From ground measurements to global models : temporal and spatial characteristics of drought and extreme precipitation under historical and future global warming
Keywords: Earth System Science.

Drought and extreme precipitation events lead to devastating damages to human and natural systems. The characteristics of these extreme climate events, including the frequency, intensity, and spatial extent of extreme wet conditions or extreme dry conditions can shape the risk posed to a region by an event. There has been substantial evidence that global warming has altered and will continue to alter atmospheric, land, and ocean conditions, which in turn lead to changes in the characteristics of extreme climate events. Additional anthropogenic emissions are expected to modify the ocean, land, and atmosphere in the future, though the role that these changes will have on future extreme precipitation and drought events is still uncertain. In my dissertation, I employ a multitude of observed and modeled climate datasets available to quantify the characteristics of drought and extreme wet events under past and future anthropogenic warming. By systematically quantifying the intensity, frequency, duration and spatial extent of extreme climate events across datasets, I aim to provide a climatological assessment of extreme climate event characteristics, and identify spatial or temporal variations of these characteristics. The findings from this dissertation will allow further insight into the impact past and future global warming have on extreme climate events, and subsequently, the risks to human and natural systems. In my first chapter, I investigate the expected changes in the frequency, duration, and spatial extent of drought over the globe in the 21st century under increased anthropogenic warming. To capture drought index-based uncertainty, I calculate four different drought indices which use precipitation, runoff, and moisture deficit, and to capture model-based uncertainty, I use climate simulations from 15 global climate models (GCMs) in the CMIP5 database. By comparing drought characteristics in the historical (1961-2005) simulations to the RCP 8.5 scenario (2010-2099) simulations, I show increases in the frequency and spatial extent of drought over the 21st century. These increases over the tropics and subtropics, reaching 80 more percentage points in the spatial extent and 40 more drought events, are robust among climate models when using deficit-based drought indices, but less robust when using precipitation or runoff-based indices. Given that deficit-based indices directly account for variations in temperature, these indices capture the significant increases in temperature simulated under high levels of anthropogenic emissions throughout the 21st century. These findings suggest that there is an increasing risk in drought stresses in many regions given the current trajectory of greenhouse gas emissions and associated warming temperatures. In my second chapter, I develop a geostatistical method to assess the spatial extent, or length scales, of extreme precipitation in GCHN-D station data. A climatological assessment over the US from 1965-2014 reveals significant seasonal and regional variations in the length scales of extreme daily precipitation. The eastern half of the US has daily extreme precipitation length scales reaching 400km during the winter months, but the length scales are halved during the summer months. The Northwest region, on the other hand, has little seasonal variation, with short extreme precipitation length scales of approximately 150km year-round. Though the magnitude of extreme precipitation length scales can be sensitive to certain choices in my method, the seasonal and regional variations remain relatively intact and can plausibly be explained by well-known atmospheric phenomena. This chapter introduces a valuable framework that can be used to quantify changes in the spatial extents of extreme climate events in the US and globally, and examine the impacts of varying spatial extents of extreme precipitation events on human and natural systems. In my third chapter, I use the geostatistical method I developed in my second chapter to quantify the intensity and length scales of tropical cyclone precipitation (TCP) along US-landfalling Atlantic tropical cyclone tracks from 1900-2017. Using GHCN-D precipitation station data and HURDAT2 tropical cyclone track data, I find significant variations in TCP intensity and length scales across different tropical cyclone strengths. TCP intensity and length scales are largest along major hurricane tracks. The highest values of TCP intensity (more than 150 mm/day) are found along major hurricane tracks, and the TCP intensity distribution is shifted positively during the weaker phases along the tracks. During these weaker phases, I also find the longest TCP length scales. However, the longest length scales of extreme TCP (> 75 mm/day) are found during the strongest phases. TCP intensity and length scales are significantly smaller along tropical storm and minor hurricane tracks. I also find varied changes in TCP intensity and length scales between the first and second halves of the 20th century. The upper quartiles of minor hurricane and tropical storm TCP intensity have significantly increased, alongside significant increases in the extent of extreme TCP. The intensity and extent of extreme TCP during major hurricanes have significantly decreased. The climatology of TCP intensity and length scales can be linked to the strength, speed, and symmetry of a tropical cyclone, as well as interactions with the land surface and topography. Moreover, the changes in TCP intensity and length scales can be explained by changes in precipitable water and vertical wind shear. This chapter provides important insights into the intensity and spatial extent of precipitation across different tropical cyclone strengths, allowing us to better understand the possible changes in TCP under future global warming. Taken together, these chapters advance our understanding of the risks posed by extreme climate events, and inform future research directions in this area of climate science. Given that these events cause severe losses to communities and their surrounding ecosystems, it is imperative that we advance our ability to address the effects of extreme climate events.