The cryosphere plays an important role in moderating the global climate. Altimeter data is a powerful tool for measuring sea ice (and both the dynamics and mass balance of ice sheets). The Cryosat mission was specifically designed to study such phenomena.
Note that, at this date (late 2010), Cryosat data are still preliminary and will be re-processed and completed at a later date, in particular with mean surfaces and such.
Low Resolution mode data
The Low Resolution Mode (LRM) is in the most classical (pulse-limited) altimeter mode. This mode is generally selected for CryoSat wherever no margins are expected (outside some test areas), i.e. on ocean and in the interior of ice lands, including Greenland and Antarctica.
SAR mode enables to increase along-track resolution, and thus allows to better capture abrupt height variations (including those due to sea ice). Note that this is not an imagery mode as the SAR data may be.
SAR interferometry enables to retrieve slope values where measurements were taken and are mainly used over ice caps margins/glaciers.
Geographical / Temporal extraction
We will use one file over the Arctic, one over the Atlantic off Portuguese coasts (SAR mode) and one over the Atlantic (LRM mode). Note that CryoSat L1b data are not provided as tracks or half-orbits, since the mode may change (L2 “GDR” data include, however, a flag to indicate the mode). Files are the following:
- CS_OFFL_SIR_LRM_1B_20101111T084452_20101111T085219_A001.DBL (LRM mode data, taken on 2010/11/11, in the Atlantic)
- CS_OFFL_SIR_SAR_1B_20101111T001539_20101111T002242_A001.DBL (SAR mode data, taken on 2010/11/11, in the Arctic)
- CS_OFFL_SIR_SAR_1B_20101111T195241_20101111T195604_A001.DBL (SAR mode data, taken on 2010/11/11,off the coast of Portugal)
In the “Datasets” tab, we select the three files, each in a different dataset.
In the “Operations” tab, we create four operations as follows: one using LRM mode data, one SAR mode data off the coast of Portugal and two using SAR mode data in the Arctic.
For all four operations, select the latitude that is in the for “X” within the “time_orb_data” record (in order to have high-resolution latitude), and as “Data expression”, the field “avg_pow_echo_wavef” (in the “wavef_data” record), which contains the 128 Ku-band waveform samples for each waveform.
In the “Selection criteria” expression, the geographical boundaries are limited as follows for the SAR mode data in the Arctic (CS_OFFL_SIR_SAR_1B_20101111T001539_20101111T002242_A001.DBL):
– is_bounded(-180, lon, -90) (for the first part of the track)
– is_bounded(110, lon, 180) (for the second part of the track)
this is needed in order to see the waveforms vs latitude in a chronological sequence (since the satellite goes from (-90°E, 82°N) to (110°E, 72°N) through 88°N. Otherwise, you can put in “mdsr_time” X, but you will not be able to localize immediately your waveforms in the plot.
Execute the Operations
In the “View” tabs, you can choose between two different kind of graphs:
- Y=F(X) which plots the waveforms individually and represents the return echo power in function of time every 1/20th of a second.
- Z=F(X,Y) which plots a set of cumulated waveforms in function of latitude, as if seen from above.
We are showing here a few examples of Cryosat waveforms over ocean (for comparison purposes), and over sea ice.
fig 1.The three ground tracks visualized here (red LRM, green SAR, blue SARIn)
Since the best-known shape of a waveform is over ocean here is a comparison between one “LRM” (i.e. pulse-limited, or the “classical” altimetry) waveform and one “SAR” waveform, both collected in the Atlantic off the coasts of Portugal, and plotted in a Y=F(X) mode. Most waveforms for either mode are looking like this (you can try and look at the whole series for each file, either by using the arrows to increase the index or by clicking on the “animate” button that browses automatically through the whole series). As expected, the SAR waveform is peakier than the LRM one (respecting the Brown-model waveform, see Delay-Doppler (or SAR) Altimeter)
|fig 2.Three waveforms over ocean: in SAR mode (left) and, for comparison since it is the most usual shape of a waveform, in LRM mode (middle). (right) A waveform in SAR-In mode over ocean (South of Iceland). The number of gates in the SAR-In mode is 512 (vs 128 in the other two modes).|
We look at the SAR waveforms over the Arctic. In November, most of the surface of the Arctic Sea is frozen except a small area near the coasts of Siberia (Taïmyr peninsula). Compared with ocean SAR waveforms, the ones over sea ice are much rougher, since the surface where the radar wave reflects is more irregular than water. In the second part of the track (180°E-110°E), note that the data between 72°N and 76.75°N are over land, and much rougher.
Some waveforms seem to feature little peeks just before the main one. This might be the signature of a surface higher than the surrounding areas where the wave is reflected before being reflected by the main surface (this typically occurs when the ground track of the altimeter overflies an iceberg).
|fig 3.Waveforms over ice (Arctic) in SAR mode. Left, a very specular waveform. Middle and right two consecutive waveforms showing the signature of an early reflection with a small peak just before the main one.|
|fig 4.Part of the Arctic SAR track waveforms: the left side (up to about 76.7°N) corresponds to measurements over land. This representation also show the specular quality of the SAR waveforms with respect to classical mode (see Hydrology data use case: Altimetric waveforms for monitoring lakes level|
|fig 5. Ice Freeboard measurement|
The sea ice freeboard can be computed, by subtracting the ice height to the MSS (subtracting snow thickness also, if need be). From this freeboard, since the ice is floating, taking into account ice and snow density and following Archimedes principle, sea ice thickness can be computed.