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Streuverhalten und Morphologie der antarktischen Schneedecke aus Scatterometermessungen von ERS-1 (Backscattering Behaviour and Morphology of the Antarctic Snow Pack from Scatterometer Measurements of ERS-1)

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Rack, W. (1995): Streuverhalten und Morphologie der antarktischen Schneedecke aus Scatterometermessungen von ERS-1 (Backscattering Behaviour and Morphology of the Antarctic Snow Pack from Scatterometer Measurements of ERS-1) , Diplom thesis, Leopold-Franzens-Universität Innsbruck.
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Abstract:

Improved knowledge on mass balance and dynamics of Antarctic ice is needed to understand the response and sensitivity of the ice sheet to climate change. The aim of this study is to give insight in physical surface and subsurface properties of polar snow and ice by means of the low resolution Active Microwave Instrument (AMI) in wind mode, operating onboard the European Remote Sensing Satellite-1 (ERS-1). In addition to basic research on microwave interaction mechanisms, knowledge of backscattering properties of polar firn is of significant interest for analysis of SAR imagery and for radar altimetry over the ice sheets.The ERS-1 wind scatterometer (WSC) operates in C-Band (5.3 GHz), VV polarization, and is primarily designed for estimating near surface wind vectors over the ocean. The three sidelooking beams of the scatterometer and overlapping beams of adjoining orbits enable the observation of the surface under different incidence and azimuth angles. The illuminated swath width is 500 km corresponding to incidence angles from 18° to 59° degrees. Due to the large resolution cells (about 50 km) the application of these data is limited to extended and homogeneous areas. The high resolution SAR of ERS-1 can be used only over regions with direct downlink to ground receiving stations. Complementary to this, the WSC gives useful information at high temporal resolution which can be applied for studies from regional to global scale.WSC data, acquired between July 1992 and April 1993 over Antarctica, were the basis of this study. For comparison climatological and meteorological data and in situ measurements were available. The WSC data were made available by ESA within the ERS-1 AO Experiment A2 on "Backscattering properties of the Antarctic firn by means of ERS-1 AMI".Preparatory and comparative field campaigns for the ERS-1 experiment were carried out by scientists of the University of Innsbruck in cooperation with the Alfred-Wegener Institute for Polar and Marine Research in 1990 and 1992. During these field campaigns in Dronning Maud Land and on Ronne Ice Shelf backscattering and emission of various firn types were measured with a groundbased scatterometer at C- and X-Band in dependence of incidence angle and polarization. Observations of physical properties of snow and ice from these field measurements and published data provide basic information for interpretation of the spaceborne scatterometer measurements.For characterizing different regimes of polar firn by means of ERS-1 WSC-data four parameters were defined: the mean intensity, the incidence angle dependence, the azimuthal anisotropy and the azimuth angle of maximum backscattering. Spatial variations of these parameters are related to the snow metamorphic state which depends on accumulation rate, temperature and wind. Using these backscattering parameters ten different classes of snow and ice were discriminated over Antarctica.The highest backscattering coefficients are observed for refrozen firn along the coast where ice lenses and ice pipes act as strong scatterers (percolation zone). Comparatively high s° values are also found on the high central plateau where the accumulation rates are low and the snowpack shows pronounced layering. Backscattering is low in zones with permanently dry snow but high accumulation. In this regions the morphology of the snow is more homogeneous than in the low accumulation regions. The lowest s° values are observed for melting snow.The regional distribution of incidence angle dependence of backscattering, expressed as ratio of s° at two different incidence angles, is rather complicated. Strongest angular decrease is observed in zones with high accumulation in West Antarctica. Low values are observed on the high central plateau. The backscattering coefficients s° of the percolation zones show the lowest values of incidence angle dependence and almost no dependence on the azimuth angle.The azimuthal dependence can be explained by variations in the orientation of reflecting interfaces at the surface and in the top meters of the snow pack. The highest values of anisotropy are found over regions with katabatic winds with high directional persistence over the gently sloped areas of the East Antarctic Plateau. The azimuth angle of maximum s° is typically oriented perpendicular to the direction of the dominating surface wind. The direction of minimum backscattering was plotted on a grid and compared with a model of East Antarctic katabatic winds. In regions with strong winds the wind direction from the model and the direction of minimum backscattering show in general good agreement. Differences may be caused by various features influencing the formation of sastrugi and are also due to limited azimuthal resolution of the WSC data and due to local errors of the katabatic wind model.The backscattering signatures were found to be very stable in time over the large interior parts of Antarctica which are permanently dry. This offers opportunities for monitoring sensor stability and for sensor intercalibration. Significant temporal variations were observed only during summer in coastal areas and on the Antarctic Peninsula. Comparisons with the temporal sequences of SSM/I (Special Sensor Microwave/Imager of DMSP-satellite) brightness temperatures at 19 GHz were used to describe the temporal dynamics of the melt-freeze cycles. Time series of the scatterometer measurements provide informations about the extent and duration of summer melt.The investigations clearly demonstrated the high potential of ERS-1 scatterometer for studies of the polar ice sheets. The excellent stability of the instrument, the high frequency of observations, and the possibility to observe the surface under a range of incidence and azimuth angles emphasize the value of the scatterometer as complementary sensor for land surface investigations.

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