ENVISAT Radar Altimeter

Organizations

European Space Agency, Sea Ice Climate Change Initiative

Nansen Center for Remote Sensing, Finnish Meteorological Institute, University of Hamburg

Principal contacts

 

Data web site

http://icdc.zmaw.de/esa-cci_sea-ice-ecv0.html?&L=1

Methodology

Radar Altimeter

Location

Arctic Ocean and Fram Strait

Time intervals

2002-2012, October to March

Data processing notes

The method used to extract sea ice thickness from Envisat altimetry data is
based on the method used for ERS-2 [Peacock and Laxon, 2004; Laxon et al., 2003]. Only Ku band echoes acquired at the full 320 MHz bandwidth are processed. The pulse limited footprint of the radar altimeter is several
kilometres wide.

Note that the given values are mean values of successful altimeter
measurements inside the grid cell. They do not consider the fraction of open
water – if only one 3 m floe is measured in a 100 km x 100 km, it will result
into the sea_ice_thickness of 3 m.

In order to make the ice thickness data more comparable with the other observations in the data base, we have added an additional variable that is the ice thickness times the ice concentration. The ice concentration is from the NSIDC monthly averaged Sea Ice Concentration Climate Data Record (http://nsidc.org/data/NSIDC-0051)

The three variables included in the summary file (averages only) are

  • Freeboard
  • Sea ice thickness from the SICCI data set, unaltered (Avg_h)
  • Effective sea ice thickness, heff = Avg_h * ice_concentration

Note that the Envisat SIT is by nature the average thickness of the ice Envisat is able to classify as ice. To classify as ice the surface must have deformation features. Thus it never "sees" new ice that has not deformed or has only rafted and not ridged and hence it is not included the SIT product. However this ice is included (as it should be) in the SIC estimate. To obtain a more accurate estimate of the overall ice volume one should therefor have some kind of estimate on how much new undeformed ice there is.

The number of measurements averaged to retrieve a freeboard value is vital
to know when estimating effect of radar speckle in freeboard retrieval. Thus
this number is provided as a Nsamps. A low number of measurements will
result in higher uncertainties and cells with only a few measurements should
be used with caution.

The values reported are monthly averages for 100-km grid boxes, so the spatial averaging, if there are many point values, is somewhat larger than for other data sets.

The monthly mean freeboard and ice thickness values for all valid ice thickness data points are copied directly into the summary file with no further averaging. The mean year day is the center day of each month and the first and last days are the first and last days of the month. This may not correspond to the actual data included in the monthly average. Similarly the latitude and longitude are the grid box locations reported for each average value and may not correspond to the actual locations of the samples. Just the mean freeboard and the mean ice thickness are in the data set, so only these values are in the summary file. There is no information on the minimum or maximum or the ice thickness distribution.

V20141003: first edition

Number of samples

10 years, 60 months, 25036 clusters, 3,864,201 point measurements

Documentation

Point data

http://icdc.zmaw.de/esa-cci_sea-ice-ecv0.html?&L=1

Citations

Laxon, S. W., Peacock, N. R. & Smith, D. M. (2003). High interannual
variability of sea ice thickness in the Arctic region. Nature, doi:10.1038/nature2050, 947-950.

Peacock, N. R. & Laxon, S. W. (2004) Sea surface height determination in
the Arctic Ocean from ERS altimetry. J Geophys. Res., 109, doi:10.1029/2001JC001026.

 

Acknowlegements

Europea Space Agency

 

two maps of data locations

Data locations for two dates, one in the fall and one in the spring.

 

 

ICESat box plots

These are box plots of the corrected mean ice thickness for each month. The median, quartiles, and 5th and 95th percentile values are shown.