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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).
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:
Average albedo and surface temperature:
Averaging involves summing the non-missing cells in each 5 km 5 x 5 cell block, then dividing the sum by the number of non-missing cells in the block.
If all cells in the block are missing, the alba or tema value is 0.
Valid fraction file:
Represents the number of non-missing cells in each 5 km 5 x 5 cell block divided by the number of cells in the block (25) times 100.
If all cells in the block are missing, the vfrc value is 0.
Cloud fraction files:
cfr1: Cloud fraction for cmsk bit 1
cfr2: Cloud fraction for cmsk bit 2 (1981 - 1993 data; version 1 and 2 only)
Each cloud fraction file is one byte per cell, nominally in the range 0 - 100.
Represents the number of cloudy cells in each 5 km 5 x 5 cell block divided by the number of non-missing cells in the block times 100.
If all cells in the block are missing, the cloud fraction value is 128.
The Graphical Interface for Subsetting, Mapping, and Ordering (GISMO) allows users to select specific temporal and spatial subsets of gridded data from NSIDC, including AVHRR Polar Pathfinder Twice-Daily 25 km EASE-Grid Composites. The GISMO was designed to allow users to order spatial and temporal subsets of data. It is not an appropriate tool for ordering entire data sets or entire grids for long time series. Users who wish to order a full time series, or several years of AVHRR data, should contact NSIDC User Services. Large orders will be filled as resources allow.
NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org
National Snow and Ice Data Center (NSIDC)
Key, J. 1999. The cloud and surface parameter retrieval (CASPR) system for polar AVHRR. Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison.
Key, J., J. Collins, C. Fowler, and R.S. Stone. 1997. High-latitude surface temperature estimates from thermal satellite data. Remote Sensing of the Environment 61:302-309.
Kidwell, K.D. 1995. NOAA Polar Orbiter Data User's Guide, U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, NESDIS.
Maslanik, J., C. Fowler, J. Key, T. Scambos, T. Hutchinson, and W. Emery, 1998. AVHRR-based Polar Pathfinder products for modeling applications. Annals of Glaciology 25:388-392.
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: September 2002
Document Review Date: March 2002
To broaden awareness of our services, NSIDC requests that you acknowledge the use of data sets distributed by NSIDC. Please refer to the citation below for the suggested form, or contact NSIDC User Services for further information. We also request that you send us one reprint of any publication that cites the use of data received from our Center. This helps us to determine the level of use of the data we distribute. Thank you.
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.
