David A. Braaten

Coherent radar ice thickness measurements over the Greenland ice sheet

We developed two 150-MHz coherent radar depth sounders for ice thickness measurements over the Greenland ice sheet. We developed one of these using connectorized components and the other using radio frequency integrated circuits (RFICs). Both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice. We used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers. We operated both radar systems on the NASA P-3B aircraft equipped with GPS receivers. Radar data are tagged with GPS-derived location information and are collected in conjunction with laser altimeter measurements. We have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of Program for Regional Climate Assessment (PARCA) investigations and the North Greenland Ice Core Project. Radar echograms and derived ice thickness data are placed on a server at the University of Kansas (http://tornado.rsl.ukans.edu/Greenlanddata.htm) for easy access by the scientific community. We obtained good ice thickness information with an accuracy of ±10 m over 90% of the flight lines flown as a part of the PARCA initiative. In this paper we provide a brief description of the system along with samples of data over the interior, along the 2000-m contour line in the south and from a few selected outlet glaciers.

Widespread Persistent Thickening of the East Antarctic Ice Sheet by Freezing from the Base

A large fraction of the ice at Dome A, Antarctica, did not form by the usual process of snowfall compaction. An International Polar Year aerogeophysical investigation of the high interior of East Antarctica reveals widespread freeze-on that drives substantial mass redistribution at the bottom of the ice sheet. Although the surface accumulation of snow remains the primary mechanism for ice sheet growth, beneath Dome A, 24% of the base by area is frozen-on ice. In some places, up to half of the ice thickness has been added from below. These ice packages result from the conductive cooling of water ponded near the Gamburtsev Subglacial Mountain ridges and the supercooling of water forced up steep valley walls. Persistent freeze-on thickens the ice column, alters basal ice rheology and fabric, and upwarps the overlying ice sheet, including the oldest atmospheric climate archive, and drives flow behavior not captured in present models.

Risk of rising sea level to population and land area

Low-elevation land areas and their populations are at risk globally from rising sea level. Global sea level has risen by about 2 millimeters per year over the past century. About half of this rise may be attributed to thermal expansion of the ocean and the melting of temperate-latitude glaciers [Dyurgerov and Meier, 1997]. The remainder of the rise is believed to come from a net loss of mass from the Antarctic and Greenland ice sheets, although the exact contribution is unknown.

Advanced Multifrequency Radar Instrumentation for Polar Research

This paper presents a radar sensor package specifically developed for wide-coverage sounding and imaging of polar ice sheets from a variety of aircraft. Our instruments address the need for a reliable remote sensing solution well-suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution. The sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance. The instruments operate simultaneously over different frequency bands within the 160 MHz-18 GHz range. The sensor package has allowed us to sound the most challenging areas of the polar ice sheets, ice sheet margins, and outlet glaciers; to map near-surface internal layers with fine resolution; and to detect the snow-air and snow-ice interfaces of snow cover over sea ice to generate estimates of snow thickness. In this paper, we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance. We also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform. Finally, we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features.

GIS analysis of global impacts from sea level rise.

Future sea level rise caused by climate change would disrupt the physical processes, economic activities, and social systems in coastal regions. Based on a hypothetical global sea level increase of one to six meters, we developed GIS methods to assess and visualize the global impacts of potential inundation using the best available global datasets. After susceptible areas were delineated, we estimated that the size of the areas is between 1.055 (one meter) to 2.193 million km 2 (six meters). Population in the susceptible areas was estimated to range from 108 (one meter) to 431 million (six meters) people. Among the seven land-cover types in the susceptible areas, forest and grassland account for more than 60 percent for all the increments of sea level rise. A suite of interactive visualization products was also developed to understand and communicate the ramifications of potential sea level rise.

A wideband radar for high-resolution mapping of near-surface internal layers in glacial ice

Snow accumulation rate is an important parameter in determining the mass balance of polar ice sheets. Accumulation rate is currently determined by analyzing ice cores and snow pits. Inadequate sampling of the spatial variations in the ice sheet accumulation has resulted in accumulation rate uncertainties as large as 24%. We designed and developed a 600-900-MHz airborne radar system for high-resolution mapping of the near-surface internal layers for estimating the accumulation rate of polar ice sheets. Our radar system can provide improved spatial and temporal coverage by mapping a continuous profile of the isochronous layers in the ice sheet. During the 2002 field season in Greenland, we successfully mapped the near-surface layers to a depth of 200 m in the dry-snow zone, 120 m in the percolation zone, and 20 m in the melt zone. We determined the water equivalent accumulation rate at the NASA-U/spl I.bar/1 site to be 34.9/spl plusmn/5.1 cm/year from 1964 to 1992. This is in close agreement with the ice-core derived accumulation rate of 34.6 cm/year for the same period.

Wind Tunnel Experiments of Large Particle Reentrainment-Deposition and Development of Large Particle Scaling Parameters

Reentrainment-deposition characteristics of lycopodium spores, timothy pollen, glass microballoons, glass spheres, and nickel spheres were examined in a series of wind tunnel experiments. Particle diameter varied from 18 to 34 (μm, and particle density varied from 1.0 to 8.9 g cm−3. Adhesion force distribution of each particle type was determined using the centrifuge method, and the force required to remove 50% of the particles had a range of more than an order of magnitude with lycopodium requiring the smallest force and glass spheres requiring the largest. The mean flow speed required to reentrain 50% of each particle type had a range of more than a factor of two with the same ordering from smallest speed to largest as found for adhesion force with the exception of nickel spheres, which required the second largest wind speed and the second smallest adhesion force for 50% removal. Threshold reentrainment flow speed, and distance traveled in a single reentrainment-deposition cycle were also examined for e...

Boundary-layer flow structures associated with particle reentrainment

Measurements of near-surface longitudinal and vertical wind velocity components associated with particle reentrainment from a flat surface have been examined in a wind tunnel. Sparsely covered particle beds were used to assure that observed reentrainment events resulted primarily from the action of fluid forces. Characteristic velocity patterns were found to be associated with a majority of particle reentrainment events examined. These characteristics have been categorized and examined as ensemble averages. The flow pattern most frequently observed during particle reentrainment was termed Ejection-Sweep (E-S) and is very similar to organized fluid motions previously observed in laboratory flows and in the atmospheric boundary layer. A simple two-tiered E-S pattern recognition scheme is described which strives to identify particle reentrainment events objectively based on flow characteristics alone. The first step is to identify potential E-S patterns using criteria which identify a characteristic longitudinal acceleration, and the second step is to use threshold values of pattern characteristics to accept or reject these first-tier patterns. Pattern recognition results are presented in terms of the ability to identify reentrainment events versus false identifications, and show an exponential growth in false identifications with an increasing number of reentrainment events identified.

Freezing of ridges and water networks preserves the Gamburtsev Subglacial Mountains for millions of years

Once an ice sheet grows beyond a critical thickness, the basal thermal regime favors melting and development of subglacial water networks. Subglacial water is necessary for bedrock erosion, but the exact mechanisms that lead to preservation of subglacial topography are unclear. Here we resolve the freezing mechanisms that lead to long-term, high-altitude preservation across the Gamburtsev Subglacial Mountains in East Antarctica. Analyses of a comprehensive geophysical data set reveal a large-scale water network along valley floors. The ice sheet often drives subglacial water up steep topography where it freezes along high ridges beneath thinner ice. Statistical tests of hypsometry show the Gamburtsevs resemble younger midlatitude mountains, indicating exceptional preservation. We conclude that the Gamburtsevs have been shielded from erosion since the latest Eocene (∼34 Ma). These freezing mechanisms likely account for the spatial and temporal patterns of erosion and preservation seen in other glaciated mountain ranges.

Direct measurements of episodic snow accumulation on the Antarctic polar plateau

During a 1-year field experiment at a remote location on the Antarctic polar plateau (85.67°S, 46.38°W) influenced by moderate magnitude katabatic winds, snow accumulation was characterized at three different spatial and temporal scales using snow stakes, tracer material dispersed periodically on the snow surface, and an acoustic depth gauge. The spatial variability of snow accumulation was found to be large, on both annual and intra-annual timescales, and is attributed to the high frequency of moderate to strong winds at the site. Accumulation throughout the year was observed to be episodic in nature, with a small number of snow accumulation events producing the majority of the annual total accumulation for the site, averaging 0.174 m. In the intervals between observed accumulation events (up to several months), negative changes to snow surface height caused by sublimation and densification of the firn were quantified using an acoustic depth gauge. The rate of decrease in snow surface elevation was largest during the austral summer, as expected, and the overall change in snow surface elevation due to sublimation/densification during the year was estimated to be about −0.10 m. Using the precise timing of accumulation events provided by the acoustic depth gauge, meteorological surface observations, numerical model analyses, and satellite imagery were used to gain insights into whether the event was associated with precipitation or related exclusively to blowing snow and to diagnose the meteorological conditions producing the event. Meteorological conditions during the accumulation events were found to strongly support an association with precipitation events caused by mesoscale or synoptic-scale cyclones along the coastal margin. Dating of the accumulation profile using the dispersed tracer technique identified several other accumulation events that were not measured within the target area of the acoustic depth gauge, suggesting that snow accumulation data from a single acoustic depth gauge cannot be extrapolated over a broad area.