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., [1993]). 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. [1985] 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 is evident.

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 [1965] 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 [1993]. 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.

REFERENCES

• 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. CalTech Library
• 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, 1993.
• 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

Greenland Observed* 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.

MERS Bibliography