Studies of Greenland with Scatterometer Data
The MERS research group has been actively involved in research
involving Greenland. Seasat Scatterometer (SASS), NASA Scatterometer
(NSCAT), and ERS-1 & ERS-2 scatterometer (Escat) data have been
processed with the SIRF resolution enhancement algorithm to make
time series of images of the radar backscatter of the Greenland
ice sheet. These images have been used to identify and map the locations
of key ice zones or facies.
Sample images are avilable below.
Increasing interest in the specific role of ice sheets in regulating
global climate has resulted in a scientific mandate to accurately
record and monitor the extent and surface conditions of the earth's
major terrestrial ice bodies. Such monitoring can only be done using
spaceborne sensors. However, optical and infrared (IR) sensors have
are restricted to daylight hours. Further, studies to identify glacier
facies from optical and IR satellite imagery have concluded that
many of the boundaries between significant facies are not discernible
at the surface. The wet and dry- snow lines, for instance, can only
be discerned by measurements which penetrate several meters into
the surface of the snow and ice. However, active microwave sensors
(radars) can the ability to "see" into the surface without regard
to solar illumination and meteorological conditions. They are thus
well-suited for mapping the polar regions.
The first satellite-borne radar remote sensing systems were flown
on the Seasat satellite in 1978. Seasat carried two sensors, an
L-band synthetic aperture radar (SAR) and a Ku-band scatterometer
(SASS). The SAR provided very high resolution images, but only over
a narrow swath with limited spatial and temporal sampling. Nevertheless,
it has been a useful tool in studying polar ice (e.g. Jezek et al.,
). However, unlike the SAR, SASS was designed provided frequent,
global coverage of the 14.6 GHz radar back scatter (sigma-0) with
a nominal resolution of 50 km. Over the ocean, the measurements
of sigma-0 were used to determine the near-surface wind vector.
While 50 km resolution is adequate for ocean studies, it limits
applications of the data in ice studies. Despite this drawback,
50 km SASS data has been successfully used by Thomas et al. 
to illustrate the application of ice sheet-wide coverage by a microwave
sensor for mapping regional melting. Improved resolution would significantly
enhance the utility of the SASS data in polar ice studies. In particular,
SASS provides an important historical data point in long-terms studies
of changes in the polar regions.
To address the need for improved resolution with frequent global
coverage, a new method of enhancing the resolution of the SASS data
has been developed as part of this research [Long, Hardin, and Whiting,
1993]. The method can also be applied to other sensors. The resolution
enhancement method is capable of the generation of images with a
resolution as fine as 4-5 km from the original 50 km resolution
SASS measurements using special signal processing techniques which
take advantage of the spatial overlap of measurements from multiple
orbit passes. Although the resulting enhanced resolution images
can only be considered "high" resolution when compared to the intrinsic
resolution of the scatterometer, the resulting high resolution images
have proven remarkably useful in large-scale studies of polar ice.
For example, Figure
1 presents a time series of resolution radar images of Greenland
in which seasonal change over the time period of July-Sept., 1978
We note first that Central Greenland exhibits a relatively high
radar backscatter at 14 GHz and has very little change over the
three month SASS data set. The brightness in backscatter maps correlates
extremely well with maps of annual snow accumulation. The dark grey
patch in central Greenland occurs in the region of highest annual
accumulation of solid precipitation. The backscatter brightness
increases gradually up to the summit as the accumulation is reduced
to one-half of its highest value. The largest backscatter values
occur in the dry snow facies in the north-east of the ice sheet.
Though this is not the highest elevation part of the ice sheet,
the north-eastern catchment of the ice sheet is in the precipitation
shadow of the major ice divides. As a result the annual accumulation
is much smaller.
Seasonal variation is dramatically evident in the image time series
along the ice sheet periphery. This variation is attributed to the
progression in the extent and influence of the melt zone in summer.
In early July, during the height of the ablation season, the black
swath around western and southern Greenland corresponds to surface
melt. The outer limit of this dark swath corresponds to the zone
of wet bare ice in the ablation zone at the fringe of the ice sheet.
The inner limit corresponds to the transition from wet snow to percolation
facies at the wet snow line. In the sequence of images shown in
Fig. 1, as the fall freeze-up takes place from mid-August onwards,
the original wet snow and percolation zones coalesce to become a
bright band of extremely high backscatter. This zone has one of
the largest values of microwave backscatter observed anywhere in
the solar system. Normalized radar response at 40 deg incidence
(termed "A") are close to 0 dB in the percolation zone. By late
September the upslope limit to the percolation zone is well defined.
This is the limit of summer melting on the ice sheet, i.e., the
dry snow line where the transition from percolation facies to dry
snow facies occurs.
Recent controversy over changes in Greenland ice sheet elevation
have increased interest in monitoring this critical ice sheet. Whether
ice sheet growth results from reduced ablation or increased precipitation
is a thorny issue. These inconsistencies make it imperative to build
up a spatial picture of areas of ice sheet ablation and accumulation
in addition to baseline surveying by altimetric techniques. Recent
field programs have made it possible to link physical models of
the snow and ice facies first identified by Benson  and the
radar response. Thus, the radar response can be used as a basis
for delineating snow and ice regimes on the Greenland ice sheet.
The main factor affecting the diagenesis and resulting stratigraphy
of the snow and ice is thermal forcing during seasonal change, and
especially the presence of meltwater. The principal parameter affecting
the microwave response to the snow and ice surface is the presence
of liquid water. It changes the dielectric properties of the medium
so significantly that it regulates the reflection or transmissivity
at the surface and limits the contribution of subsurface or "volume
scattering" effects, by absorption and extinction within the upper
layers. The seasonal variations in the sigma-0 in the enhanced resolution
SASS images demonstrate the sensitivity of sigma-0 to the transitions
in surface and subsurface properties. It is thus possible to use
the enhance resolution SASS image sequence to map key facies. The
details of this mapping are described in Long and Drinkwater .
The resulting map is shown in Figure 2.
(Figure 2. Postscript version)
(Figure 2. gif version) The dry snow regime has been segmented
into two separate regions which differ in radar response due to
annual accumulation. NASA Scatterometer (NSCAT) data makes it possible
to study interannual variations in the radar response and identify
long-term changes in the location of these facies.
- Ashcraft, I.S., and D.G. Long, Relating Microwave Backscatter
Azimuth Modulation to Surface Properties of the Greenland Ice Sheet,
Journal of Glaciology, Vol. 52, No. 177, pp. 257-266, 2006.
- Ashcraft, I.S., and D.G. Long, Observation and Characterization of
Radar Backscatter over Greenland, IEEE Transactions on Geoscience
and Remote Sensing, Vol. 43, No. 2, pp 237-246, 2005.
- Benson, C.S., Stratigraphic Studies in the Snow and Firn of
the Greenland Ice Sheet, SIPRE Research Report, No. 70, 1962.
(83 MB pdf)
- Benson, C.S., Stratigraphic Studies in the Snow and Firn of
the Greenland Ice Sheet, Ph.D. Dissertation, California Institute of Technology, 1960.
- Early D.S., and D.G. Long, Image Reconstruction and Enhanced
Resolution Imaging from Irregular Samples, IEEE Transactions on
Geoscience and Remote Sensing, Vol. 39, No. 2, pp. 291-302, 2001.
- Jezek, K. C., M.R. Drinkwater, J.P. Crawford, and R. Kwok,
Analysis of Synthetic Aperture Radar Data Collected Over the Southwestern
Greenland Ice Sheet, Journal of Glaciology, Vol. 39, No. 131,
- Long, D.G., and M. Drinkwater, Greenland Observed at Enhanced
Resolution by the Seasat-A Scatterometer, Journal of Glaciology,
Vol. 40, No. 135, pp. 213-230, 1994.
- Long, D.G., P. J. Hardin, and P. T. Whiting, Resolution Enhancement
of Spaceborne Scatterometer Data, IEEE Trans. Geosci. Remote Sensing,
Vol. 31, No. 3, pp. 700-715, May 1993.
- Long, D.G., and P. J. Hardin, Vegetation Studies of the Amazon
Basin Using Enhanced Resolution Seasat Scatterometer Data,
IEEE Trans. Geosci. Remote Sensing, Vol. 32, No. 2, pp. 449-460, 1994.
- Thomas, R.H., R.A. Bindschadler, R.L. Cameron, F.D. Carsey,
B.Holt, T.J. Hughes, C.W.M. Swithinbank, I.M. Whillans, and H.J.
Zwally, Satellite Remote Sensing for Ice Sheet Research, NASA
Technical Memorandum, 86233, 27, 1985.
SASS A & B image time series of Greenland
Display Image (401K)
Comparison of SASS and ERS-1 A images of Greenland
Display image (336K)
NSCAT A image of Greenland
Display image (23K)
Studies of Greenland with Radiometer Data
Seasat SMMR (radiometer) time series image of Greenland
Display image (119K)
Selected Papers from the MERS Group
at High Resolution by the Seasat-A Scatterometer
D.G. Long and M.R. Drinkwater, Journal of Glaciology, Vol. 32,
No. 2, pp. 213-220, 1994.
Comparison of Methods
for Melt Detection over Greenland using Active and Passive Microwave Measurements
I.S. Ashcraft and D.G. Long, International Journal of Remote Sensing, Vol. 27, No. 12, pp. 2569-2488, 2006.
Differentiation Between Melt and Freeze*
Stages of the Melt Cycle Using SSM/I Channel Ratios
I.S. Ashcraft and D.G. Long, EEE Transactions on Geoscience and Remote Sensing,, Vol. 43, No. 6, pp. 1317-1323, 2005.
*note: selected PDF files are provided as a convenient public service under fair-use copyright restrictions. Copyright is retained by the original owner.