Prasad Gogineni

Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf

acceleration exceeds 27 km 3 per year, and ice is thinning at rates of tens of meters per year. We attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf. The magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change. INDEX TERMS: 1827 Hydrology: Glaciology (1863); 1863 Hydrology: Snow and ice (1827); 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 6924 Radio Science: Interferometry; 9310 Information Related to Geographic Region: Antarctica. Citation: Rignot, E., G. Casassa, P. Gogineni, W. Krabill, A. Rivera, and R. Thomas (2004), Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B ice shelf, Geophys. Res. Lett., 31, L18401, doi:10.1029/ 2004GL020697.

Observation and analysis of ice flow in the largest Greenland ice stream

We have applied satellite radar interferometry methods to map the velocity field of the recently discovered Northeast Greenland Ice Stream. We have used these data in conjunction with ice thickness and accumulation data to determine that the ice stream is in balance within the measurement errors. We used control methods to invert a finite element model of ice sheet flow constrained by the data to infer the ice streams basal shear stress distribution. Our results reveal that flow in a section of the downstream end has much in common with the streaming flow of the Ross Ice Streams of West Antarctica (e.g., a weak bed and fast flow in the presence of low driving stresses). For several hundred kilometers along the middle of the ice stream, the basal shear stress balances the driving stress. In the upstream area, where the ice stream is first visible in the velocity data, the bed appears to be weak, which may contribute to the initiation of the ice stream.

Radar measurements of melt zones on the Greenland Ice Sheet

Surface-based microwave radar measurements were performed at a location on the western flank of the Greenland Ice Sheet. Here, firn metamorphosis is dominated by seasonal melt, which leads to marked contrasts in the vertical structure of winter and summer firn. This snow regime is also one of the brightest radar targets on Earth with an average backscatter coefficient of 0 dB at 5.3 GHz and an incidence angle of 25°. By combining detailed observations of firn physical properties with ranging radar measurements we find that the glaciological mechanism associated with this strong electromagnetic response is summer ice lens formation within the previous winters snow pack. This observation has important implications for monitoring and understanding changes in ice sheet volume using space-borne microwave sensors.

Ice-sheet bed 3-D tomography

Information on bed topography and basal conditions is essential to developing the next- generation ice-sheet models needed to generate a more accurate estimate of ice-sheet contribution to sea-level rise. Synthetic aperture radar (SAR) images of the ice-bed can be analyzed to obtain information on bed topography and basal conditions. We developed a wideband SAR, which was used during July 2005 to perform measurements over a series of tracks between the GISP2 and GRIP cores near Summit Camp, Greenland. The wideband SAR included an eight-element receive-antenna array with multiple-phase centers. We applied the MUltiple SIgnal Classification (MUSIC) algorithm, which estimates direction of arrival signals, to single-pass multichannel data collected as part of this experiment to obtain fine-resolution bed topography. This information is used for producing fine- resolution estimates of bed topography over a large swath of 1600 m, with a 25 m posting and a relative accuracy of approximately 10 m. The algorithm-derived estimate of ice thickness is within 10 m of the GRIP ice-core length. Data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9.1 and 10.3 m for the parallel and perpendicular tracks, respectively.

High-Altitude Radar Measurements of Ice Thickness Over the Antarctic and Greenland Ice Sheets as a Part of Operation IceBridge

The National Aeronautics and Space Administration (NASA) initiated a program called Operation IceBridge for monitoring critical parts of Greenland and Antarctica with airborne LIDARs until ICESat-II is launched in 2016. We have been operating radar instrumentation on the NASA DC-8 and P-3 aircraft used for LIDAR measurements over Antarctica and Greenland, respectively. The radar package on both aircraft includes a radar depth sounder/imager operating at the center frequency of 195 MHz. During high-altitude missions flown to perform surface-elevation measurements, we also collected radar depth sounder data. We obtained good ice thickness information and mapped internal layers for both thicker and thinner ice. We successfully sounded 3.2-km-thick low-loss ice with a smooth surface and also sounded about 1-km or less thick shallow ice with a moderately rough surface. The successful sounding required processing of data with an algorithm to obtain 56-dB or lower range sidelobes and array processing with a minimum variance distortionless response algorithm to reduce cross-track surface clutter. In this paper, we provide a brief description of the radar system, discuss range-sidelobe reduction and array processing algorithms, and provide sample results to demonstrate the successful sounding of the ice bottom interface from high altitudes over the Antarctic and Greenland ice sheets.

Ice flow of Humboldt, Petermann and Ryder Gletscher, northern Greenland

Radar interferometry, ice-penetrating radar profiles and an elevation model are used to determine the velocity fields, rates of ice discharge, approximate states of balance and catchment area for three large outlet glaciers in northeast Greenland. Discharge through flux gates is calculated for Humboldt and Petermann Gletscher, which are found to be in balance (at the level that the accumulation is known). A large difference between the measured and estimated fluxes for Ryder Gletscher may be a reflection of unsteady flow behavior for this glacier. The patterns ofice flow for the three glaciers considered are each unique, showing that the nature of ice discharge varies substantially from basin to basin, controlled by bed conditions and the presence of subglacial troughs and obstructions.

Validation of Airborne FMCW Radar Measurements of Snow Thickness Over Sea Ice in Antarctica

Antarctic sea ice and its snow cover are integral components of the global climate system, yet many aspects of their vertical dimensions are poorly understood, making their representation in global climate models poor. Remote sensing is the key to monitoring the dynamic nature of sea ice and its snow cover. Reliable and accurate snow thickness data are currently a highly sought after data product. Remotely sensed snow thickness measurements can provide an indication of precipitation levels, predicted to increase with effects of climate change in the polar regions. Airborne techniques provide a means for regional-scale estimation of snow depth and distribution. Accurate regional-scale snow thickness data will also facilitate an increase in the accuracy of sea ice thickness retrieval from satellite altimeter freeboard estimates. The airborne data sets are easier to validate with in situ measurements and are better suited to validating satellite algorithms when compared with in situ techniques. This is primarily due to two factors: better chance of getting coincident in situ and airborne data sets and the tractability of comparison between an in situ data set and the airborne data set averaged over the footprint of the antennas. A 2-8-GHz frequency modulated continuous wave (FMCW) radar loaned by the Center for Remote Sensing of Ice Sheets to the Australian Antarctic Division is used to measure snow thickness over sea ice in East Antarctica. Provided with the radar design parameters, the expected performance parameters of the radar are summarized. The necessary conditions for unambiguous identification of the air/snow and snow/ice layers for the radar are presented. Roughnesses of the snow and ice surfaces are found to be dominant determinants in the effectiveness of layer identification for this radar. Finally, this paper presents the first in situ validated snow thickness estimates over sea ice in Antarctica derived from an FMCW radar on a helicopterborne platform.

Linear chirp generator based on direct digital synthesis and frequency multiplication for airborne FMCW snow probing radar

This paper presents a linear chirp generator for synthesizing ultra-wideband signals for use in an FM-CW radar being used for airborne snow thickness measurements. Ultra-wideband chirp generators with rigorous linearity requirements are needed for long-range FMCW radars. The chirp generator is composed of a direct digital synthesizer and a frequency multiplier chain. The implementation approach combines recently available high-speed digital, mixed signal, and microwave components along with a frequency pre-distortion technique to synthesize a 6-GHz chirp signal over 240 μs with a <;0.02 MHz/μs deviation from linearity.

Identifying and Compensating for Phase Center Errors in Wing-Mounted Phased Arrays for Ice Sheet Sounding

Highly crevassed ice surfaces at ice-sheet margins and fast-flowing glaciers significantly scatter radar signals. The scattered signals, often known as surface clutter, mask weak echoes from the ice-bed interface. Large wing-mounted antenna arrays are essential to synthesizing low-sidelobe patterns to reduce surface clutter. However, wing-mounted arrays are susceptible to structural flexure, which causes amplitude and phase errors that result in shifting and filling of desired array pattern nulls. In this communication, we characterize the effects of wing flexure on array beamformation using a scaled array model, and we present a compensation method to mitigate phase center errors caused by wing flexure. The compensation greatly improves clutter suppression through improved null formation. Experimental results show that we obtain an average of 7.5 dB improvement in the signal-to-interference noise ratio.

Development of a multi-frequency airborne radar instrumentation package for ice sheet mapping and imaging

We have developed improved versions of three different radar systems and integrated them as an airborne instrumentation suite for sounding and imaging Polar ice sheets. The first instrument consists of a multi-channel, coherent, pulsed radar operating at VHF with up to 20 MHz bandwidth. This instrument is capable of sounding a few-kilometer thick ice while flying at altitudes up to 10 km above mean sea level. The second instrument is designed to operate at UHF using a burst of narrow-bandwidth signals to digitally synthesize a bandwidth of in excess of 300 MHz. This apparatus is used to measure internal layers of the ice sheet to a depth close to 100 m. The third component to the instrumentation package is based on a frequency-modulated continuous wave (FMCW) radar, which operates at microwave (Ku band) frequencies with up to 1 GHz of instantaneous bandwidth. This radar set is used to measure the ice sheet surface elevation profile with centimeter accuracy. We are presenting a description of each system, with emphasis on the VHF depth sounder. We also present sample field test results obtained during the 2009 austral summer season in Antarctica, as a validation of the performance of the instrument package.