Trends in Precipitation Maxima at U.S. Historical Climatology Network Stations 1893-2010

摘要

Many recent studies predict changes in annual precipitation patterns and impacts on water supplies based on global climate modeling. This research is less valuable for forecasting changes in severe storms, which is of primary interest for urban infrastructure studies, such as flood control and water quality planning. Examination of extreme rainfall trends over the last century can be an indicator of likely future patterns, and can illuminate deficiencies in available frequency statistics that are usually based on assumed stationarity in the historic data. This study analyzes daily data from 1893 to 2010 for the 1,100-station U.S. Historical Climatology Network (USHCN). Stations were grouped according to nine climate regions for the contiguous US. Each USHCN station dataset was analyzed to identify 1-year and 5-year rainfall at 1-day and 7-day durations. The methodology follows the analysis presented by Kunkel et al. (1998) in an analysis of 1931-1996 data. Each station dataset was considered stationary for establishment of comparative precipitation thresholds. Each partial duration series of maxima was fitted to the generalized extreme value function using CDM Smith's NetSTORM rainfall analysis software to identify station-specific rainfall maxima. In the five eastern-US climatic regions (Northeast, Southeast, East North Central, Central, and South), the annual incidence of 5-year 1-day rainfall in the 40 years since 1970 averages 26 percent higher than in the preceding 77 years. The annual incidence of 5-year 1-day rainfall in the 20 years since 1990 is 38 percent higher than in the preceding 97 years. In the Northwest, Southwest, and West, the annual incidence of 5-year 1-day rainfall is nearly identical for the pre- and post-1970 periods. Comparable trends are apparent even for 1-year 1-day rainfall: the four decades since 1970 have 19 percent more events per year than the preceding eight decades across the Northeast, South, Central, and East North Central regions. Since 1990, there have been 23 percent more 1-year 1-day rainfall events per year than during the previous century in those same regions. Analysis of 7-day rainfall yielded results similar to the 1-day findings in most climate regions. The increases correlate with shifts in annual precipitation - the eastern stations recorded seven percent more precipitation in the four decades since 1970 compared with the prior eight decades, while western stations recorded four percent more precipitation than previously over those same periods. The changes in large storm frequency are greatest in the northeastern quarter of the country; the incidence of 5-year, 1-day events since 1990 compared with the preceding nine decades increased by 51, 39, and 44 percent for the Northeast, Central, and East North Central climatic regions. While these changes may in part be driven by random variation, inter-decadal patterns, and artifacts of the monitoring methodologies, they also indicate larger climatic trends, and demonstrate that the assumption of stationarity in existing extreme rainfall atlases is inappropriate in many places.