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The AVHRR Polar Pathfinder Twice-Daily 25 km EASE-Grid Composites are a collection of products for both poles, consisting of twice-daily gridded and calibrated satellite channel data and derived parameters. Data include five Advanced Very High Resolution Radiometer (AVHRR) channels, clear sky surface broadband albedo and skin temperature, average albedo and temperature, valid fraction file, solar zenith angle, satellite elevation angle, sun-satellite relative azimuth angle, surface type mask, cloud mask, cloud fraction files, and Universal Coordinated Time (UTC) of acquisition. The 25 km data are derived from the AVHRR Polar Pathfinder Twice-Daily 5 km EASE-Grid Composites, and extend poleward from 48.4 degrees north and 53.2 degrees south latitudes, spanning July 1981 through December 2000. Data are in 1-byte and 2-byte integer grid format and are available by FTP.
Note: NSIDC reccommends the use of the Extended AVHRR Polar Pathfinder (APP-x) Product instead of our AVHRR Polar Pathfinder Twice-Daily 25 km EASE-Grid Composites. The APP-x 25 km product has albedo and temperature calculations for all sky conditions. Cloud properties are modeled to estimate the albedo and temperature under the clouds. An improved cloud detection algorithm is also used in the APP-x product.
For parameters other than the cloud mask, data characteristics are the same as for the corresponding 5 km product. Please refer to the AVHRR Polar Pathfinder Twice-Daily 5 km EASE-Grid Composites documentation for more details. The 25 km grid values are generated by subsampling the 5 km grids. Each 25 km cell is copied from the center cell of a corresponding 5 x 5 cell region in the 5 km parameter. The grid projection is based on a Lambert Azimuthal Equal-Area projection (the NSIDC EASE-Grid). Dimensions of the 25 km grids are 361 pixels wide by 361 pixels high for the Northern Hemisphere grid, and 321 pixels wide by 321 pixels high for the Southern Hemisphere grid. File sizes for the Northern Hemisphere grids are 230,642 bytes (two-byte grid) and 130,321 bytes (one-byte grid). File sizes for the Southern Hemisphere grids are 206,082 bytes (two-byte grid) and 103,041 bytes (one-byte grid).
Each of the 25 km parameters that have the same name as a corresponding 5 km parameter have been resampled using a nearest-neighbor algorithm from the 5 km grid to the 25 km grid.
Bit values of the 25 km cloud mask are the same as those in the 5 km cloud mask, except that bit 7 represents missing data. The following are additional parameter files provided with the 25 km product:
Graphs spanning the entire temporal coverage of the data indicate that sensor calibration errors have occurred. Albedo measurements in areas where only small variances are expected consistently show a substantial dip in 1995 and again in 2001-2005. This unusual pattern is most likely resulting from incorrect sensor calibration during those periods. Figures 2 through 4 show the albedo dip patterns at Summit, South Dome, and Humboldt glaciers in Greenland.
Figure 2. Albedo Measurements at Summit Glacier
Figure 3. Albedo Measurements at South Dome Glacier
Figure 4. Albedo Measurements at Humbolt Glacier
Projection inconsistencies were found between the CASPR processing Versions 2 and 3, resulting in slight geolocation variance. Instances in which the two processing versions are intermixed can cause animated images to appear to jitter.
The swath compositing seems to be in error between 26 July through 2 August, 1983.
Isolated instances of misidentified data parameters were found. In these cases, either the land masking or the IR/Vis channels is interchanged.
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National Snow and Ice Data Center (NSIDC)
Rao, C.R.N. and J. Chen. 1999. Revised post-launch calibration of channels 1 and 2 of the Advanced Very High Resolution Radiometer on board the NOAA-14 spacecraft. on-line version .
Schweiger, A., C. Fowler, J. Key, J. Maslanik, J. Francis, R. Armstrong, M.J. Brodzik, T. Scambos, T. Haran, M. Ortmeyer, S. Khalsa, D. Rothrock, and R. Weaver. 1999. P-Cube: A multisensor data set for polar climate research. Proceedings on the 5th Conference on Polar Meteorology and Oceanography, American Meteorological Society, Dallas, TX, 15-20 January 1999, 136-141.
Document Revision Date:
May 2012 - added note about data not being available.
We kindly request that you cite the use of this data set in a publication using the following citation. For more information, see our Use and Copyright Web page.
The following example shows how to cite the use of this data set in a publication. List the principal investigators, year of data set release, data set title and version number, dates of the data you used (for example, March to June 2004), publisher: NSIDC, and digital media.
Fowler, C., J. Maslanik, T. Haran, T. Scambos, J. Key, and W. Emery. 2002. AVHRR Polar Pathfinder Twice-daily 25 km EASE-Grid Composites. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.