Thursday, October 12, 2006
Surface temperature patterns in complex terrain: Daily variations and long-term change in the central Sierra Nevada, California
A realistic description of how temperatures vary with elevation is crucial for ecosystem studies and for models of basin-scale snowmelt and spring streamflow, which supply over 50% of water resources to the western United States. This presentation explores surface temperature variability using densely sampled temperature data from an array of 37 sensors, called the Yosemite Network, which traverses both slopes of the Sierra Nevada in the vicinity of Yosemite National Park, California. These data indicate that a simple lapse rate is often a poor description of the spatial temperature structure. Rather, the spatial pattern of temperature over the Yosemite Network varies considerably with synoptic conditions. Empirical orthogonal functions (EOFs) were used to identify the dominant spatial temperature patterns and how they vary in time. Temporal variations of these surface temperature patterns were correlated with large-scale weather conditions, as described by NCEP-NCAR reanalysis data. Regression equations were used to downscale larger-scale weather parameters, such as reanalysis winds and pressure, to the surface temperature structure over the Yosemite Network. These relationships demonstrate that strong westerly winds are associated with relatively warmer/cooler temperatures on the east/west slope of the Sierra, and weaker westerly winds are associated with the opposite pattern. Reanalysis data from 1947 to 2005 indicate weakening westerlies over this time period, a trend leading to relatively cooler temperatures on the east slope over decadal time scales. This trend is corroborated by observations and demonstrates the need to consider topographic effects when examining long-term changes in mountain regions.
Jessica Lundquist is an Assistant Professor in
hydrology and hydroclimatology at the University of Washington's Department of
Civil and Environmental Engineering.