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Rosenau R.,TU Dresden | Schwalbe E.,TU Dresden | Maas H.-G.,TU Dresden | Baessler M.,Institute For Methodik Der Fernerkundung | Dietrich R.,TU Dresden
Journal of Geophysical Research: Earth Surface | Year: 2013

We performed three field campaigns in 2004, 2007, and 2010 at the southern margin of the Jakobshavn Isbræ, West Greenland, in order to infer flow velocities and their changes from photogrammetric time-lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface. Area-wide analysis of more than 3000 three-dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events. From 2004 to 2010, the grounding line of Jakobshavn Isbræ retreated 3.5 ± 0.2 km. Considering previously published results, the grounding line retreat amounts to 6 km since 1985. The glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events. Observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m/d can be reached. Moreover, large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event. After the break up, the glacier slowly adjusts to the new boundary conditions within the next 4-5 days. Flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large-scale glacier dynamics. Key Points Development of a method to determine the location of the grounding line Inland migration of the grounding line of Jakobshavn Isbrae Observation of high-resolution flow pattern during calving event ©2012. American Geophysical Union. All Rights Reserved.

Van Roozendael M.,Belgian Institute for Space Aeronomy | Spurr R.,Rt Solutions, Llc | Loyola D.,Institute For Methodik Der Fernerkundung | Lerot C.,Belgian Institute for Space Aeronomy | And 10 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

The Global Ozone Monitoring Instrument (GOME) was launched in April 1995 on ESA's ERS-2 platform, and the GOME Data Processor (GDP) operational retrieval algorithm has produced total ozone columns since July 1995. We report on the new GDP5 spectral fitting algorithm used to reprocess the 16-year GOME data record. Previous GDP total ozone algorithms were based on the DOAS method. In contrast, GDP5 uses a direct-fitting algorithm without high-pass filtering of radiances; there is no air mass factor conversion to vertical column amount. GDP5 includes direct radiative transfer simulation of earthshine radiances and Jacobians with respect to total ozone, albedo closure and other ancillary fitting parameters - a temperature profile shift, and amplitudes for undersampling and Ring-effect interference signals. Simulations are based on climatological ozone profiles extracted from the TOMS Version 8 database, classified by total column. GDP5 uses the high-resolution Brion-Daumont-Malicet ozone absorption cross-sections, replacing older GOME-measured flight model data. The semi-empirical molecular Ring correction developed for GDP4 has been adapted for direct fitting. Cloud preprocessing for GDP5 is done using updated versions of cloud-correction algorithms OCRA and ROCINN. The reprocessed GOME GDP5 record maintains the remarkable long-term stability of time series already achieved with GDP4. Furthermore, validation results show a clear improvement in the accuracy of the ozone product with reduced solar zenith angle and seasonal dependences, particularly in comparison with correlative observations from the ground-based network of Brewer spectrophotometers. Copyright 2012 by the American Geophysical Union.

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