UC Santa Barbara

Atmospheric Rivers (ARs), a global phenomenon, play an important role in the hydroclimate as they transport large amounts of moisture poleward via plumes in the lower troposphere across mid-latitudes and into Polar regions. Multiple studies have demonstrated that ARs are related to precipitation extremes, flooding, seasonal snowpack, and water availability where they occur. Despite ARs occurring about 10\% of the time during the winter and spring in High Mountain Asia (HMA), little is known about these unique inland penetrating ARs, their resulting orographic precipitation and their association with precipitation-related hazards such as landslides and lightning. This research demonstrates the importance of HMA ARs to annual precipitation, summarizes their spatial and temporal variability in recent decades, and contrasts the synoptic and mesoscale conditions of three unique and hazardous ARs. The first part of this research explores the connection between ARs and lightning via a timely case study on an AR in Santa Barbara, CA that simultaneously occurred with over 8,000 lightning flashes in under 24 hours. Thermodynamic analysis revealed that orographic forcing and the warm conveyor belt lifted the abnormally high water vapor content in the AR in a convectively unstable atmosphere, resulting in hail formation and enhanced electrification. The second part of this research uses 40 years of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalyses of the global climate (ERA5) to develop a climatology of ARs that reach HMA during winter and spring and quantify their contribution to seasonal precipitation. Combined empirical orthogonal function (cEOF) and k-means clustering applied to meridional and zonal integrated vapor transport identified three distinct HMA AR types with unique circulation and precipitation patterns. Synoptic composites revealed that each HMA AR type results in above-average precipitation in Northwestern, Western, and Eastern HMA, respectively, when they occur. The differences in the precipitation patterns is largely due to the location and magnitude of the upper-level troughs and ridges, influencing the low-level moisture circulation in the ARs. Further, we found that large-scale climate modes, such as the El Ni{\~n}o Southern Oscillation, Arctic Oscillation, and Siberian High significantly influence the frequency of Northwestern and Western HMA ARs. During HMA ARs in recent decades, trends in moisture transport and the height of the freezing level have both increased significantly, a concern considering that during HMA ARs with a higher than average freezing level, there is significantly less frozen precipitation. The third part of this research contrasts the mesoscale characteristics of two Western HMA ARs that both resulted in extreme precipitation. With Climate Forecast System Reanalysis (CFSR) dynamically downscaled to 6.7 km spatial resolution, we show that the orientation of the AR axis relative to the topography increases the efficiency of the precipitation more than the amount or duration of the moisture within the AR. However, the 6.7 km data does not quite resolve the extreme precipitation associated with these ARs; therefore, the final part of this research uses the Advanced Weather Research and Forecasting (ARW-WRF, hereafter WRF) model to improve the accuracy of simulated extreme precipitation at a finer-scale resolution (3 km). This dissertation demonstrates the relevance of ARs to the winter and spring hydroclimate and precipitation-related hazards in HMA, and provides information that can be used for future projections of precipitation across multiple timescales.