Impacts of climate changes on the spatiotemporal distribution of precipitation in the western United States.

摘要

Precipitation in the Intermountain West is characterized by its great variability in both spatial and temporal distributions. Moreover, the spatiotemporal distribution of the precipitation is changing due to the climate changes. In this dissertation, three studies are conducted to investigate the multi-scale temporal variability of precipitation, the performance of current climate models on this variability, the influence of large-scale ocean oscillations on heavy precipitation, and the impact of human induced global warming on storm properties.;The first study is to examine the performance of current climate models on the simulation of the multi-scale temporal variability determined from the observed station precipitation data. The results show that the studied Global Circulation Models/Regional Climate Models (GCMs/RCMs) tend to simulate longer storm duration and lower storm intensity as compared to those from observed records. Most GCMs/RCMs fail to produce the high-intensity summer storms caused by local convective heat transport associated with the summer monsoon. Both inter-annual and decadal bands are present in the GCM/RCM-simulated precipitation time series; however, these do not line up to the patterns of large-scale ocean oscillations such as El Nino/La Nina Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). The results also show that these GCMs/RCMs can capture long-term monthly mean as the examined data is bias-corrected and downscaled, but fail to simulate the multi-scale precipitation variability including flood generating extreme events, which suggests their inadequacy for studies on floods and droughts that are strongly associated with the multi-scale temporal precipitation variability.;The second study investigates the integrated effect of large-scale ocean oscillations including ENSO, PDO, Atlantic Multi-decadal Oscillation (AMO), and North Atlantic Oscillation (NAO) on the multi-scale temporal variability and spatial distribution of heavy precipitation expressed as total precipitation when daily precipitation is larger than 95th percentile (R95) in the western United States using Empirical Orthogonal Function (EOF) analysis. The analysis has shown that the leading modes of R95 variability and the connections between local R95 and Sea Surface Temperature (SST) over Western United States are seasonally dependent. The first EOF mode of summer R95 is associated with AMO. The first two EOF modes of winter R95 are related to an integrated effects of ENSO, PDO, and NAO which explain nearly half (49%) of the spatial and temporal variance in R95 in this region. Additionally, the coupled effects of these three oceanic-atmospheric oscillations on winter R95 are evaluated by investigating the ENSO-R95 responses modulated by a combination of different PDO and NAO phases. The results have implications for predicting the seasonal precipitation extremes for next few decades over the western United States, which may be useful to forecasters and water managers.;In the third study, the potential changes of storm properties including storm duration, inter-storm period, average storm intensity, and within-storm pattern from 10 North American Regional Climate Change Assessment Program (NARCCAP) RCMs with both historical simulations (1968--2000) and future simulations (2038--2070) are evaluated. Results illustrate that NARCCAP RCMs are consistent with observed precipitation in the seasonal variation of storm duration and inter-storm period. The ability to simulate the seasonal trend of average storm intensity varies among locations. Within-storm patterns from RCMs exhibit greater variability than from observed records. Comparisons between historic and future simulations of storm properties indicate that most regions of United States will experience future precipitation projections with shorter storm duration, longer inter-storm period, larger average storm intensity, and unchanged within-storm patterns.;The western United States is undergoing rapidly changing social dynamics, pressure from an expanding population and a greater risk of water shortage and flooding. Gaining better knowledge of how climate changes will impact on the spatiotemporal distribution of precipitation will help us on hydrologic modeling and assessment of uncertainty of water sustainability in this region.