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Abstract
Analysis of Sub-Ice-Shelf Tides in the Weddell Sea Using SAR Interferometry Laurie Padman Doug MacAyeal Eric Rignot Tides near and under floating ice shelves can influence heat transport into the sub-ice-shelf cavity, mixing of the under-ice water column, and the calving and subsequent drift of tabular icebergs. It is extremely difficult to collect ocean data in these environments. Fortunately, synthetic aperture radar (SAR) can provide information on ocean height variability under the ice shelves. The procedure involves creating a "differential SAR interferogram" ("DSI"). A simple SAR interferogram is the difference between two SAR images, and can be analyzed to show a combination of the time-varying vertical motion due to tides and other ocean processes, and the lateral ice flow of the glacial sheet. Then, by differencing two SAR interferograms to give a DSI, and assuming that the lateral ice flow is time-independent, the remaining signal can be attributed to ocean-forced vertical motion of the ice sheet. For simplicity, for most of this paper we will refer to the ocean-forced motion as "tidal motion" or ‘tides", however we will show later that other signals can be significant. A DSI image is not a simple map of ocean tides, being a finite-second-difference of the ice vertical displacement. Depending on the satellite orbit parameters, the DSI can also exclude displacement signals due to specific tidal constituents. For example, for a 1.0-day image separation, the S2 tide (with period 12.00 h) cannot be detected, since the tidal wave has exactly the same phase at each satellite pass. The K1 tide ( 23.93 h) is also only poorly resolved. Nevertheless, with appropriate orbit parameters and a sufficient number of DSI images for the same region, it is theoretically possible to determine the amplitude and phase of all major tidal constituents. With present limitations on DSI images available to us, just one DSI for each of the Ronne and the Filchner Ice Shelves in the southern Weddell Sea, we cannot map individual tidal constituents based only on SAR data. We can, however, use comparisons of the DSI images with synthesized tidal DSI fields based on numerical ocean tidal models to demonstrate that the DSI method can actually provide useful tidal and other ocean signals. |
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