HR: 0800h
AN: U21C-0626    [Abstracts]
TI: Results from the GRACE Prime Mission: Monitoring Water Comparison of GRACE Monthly Estimates with Surface Gravity Variations at North American Sites
AU: Lambert, A
EM: tlambert@nrcan.gc.ca
AF: Geological Survey of Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: Huang, J
EM: jianliang.huang@nrcan.gc.ca
AF: Geodetic Survey Division, Natural Resources Canada 615 Booth Street, Ottawa, ON K1A 0E9, Canada
AU: Courtier, N
EM: courtier@nrcan.gc.ca
AF: Geological Survey of Canada, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: Liard, J
EM: jliard@nrcan.gc.ca
AF: Geodetic Survey Division, Natural Resources Canada 615 Booth Street, Ottawa, ON K1A 0E9, Canada
AU: Henton, J
EM: jhenton@nrcan.gc.ca
AF: Geodetic Survey Division, Natural Resources Canada 615 Booth Street, Ottawa, ON K1A 0E9, Canada
AU: * Klatt, C
EM: cklatt@nrcan.gc.ca
AF: Geodetic Survey Division, Natural Resources Canada 615 Booth Street, Ottawa, ON K1A 0E9, Canada
AU: Winester, D
EM: daniel.winester@noaa.gov
AF: National Oceanic and Atmospheric Administration, Table Mountain Gravity Observatory, 8600 North 39th Street, Longmont, CO 80503, United States
AB: One of the objectives of the GRACE mission is the monitoring of temporal changes in water mass distribution on the continents. To a first approximation, variations in the mean water content and the corresponding gravity signal can be expressed as a seasonal variation and a trend. We compare GRACE seasonal gravity variations with seasonal surface gravity variations at the Canadian Absolute Gravity Site (CAGS), Cantley (Quebec) and on Vancouver Island, BC. GRACE trends are compared with trends from long-series annual absolute gravity measurements at several sites in the continental interior of North America. In this comparison the monthly GRACE gravity values were derived from the CSR-RL04 spherical harmonic monthly models for the period of April 2002 to June 2007. Both least-squares fitting of the harmonic coefficient series and non-isotropic Gaussian filtering are used to extract the gravity signal from these models. The GRACE values represent estimates of mean gravity extending out to about 300 - 450 km from the comparison sites. For the comparison of seasonal variations at CAGS during the GRACE mission, we have used surface gravity variations provided by a GWR TT70 superconducting gravimeter at Cantley. Surface gravity values on Vancouver Island for comparison with GRACE were estimated at 3-month intervals by averaging absolute gravity measurements at four sites observed using absolute gravimeter FG5-106 (Natural Resources Canada). For the comparison of trends, we used surface gravity measurements carried out annually from 1993 to the present using free-fall absolute gravimeter FG5-106 and, occasionally, FG5-102 (NOAA). Some measurements made using JILA instruments were used before 1992. In the trend comparisons we assume that postglacial rebound is the primary process causing the long-term gravity change and use a theoretical, g-dot to h-dot, ratio of -0.15 μGal / mm and the free-air gradient to remove the vertical motion effect from the surface gravity. Significantly larger seasonal variations are observed at CAGS compared to the variations seen by GRACE, probably as a result of topographic effects. A closer fit of seasonal variations between GRACE data and surface gravity is obtained on Vancouver Island where the mean of four relatively closely grouped sites is used. The comparison of trends between GRACE and long-term, annual absolute gravity data in the continental interior shows remarkably good agreement over the GRACE mission period (2002 - 2007). It is clear from the absolute gravity data that at most sites significantly different five-year trends would be observed by GRACE at different epochs. The inter-annual variations do not appear to be explained by surface soil moisture variations. Deeper sources for the mass variations are suggested.
DE: 1217 Time variable gravity (7223, 7230)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1244 Standards and absolute measurements
DE: 1655 Water cycles (1836)
DE: 1855 Remote sensing (1640)
SC: Union [U]
MN: 2007 Fall Meeting