SM show high degrees of correlation (>=0.97) with MERRA2 and low mean absolute errors (<1 %) annually, seasonally, and across different ensemble members. However, some large-scale condition biases exist leading to AR biases - most significant of which are: the double-ITCZ, a stronger and/or equatorward shifted subtropical jet during boreal and austral winter, and enhanced northern hemisphere westerlies during summer. By comparing atmosphere-only and fully-coupled simulations, we attribute the sources of the biases to the atmospheric component or to a coupling response. Using relationships revealed in Dong et al., 2021, we provide evidence showing the stronger north Pacific jet in winter and enhanced northern hemisphere westerlies during summer associated with E3SM’s double-ITCZ and related weaker AMOC, respectively, are significant sources of the AR biases found in the coupled simulations. In Chapter 4, we explore how the vertical structure of ARs - specifically the core (the area of the strongest moisture transport) of an AR - can influence precipitation on the west coast states of the U.S.. The relationship between moisture transport intensity and precipitation impacts for ARs is currently well established. While vertically integrated moisture transport (IVT) is the most significant predictor in the intensity of precipitation for landfalling ARs, other aspects remain understudied. In this chapter, we find that the height of the AR core - defined as the height of the moisture flux maximum - can cause significant differences in precipitation even when controlling for IVT strength. Depending on the AR core height and the landfall terrain height, precipitation influences are varied. We find ARs with low (1000 - 950 hPa) core heights have enhanced precipitation over all terrain heights, ARs with medium heights (950 - 900 hPa) generally have reduced precipitation but particularly so over elevated terrain, and high (< 900 hPa) core heights deliver more precipitation to elevated terrain and into the interior of the U.S.. Looking at trends over the last 4 decades, the AR core height means are shifting to slightly higher altitudes with greater variance - i.e. low and high AR core heights will become more frequent while medium core heights become less frequent. In order to carry out the analysis on the AR core, we developed a novel algorithm used to identify AR sectors (core, cold sector, and warm sector) using IVT and geometric constraints. We compare characteristics of these detected sectors in 41 years of reanalysis data to recent dropsonde observations and find agreement in key characteristics for all sectors although there are lower moisture flux and windspeed values in the cold and warm sector due to threshold differences and reanalysis biases.

Advisor: Chiang, John C. H.