The cryosphere plays an important role in moderating the global climate. Altimeter data is a powerful tool for measuring both the dynamics and mass balance of ice sheets (and sea ice). The Cryosat mission was specifically designed to study such phenomena.

Note that, at this date (end of 2010), Cryosat data are still preliminary and will be re-processed and completed at a later date, in particular with mean surfaces.

Data used

Low Resolution Mode (LRM) data
The Low Resolution Mode (LRM) data are the data from the time Cryosat 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 iced lands, including Greenland and Antarctica.

See Cryosat pages on this site for more information
Data can be obtained from EOHelp

Methodology

Two parameters are used here to map ice sheets:
– the radar altimetric backscatter coefficient which corresponds to the waveform integrated power and gives information on surface characteristics.
– the surface height with respect to the reference ellipsoid (i.e. altitude – (satellite-surface distance), corrected for instrument effects, propagation delays, measurement geometry, and other geophysical effects such as tides).

Time period

We use data measured during the period  September 10 – October 9, 2010 period (roughly one Cryosat 30 days pseudo-cycle).
For comparison, we took the Envisat IGDR data over the same period.

Operations

We use the Broadview Radar Altimetry Toolbox to read and visualize the Cryosat and Envisat data.

Cryosat Low resolution Mode (LRM) data
In the “Datasets” tab, we have selected a series of files from September 10, 2010 to October 9, 2010. All are “LRM” data, so mostly over the ocean and inland the ice caps.
Select longitude for “X”, latitude for “Y”.
As Data expression, we choose:
– surf_height (note that this is a high-resolution (20 Hz) data)
– bkscat_sigma_0 (note that this is a high-resolution (20 Hz) data) The following selection criteria limits the backscatter coefficient boundaries to exclude the most erroneous data and applies an ocean mask: (surf_type_flags > 0) && (is_bounded(0, bkscat_sigma_0, 45)).

Envisat IGDR data

For Envisat, we make a second operation, and take the corresponding variables in Ku band.
– corrected height: alt_cog_ellip – hz18_ku_band_ocean – mod_dry_tropo_corr – solid_earth_tide_ht – geocen_pole_tide_ht – ra2_ion_corr_ku – mod_wet_tropo_corr
– hz18_ku_ice1_bscat (the “ice1” or “OCOG” retracking being the closest to Cryosat one, and the Ku band the frequency used by Cryosat; note that this is a high-resolution (18 Hz) data)
And the selection criteria: ((altim_landocean_flag > 0) && (is_bounded(0, hz18_ku_ice1_bscat, 45))) && (is_bounded(0, hz18_ku_ice2_bscat, 45)).

In the “Views” tab, select each variable to be plotted and click on “Execute”, one after the other.

 

The cryosphere plays an important role in moderating the global climate. Altimeter data is a powerful tool for measuring both the dynamics and mass balance of ice sheets (and sea ice). The Cryosat mission was specifically designed to study such phenomena.

Results

Once plotted, the two parameters can be looked at over Greenland and over Antarctica. Note that the margins of those lands are not visible within Cryosat, since we are using “only” LRM data. The borders of those lands are measured using the SAR-interferometric mode.

NB. Results illustrate just the methodology as much more work is needed to study these phenomena with CryoSat (including the use of longer time series).

Sigma0 gives an indication of surface roughness: the rougher the surface, the less signal will be received back by the altimeter. Monitoring this quantity thus enables to follow surface roughness variations (e.g.snowmeltt or fall, freezing of surfaces or breaking of the ice, flooding…)

 

En_sigma0_ice1_Antarctica_orthographic_sm En_sigma0_ice1_Greenland_orthographic_sm
fig 1.Sigma0 in Ku band over Greenland and Antarctica from Envisat (ice1) IGDRs.

 

(Corrected) Surface height is one of the fields available within the Cryosat Level 2 data and can be computed in Envisat.

Cs_corrHeight_Antarctic_orthographic_sm Cs_corrHeight_Greenland_orthographic_sm
En_corrHeight_Antarctic_orthographic_sm En_corrHeight_Greenland_orthographic_sm
fig 2.Surface height in Ku band over Greenland and Antarctica from Cryosat LRM mode (top) and from Envisat (ice1) IGDRs (bottom). Note that Cryosat gives measurements much closer to the pole (margins of the iced lands being in SAR-In mode, there are not represented in the plot even though the satellite was measuring over the whole Earth).