HR: 0830h
AN: C31B-0407    [PDF]
TI: A Snow Accumulation Map of Part of Greenland Derived From InSAR Correlation Observations
AU: Oveisgharan, S
EM: shadio@stanford.edu
AF: Departments of Geophysics and Electrical Engineering, Stanford University, Packard Building, 350 Serra Mall, Stanford, CA 94305 United States
AU: * Zebker, H A
EM: zebker@stanford.edu
AF: Departments of Geophysics and Electrical Engineering, Stanford University, Packard Building, 350 Serra Mall, Stanford, CA 94305 United States
AB: We present here an improved method for estimating snow accumulation rates in the Earth's polar ice sheets from remote sensing observations. Microwave methods have often been used to extrapolate point accumulation rates to obtain continent-wide maps. Because ice is semi-transparent to microwave radiation, observations of the ice sheets reveal composition and structure of the top 10's of meters of firn when interpreted using electromagnetic scattering models. These in turn constrain estimates of ice accumulation rates, critical for determining the mass balance of the ice sheets. Typical approaches use radar reflectivity or emissivity measurements to obtain snow grain sizes, which are then related to accumulation rate. But reflectivity and emissivity depend not only on grain size, but also on the thickness of annual layers in the top few meters of firn. A radar image may appear bright because grain size is large, implying slow accumulation, or because the annual layers are thick, implying high accumulation. Using InSAR measurements of both reflectivity and correlation permits solution for both the layer thickness and the grain size at depth, hence the ambiguity in the brightness measurements is avoided. We developed an interferometric ice scattering model to relate ice grain size and layer thickness to accumulation. We propose a buried layer model based on the incoherent addition of echoes from hoar layer interfaces, in which the scattering from a single layer is found by small-perturbation methods. Because the model also depends on the total thickness of the layered region, we assumed a constant 6 m thickness for the layered region in Greenland's dry snow zone. Penetration depth estimates, also derived from InSAR, in this area are typically 20 m or more, so that much of the observed echo is from depths below the layers-scattering from individual grains is also significant in this region. The interferometric model describes the ERS amplitude and correlation data simultaneously and consistently, with variations in hoar layer spacing of ~11-26 cm and grain radius of 0.6-2 mm providing the needed structural fluctuations for the observed range of cross-section and correlation. In situ accumulation measurements varies from about 15-25cm w.e./yr in our test area. Comparison of our model inversion results with in situ core measurements show that the spatially-varying trend in accumulation rates over our study area is consistent between both data sets, which is not the case in many published maps derived from radar reflectivity or radiometer data alone. Interestingly, straight contouring of the Greenland-wide ice core data disagrees in this region also as the data are very sparse. Because the remote sensing InSAR method using both correlation and reflectivity data provides closer agreement to the truth measurements inferred from the cores, it is likely that a full continent accumulation map would be more accurate also.
DE: 0669 Scattering and diffraction
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 6924 Interferometry
DE: 6969 Remote sensing
SC: Cryosphere [C]
MN: 2003 Fall Meeting