HR: 09:45h
AN: GC21A-08    [Abstracts]
TI: Trends in Snowfall versus Rainfall for the Western United States
AU: * Knowles, N
EM: nknowles@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 496, Menlo Park, CA 94025 United States
AU: Dettinger, M D
EM: mddettin@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 496, Menlo Park, CA 94025 United States
AU: Dettinger, M D
EM: mddettin@usgs.gov
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Cayan, D R
EM: dcayan@ucsd.edu
AF: U.S. Geological Survey, 345 Middlefield Road, MS 496, Menlo Park, CA 94025 United States
AU: Cayan, D R
EM: dcayan@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AB: The water resources of the western U.S. depend heavily on snowpack to store part of wintertime precipitation into the drier spring and summer months. A well-documented shift towards earlier runoff in recent decades has been attributed to 1) more precipitation falling as rain instead of snow, and 2) earlier/faster snowmelt. The present study addresses the former, documenting a regional trend during the period 1949-2004 toward smaller ratios of winter-total snowfall water equivalent (SFE) to winter-total precipitation (P). The most pronounced reductions in this ratio have occurred in the Sierra Nevada and the Pacific Northwest, with more varied changes (still predominantly reductions) in the Rocky Mountains. The trends toward reduced SFE/P ratios are a response to warming across the region, with the most significant reductions occurring where winter-average wet-day minimum temperatures were warmer than -5C. Most of the SFE reductions were associated with winter wet-day temperature changes between 0 and +3°C over the study period. Larger warmings have occurred mainly at sites where the mean temperature was cool enough that precipitation form has been less vulnerable to warming trends. At the monthly scale, trends toward reduced SFE/P have been most pronounced in March regionwide and near the West Coast in January, corresponding to widespread warming in these months. Mean temperatures were colder in January at higher elevations, restricting SFE/P impacts to the lower-elevation sites near the West Coast, whereas warmer mean temperatures in March allowed the recent warming to produce SFE/P declines at sites across the study region.
DE: 1630 Impacts of global change (1225)
DE: 1655 Water cycles (1836)
DE: 1807 Climate impacts
DE: 1833 Hydroclimatology
DE: 1863 Snow and ice (0736, 0738, 0776, 1827)
SC: Global Climate Change [GC]
MN: Fall Meeting 2005