RESTRICTED only accessable in CEN/MPI network or via CliSAP login What does that mean?
Data access via file system: /data/icdc/ice_and_snow/arctic_lead_fraction_amsre
Advanced Microwave Scanning Radiometer aboard EOS (AMSR-E) data are used to produce a data set of the lead area fraction for Arctic sea ice for the freezing season (November - April) on the NSIDC polar-stereographic grid with 6.25 km grid resolution.
For this purpose AMSR-E brightness temperatures observed at 19 GHz and 89 GHz (TB19 and TB89) are interpolated onto the 6.25 km NSIDC polar-stereographic grid. From these the ratio r = TB89/TB19 is computed. This ratio takes considerably larger values over leads and thin ice than over the adjacent thicker sea ice. By moving a 7x7 grid cell search window over maps of r, grid cells are identified which exhibit a substantially larger r than the background value; the latter is computed as Median(r) and subsequently the difference between actual (the center grid cell of the 7x7 window) and the background is computed: r' = r - Median(r). By using coincident MODIS visible imagery tie points are selected which permit to translate r' into the lead ice concentration or lead area fraction which is computed for every grid cell. More details about the approach can be found in Roehrs and Kaleschke (2012) and Roehrs et al. (2012) (see References).
This data set has been put into our data base on Dec. 19, 2013.
|Lead Area Fraction||0-100%|
|Status Flag||0, 1, 100, 110, 120|
Period and temporal resolution:
- November 1, 2002, to April 30, 2011
- Daily for freezing season: November 1 to April 30
Coverage and spatial resolution:
- Northern hemisphere
- Spatial resolution: 6.25 km x 6.25 km, polar-stereographic grid from NSIDC, tangential plane at 70°N and 70°S, respectively
- Geographic longitude: -180°E to 180°E
- Geographic latitude: from about 41°N to 90°N, in the corners of the grid further equatorward
- Dimension: 1216 columns x 1792 rows
- Altitude: 0.0 m
The data set does not yet contain uncertainty estimates.
A number of comparisons with independent data (Envisat ASAR, MODIS, Cryosat-2) has been carried out. These show that about 50% of the leads present in MODIS imagery (500 m grid resolution) can be identified with the approach used here (6.25 km grid resolution). The minimum width down to which leads can be identified is about 3 km. Note that the leads need to be dominantly covered by thin ice; if too much open water is present the approach provides a less reliable result.
Because of the size of the searching window the approach cannot generally applied to identify larger thin ice areas or polynyas.
More details are found in the references.
Alfred Wegener Institute for Polar and Marine Research / Bremerhaven
email: lars.kaleschke (at) awi.de
ICDC / CEN / University of Hamburg
email: stefan.kern (at) uni-hamburg.de