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NCEP Land for Sea Ice Uses

Part I: Land corresponding to 25.4 km masks

Since we can't have sea ice on land, it is a requirement for sea ice products that land be represented reasonably.  This turns out to be a nontrivial process.  Over the years that NCEP sea ice products have been constructed, different versions of land have been used in response to interests and concerns of users.  This has resulted in improved products for daily usage, but has introduced artefacts into the record for those users who are interested in long-term effects.

The potential effects of different land definition policies are not trivial.  Approximately 1.6 million km^2 can be moved into or out of the 'land' category, depending on how 'land' is defined.  Much of this occurs in areas with complex coastlines, such as the Baltic Sea or the Canadian Archipelago.  Since both can include an ice cover, this represents a significant fraction of the total Arctic ice pack (which reaches a maximum extent of approximately 14 million km^2).

In order to stabilize time series which may be derived from the NCEP ice products, we have recomputed the land masks all at the same time, and then recomputed the products.  The hemispheric products have not included (in their data files) land, but the global fields do include land as part of the processing.  Since it is also the global products which are most used for time-series purposes, this does require revisitation by those users.

Pointwise analysis of changes to the global 0.5 degree grid
Pointwise analysis of changes to the hemisphere 25.4 km polar stereographic grids

Part II: Land corresponding to 12.7 km masks

Some points on the global grid may not be filled depending on the source polar stereographic grids' land masks. In going to using a higher resolution land mask on the polar stereographic grid, some points on the global grid change their nature. Likely the most difficult for users of the global grid will be the 2 points which change from ocean to land, and the 16 which change from coast to land. Their locations are listed, as are locations for the 10 points which change from land to ocean, and the 53 which change from land to coast. The net global changes are a decrease of 58,184 km^2 in the area of land, an increase of 51,032 km^2 in the area of coast, and an increase of 7,147 km^2 in the area of ocean. The total area involved in changes is approximately 0.01% of the surface area of the earth, or 0.2% of the area of the ice pack at global maximum extent.

For the listing of points, see

Part III: Graphic illustration of the 25.4 and 12.7 km-corresponding land masks

Base Northern Hemisphere Southern Hemisphere Global - 
0.5 degree lat-long
25.4 km 25.4 km northern hemisphere land 25.4 km Southern Hemisphere Land Global Ice mask corresponding to 25.4 km land
12.7 km 12.7 km Northern Hemisphere Land 12.7 km Southern Hemisphere Land Global Land Mask corresponding to 12.7 km land masks

The data files involved may be retrieved by anonymous ftp to, directory pub/ice/support  Data are in both 1 byte per grid point binary format, and in grib for each of the files.  Please see the README file in that directory for more details.

Last Modified 13 April 2001
Last Reviewed 20 May 2014
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Page last modified: Tuesday, 20-May-2014 14:51:33 UTC