Radar altimetry methods for solid earth geodynamics studies

Satellite radar altimetry, which was initially designed for accurate measurements of sea surface height, has been demonstrated to be applicable to non-ocean surfaces as well. In this study, three different applications of satellite altimetry to geodynamics studies have been examined: solid Earth crustal deformation due to Glacial Isostatic Adjustment (GIA) over Hudson Bay, North America, water level fluctuation over vegetated wetlands of Louisiana, and ice sheet elevation change over the Amundsen Sea sector, West Antarctica. For efficient altimetry data processing, high-rate (10-Hz for TOPEX, 18-Hz for Environmental Satellite (Envisat)) regional stackfiles were developed based on the original low-rate (1-Hz) global ocean stackfile. A modified threshold retracker has also been developed for more accurate land radar waveform retracking. 90-m resolution C-band Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) plays an important role to be used as a reference surface to select an optimal retracker, to correct surface gradient errors, and to calculate land surface anomalies over Hudson Bay. As a result, the crustal vertical motion is estimated from TOPEX decadal (1992--2002) time series over land surfaces with height variation (in terms of standard deviation) less than 2 m. The estimated vertical motion has been compared with the vertical motion derived from Gravity Recovery and Climate Experiment (GRACE) satellite and several GIA models. It agrees well with the laterally varying 3D GIA model, RF3S20 (beta=0.4) with differences of -0.1 +/- 2.2 mm/year. It is anticipated that the new observation from this study can be used to provide additional constraints for GIA model improvement. The 10-Hz stackfile procedure has also been utilized to observe the Louisiana wetland water level variations over each of 10-Hz stackfile bin with along-track spacing of ∼660 meter using TOPEX altimeter measurements. The feasibility of applying retracking has also been examined. Specifically, the water level variation over the swamp forest along the Atchafalaya River basin has been examined with the SRTM DEM elevation and L-band Advanced Land Observing Satellite (ALOS) Synthetic Aperture Radar (SAR) imagery. It has been found that the water level fluctuations in terms of amplitude of each 10-Hz TOPEX time series becomes larger as the elevation decreases and the SAR backscattered power increases. Over the Amundsen Sea sector, which suffers dynamic thinning due to the recent acceleration of coastal glaciers, the 18-Hz stackfile has been built using Envisat retracked measurements. The rates of ice sheet elevation changes have been estimated over 1° x 1° regions with 1-km resolution Antarctic DEM which is used to correct for the surface gradient error. The ice mass loss from September 2002--May 2005 has been estimated to be -49 +/- 5 Gigaton/year after correcting for the firn depth changes, which correspond to equivalent sea level change of 0.14 +/- 0.01 mm/year.