Dr. Bryan Isacks
Department of Geological Sciences
Snee Hall
Cornell University
Ithaca, NY 14853
Co-Investigator:
Arthur L. Bloom, Cornell University
SIR-C/X-SAR
Analysis of Topography and Climate in the Central Andes
OBJECTIVES
Understand large-scale interactions between tectonic and climate-controlled erosional
processes that created the Andes.
Determine the modern and Pleistocene snow-line altitudes and gradients in a poorly
known but critical latitude range of the central Andes, and interpret ice-age changes
in atmospheric circulation.
PROGRESS
Since the launch of the April mission our project has focused primarily on the analysis
of the enormous amount of new
SIR-C/X-SAR
data for the Patagonian Icefields. The
North and South Patagonian Icefields represent 62% of the glacial area in the southern
hemisphere outside of Antarctica, with the South Patagonian Icefield (SPI) ranking
as the third largest ice mass on Earth after Antarctica and Greenland. Although little
studied, these glaciers are some of the most dynamic in the world, with annual precipitation exceeding 7 m in the accumulation areas and ablation rates as high as 6 cm/day
at sea level, allowing them to respond quickly to climatic variations. Monitoring
the response of these glaciers is important for the study of the regional climate
as well as the relationships between climate changes in the northern and southern hemispheres.
The new radar data will have a major impact on the understanding of these important
icefields and glaciers.
Radar interferometry analysis of the San Rafael glacier, one of the fastest moving
glaciers in the world, has produced spectacular results revealing new information
regarding its flow characteristics. The extremely fast velocities at the terminus
are not characteristic throughout its length and slow down by an order of magnitude up glacier.
Within the interior of the icefield the glacier acts as a well defined ice stream
with slower moving ice on its flanks.
Analysis of the Patagonian
SIR-C/X-SAR
data has lead us to new discoveries regarding
the spatial and temporal characterization of snow and ice conditions on the Patagonian
Icefields, and has demonstrated the utility of combining multitemporal
SAR
with meteorological and hydrological data to detect the effects of synoptic weather systems
and seasonal changes on snow and ice conditions.
The Patagonian results have spawned collaborations with the Chilean and Japanese glaciologists
who have been responsible for much of the previous research on the icefields. We
are planning to conduct a joint field trip with them to measure ice depth profiles and snow and ice conditions on the San Rafael glacier. The ice depth measurements
will be made with an ice penetrating radar and are needed in order to use the ice
velocity measurements to calculate ice flux and test models of glacier flow.
In April 1995 we received our first four
TOPSAR
scenes processed through the new
AIRSAR
integrated processor. These four scenes cover two glacier sites in the Cordillera
Real, Bolivia and the Quelccaya Ice Cap, Peru. The focus of our recent research
has been to assess the usefulness of this data for glaciological studies, especially as
a high resolution data source for detection of glacier change. We are collaborating
with researchers at the Unidad de Glaciology Recursos Hedricos de Electroperu to
use
TOPSAR
data to update the glacial inventory of the Cordillera Blanca which was originally
done using aerial photographs taken in the 1950s and 60s.
FUTURE PLANS
Spaceborne Imaging Radar (SIR-C/X-SAR) Glaciological Observations of the Patagonian
Icefields
A mosaic of the North Patagonian Icefield and the northern half of the South Patagonian
Icefield (SPI) is being constructed from the 14
SIR-C
scenes acquired during
SRL-1.
Because of the persistent cloud cover this is the first complete mosaic of the icefields since a series of
TM
acquisitions in 1986. Terminus position changes of 15
glaciers are determined and the spatial distribution of these changes correlated
with glacier characteristics including aspect, area, length, accumulation area/ablation
area ratio, equilibrium line altitude (ELA) and gradient. Snowlines are mapped and compared
to previous estimates of ELAs. Several ice divides on the
SPI
are defined for the
first time. Other previously undetected glacial features include: buried crevasses,
small icefalls, and ogives. The complete set of
SRL-2
data provides a comprehensive
view of seasonal change for the northern
SPI.
Snow and Ice Conditions inferred from Scattering Mechanism Decomposition of
SIR-C/X-SAR
images of the Patagonian Icefields
The scattering mechanism decomposition technique of Cloude, modified for
SIR-C
data
by van Zyl and Rignot, is used to interpret the snow and ice conditions of the Patagonian
Icefields. Several large areas on the icefields are dominated by a double bounce
scattering mechanism which has never been reported for a snow and ice target. We
presently interpret this to be the result of subsurface refrozen ice on a 5 cm scale.
Inverse backscatter modeling together with the limited published field data will
be used to constrain the physical parameters of the snow and ice such as: grain size distribution,
snow wetness, density, surface roughness and the orientation and structure of subsurface
refrozen ice. We hope to further constrain the snow and ice parameters with our own measurements from snow pits on the Patagonian Icefields.
Ice Velocity and Surface Topographic Mapping of the Moreno Glacier area of the South
Patagonian Icefield from Spaceborne
SAR
interferometry
Interferometric analysis similar to that done on the San Rafael Glacier will be applied
to the fast-moving Moreno Glacier and several slower glaciers in the area. The glaciers
in the southern portion of the
SPI
act as a coalescence of individual glaciers rather than outlet glaciers draining a common icefield as they do on the NPI, therefore,
comparisons of the velocity and strain maps of this area with those of the San Rafael
area will illustrate the differences in flow characteristics of these two glacial regimes.
Results from San Rafael Glacier Field Experiments
If we are successful in obtaining ice depth profiles and characterizing the snow and
ice conditions on San Rafael Glacier during our planned trip to Chile then we will
be in the position to provide important ground validation results for both the San
Rafael interferometry results (Rignot, et al., 1995) and the Patagonian radar glacier zones
(Forster and Isacks, 1995).
Mapping of Glacial Geomorphology in the Cordillera Real, Bolivia with
SIR-C/X-SAR
images
Various geomorphic features are easily identifiable on the
SIR-C/X-SAR
images that
are not as evident on
VNIR
images. The extent of Pleistocene glaciation can be mapped
from moraine positions prominently displayed on the
SAR
images because of the enhancement of subtle topography to the side-looking geometry. Rock glaciers, talus and
bedrock outcrops can be distinguished through the variations in surface roughness.
A comparison of DEMs generated from
SAR
interferometry and stereo pairs in the Cordillera
Real, Bolivia
We have a unique opportunity to compare DEMs generated from
SIR-C
and
TOPSAR
interferometry,
SPOT
stereo pairs, and from aerial photograph stereo pairs for an area with extreme
relief for which we also have good ground control from 1:50,000 maps. This will serve as a test for the accuracy and coverage of a spaceborne interferometer in
the presence of large topographic variations.
Himalayan Radar Glacier Zones observed with SIR-C/X-SAR
We will apply our interpretation of the Patagonian radar glacier zones to the
SIR-C/X-SAR
data acquired over the Himalayan Glaciers. The climatic setting in the Himalayans
is dominated by monsoonal and winter precipitation as opposed to the year-round precipitation in Patagonia. We anticipate this to be reflected in the spatial and temporal
distribution of the radar glacier zones. The seasonal changes induced by the 1994
monsoon season (mid-summer) should be recorded by the differences in the April and
October radar glacier zones. Analysis of the radar zones will not be limited to relatively
flat areas (as they were in Patagonia) because we can correct the
SAR
data for radiometric
and geometric topographic influences with the high resolution
DEM
available at Cornell.
A multitemporal analysis of the Radar Glacier Zones in Patagonia and British Columbia
with
ERS-1
and SIR-C/X-SAR
Based on the Patagonian radar glacier zones defined from our
SIR-C/X-SAR
analysis
we are observing those zones as shown by C-VV from a continuing series of
ERS-1
and 2
SAR
acquisitions of the Patagonian Icefields and British Columbia. These images
are correlated with elevation of the snow and ice surfaces and with weather conditions to
determine the dynamics of climate/glacier interactions over time scales of synoptic
weather to seasonal change.
Our research has focused primarily on assessing the geometric accuracy of the data
to determine if it can be used as part of a glaciological time series. Glacier boundaries
are clearly discernible on the
SAR
imagery and can be easily mapped. Rectification of
TOPSAR
data to other datasets is more problematic. The glacier recession that
has occurred in the region over the past 30-40 years is measured in 10's of meters,
so accurate coregistration is required. The small footprint of a
TOPSAR
scene has
hindered coregistration simply because there is often a limited number of good ground control
points that can be used to accurately define the transform. In an attempt to improve
coregistration we have begun to focus on better visualization of the radar imagery. We have also recently begun to integrate
TOPSAR
digital elevation data with
SAR
imagery to create stereo radar imagery. This combination has resulted in dramatically
improved visualization of radar data. It offers several advantages over even geocorrected data in mountainous regions. First and foremost it allows radar backscatter
signatures to be seen in their true landscape position. This is crucial for proper
interpretation of geomorphic features such as moraines or debris left by retreating
glaciers as well as accurate mapping of the position of glacier termini. Secondly, it allows
much better registration with other datasets as ground control points become much
easier to identify. Lastly, It also allows radar imagery to be used by a much wider
community. Nonradar scientists can find using
SAR
data, especially in mountainous
regions, a frustrating task as layover and foreshortening render the landscape nearly
unrecognizable. However, through the creation of stereopairs, radar data is presented
in a form identical to widely used aerial photographs that researchers, even in underdeveloped
countries, are familiar working with. We intend to use this technique with the
DEM
that will be created for the Cordillera, Real region using SLR-2 data as well.
Anticipated new data requests:
SIR-C/X-SAR
Patagonia sites (10)
Bolivia, Southern Peru and other South American sites (15)
Himalayan glaciers (10)
Integrated
TOPSAR
processing:
We require the entire coverage of the Peruvian Cordillera Blanca and the Bolivian
Cordillera Real.
PUBLICATIONS
Forster, Richard R. and Bryan L. Isacks, Spaceborne Imaging Radar Reveals South Patagonian
Icefield Responses to Seasonal and Synoptic Weather Changes, submitted to JGR
, Planetary Science, 1 October 1995.
Rignot, E. J. M., R. R. Forster and B. L. Isacks, Interferometric radar observations
of Glaciar San Rafael, Chile, J. Glaciology
, (in press), 1996.
Rignot, E. J. M., R. R. Forster and B.L. Isacks, Mapping of glacial motion and surface
topography of Hielo Patagonical Norte, Chile, using satellite
SAR
L-band interferometry
data. Annals of Glaciology
, vol. 213, 1996, (in press).
Presentations at Scientific Meetings
Forster, R. R., and B. L. Isacks, The Patagonian icefields revealed by space shuttle
synthetic aperture radar (SIR-X/XSAR), EOS
, Transactions, American Geophysical Union (Abstracts), v. 75 (44), p. 226, 1994.
Forster, R. R., B. L. Isacks, and A. G. Klein, Modern and LGM glaciation of the Bolivian
and Patagonian Andes, Geological Society of America annual meeting (Abstracts)
, v. 26 (7), p. 129, 1994.
Forster, R. R., B. L. Isacks, and A. G. Klein, Spaceborne Imaging Radar (SIR-C/X-SAR)
Glaciological Observations of the Patagonian Icefields, submitted to AGU
Fall 1995.