HR: 14:00h
AN: H53F-01    [Abstracts]
TI: A Synoptic View of Low Frequency Variability in Fall Precipitation Across the United States
AU: * Small, D
EM: David.Small@tufts.edu
AF: Department of Civil and Environmental Engineering Tufts University, 200 College Ave, Medford, MA 02155, United States
AU: Islam, S
EM: Shafiqul.Islam@tufts.edu
AF: Department of Civil and Environmental Engineering Tufts University, 200 College Ave, Medford, MA 02155, United States
AB: Several recent studies have detected large increases in precipitation and stream flow across the United States over the 20th century, with the largest increases generally being reported in fall precipitation and low flow. Our recent study demonstrated that fall precipitation exhibits strong spatially coherent decadal variations across most of the interior of the country that can be explained by decadal variations in the frequency of rain days. In this study, we examine changes in the fall atmospheric circulation over the United States and identify circulation types that can explain the large trends and decadal variations in the frequency of fall precipitation. We apply a classification and regression trees based approach to categorize the atmosphere into different synoptic patterns. The method uses upper atmospheric and surface variables to classify each fall day into one of several circulation types. This air mass based approach offers several advantages over k means clustering on dynamic variables alone. Most importantly for the current study, the atmospheric patterns have been identified independent of precipitation and may therefore represent synoptic rather than local variations. After classifying each fall day from 1948 to 2004 into one of several synoptic patterns, we disaggregate daily precipitation in different regions of the United States based on circulation type and identify the dominant atmospheric controls on trends and decadal variations in precipitation frequency and total. Our results suggest that the decadal variations in precipitation frequency can be explained by the number of days with a strong trough over the southwestern United States and ridges over the Gulf of Alaska and Great Lakes. This pattern enhances southerly moisture transport from the Gulf of Mexico and the frequency of precipitation in the central United States and Great Plains. Another pattern features a ridge over the southwest and trough over eastern Canada. On these days, meridional flow into the central United States is northerly and the frequency of precipitation is reduced by 80-90%. Changes in the distribution of daily precipitation during different synoptic conditions will be discussed.
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1637 Regional climate change
DE: 1833 Hydroclimatology
DE: 1854 Precipitation (3354)
SC: Hydrology [H]
MN: 2007 Joint Assembly