Altimetry is one of the most powerful tools for monitoring ice sheets.
The cryosphere plays an important role in moderating the global climate. Rapid changes are occurring on ice caps margins that can significantly contribute to sea level rise. Altimeter data is a powerful tool for measuring both the dynamics and mass balance of ice sheets. Moreover, altimeters also provide other parameters such as backscatter coefficient and waveform shape that give information on surface roughness and snow pack characteristics.
By giving a good overview of polar regions (-82.5°S / 82.5°N), the Envisat mission is well adapted and able to accurately map 80% of Antarctica and almost all the Greenland. Envisat altimeter (RA-2) data are supplied on DVD-ROMs and by FTP and can be obtained from EOHelp.
This dataset has another advantage. The Envisat GDR directly offers the opportunity to use other algorithms that are better suited for non-ocean surfaces. One of them is optimized for ice surfaces, the so-called ICE-2 retracker.
The RA-2 altimeter on Envisat platform is a dual-frequency radar operating at Ku-band (13.575 GHz) and at S-band (3.2 GHz). This dual-frequency can be used for better quantify wave penetration in the snowpack and improve the knowledge of the ice sheets surface topography evolution.
Two parameters were used to map ice sheets: the radar altimetric backscatter coefficient which corresponds to the waveform integrated power and gives information on surface characteristics and the leading edge width which is related to the penetration into the medium and the surface roughness.
In the “Datasets” tab, we have selected both cycles 034 and 040 in two different datasets and named them: Dataset_cy034 and Dataset_cy040.
In the “Operations” tab, we have created a first operation for the backscatter coefficient for each Dataset_cycle. Select longitude for “X”, latitude for “Y”. For the “Data expression”, use the difference between the two bands S-Ku: hz18_s_ice2_bscat – hz18_ku_ice2_bscat. The following selection criteria limits the backscatter coefficient boundaries to exclude the most erroneous data and applies an ocean mask: ((altim_landocean_flag > 0) && (is_bounded(0, hz18_s_ice2_bscat, 45))) && (is_bounded(0, hz18_ku_ice2_bscat, 45)). This first operation is duplicated to be applied on the second cycle but be careful to select the corresponding dataset.
In order to give you several examples, we choose to represent two different geographic resolutions for each cycle : 1/3° for cycle034 and 1° for cycle040. This is done by clicking on “Set Resolution/Filter AND selecting the “step” in X resolution and Y resolution.
The second operation is applied to the difference S-Ku band on the leading edge width: select longitude for “X”, latitude for “Y”. For the “Data expression”, use: hz18_s_ice2_edge_width – hz18_ku_ice2_edge_width. A selection criterium masks the data over the oceans and tends to eliminated most of the bad data:((altim_landocean_flag > 0) && (is_bounded(0, hz18_ku_ice2_edge_width, 7.7))) && (is_bounded(0, hz18_s_ice2_edge_width, 7.7)).
In the “Views” tab, select each operation to be plotted and “execute” it.
Results and comments
In the S-band, the radar waves penetrate deeper in the snow and ice than the Ku-band and it is less sensitive to the slope effect than the Ku-band. From this known physical properties, we are able to see differences on high altitude areas: over West Antarctica, the S-band waves are less constrained by the slope effect and less attenuated by the snow grain, providing a stronger return echo than the Ku-band. This is clearly visible on the map below for the cycle 040 (Aug.-Sep.2005) where, over West Antarctica, a long area with a north-south direction shows a positive difference between the two bands.
On the contrary, the Ku-band gives higher values over Central-East Antarctica.
In southern winter (Envisat cycle 034, Jan.-Feb. 2005) and in southern summer (Envisat cycle 040, August-Sept. 2005), the figures below are quite similar (except for the resolution), we find essentially the same distribution for the backscatter difference.
|Backscatter coefficient difference (in dB) between the S and the Ku-bands over Antarctica, from Envisat cycle 034 (left) and 040 (right). As explained above, we give an example with two different geographical resolutions: 1/3° on the left and 1° on the right.|
The leading edge width in S-band tends to have smaller values than the Ku-band one toward the coastal areas over Greenland. On the contrary, the leading edge width in S-band has higher values on the plateau and its distribution seems to be lightly larger during winter (Envisat cycle 034 -below, left-).
|Leading edge with difference (in m) between the S and the Ku-bands over Greenland, from Envisat cycle 034 (left) and 040 (right).|