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ARCTIC SYSTEM SCIENCE (ARCSS)
PRODUCTS AND SERVICES
LEAP YEARS
CORRECTIONS

NSIDC Notes

Issue 12, Winter 1995

Special supplement: Greenland Airborne Precise Elevation Survey (GRAPES)

ARCTIC SYSTEM SCIENCE (ARCSS)

ARCSS Information on the 'Net

A World Wide Web "Home Page" has been established by the ARCSS Data Coordination Center at NSIDC. This electronic information resource is accessible using software such as Mosaic or Netscape on any computer having a connection to the Internet. The Home Page contains names and contact information for all ARCSS investigators, bibliographies for each ARCSS component, a meeting calendar, and a catalog of ARCSS data available at NSIDC and other locations. The contents are "under construction" and will be updated, expanded, and improved during the next few months and on an ongoing basis. If you have suggestions regarding the contents, or wish to contribute information for posting or pertinent links for insertion, please let us know.

Questions about access and suggestions should be addressed to Matthew Cross, ARCSS Data Coordinator at NSIDC.

LAII Science Meeting Held

The first Arctic System Science (ARCSS) Land-Atmosphere-Ice Interactions (LAII) Science Meeting, organized by the LAII Science Management Office at the University of Alaska Fairbanks, was held 1-5 March at Orcas Island in northwest Washington State. LAII-funded investigators presented their research progress and plans, with invited representatives of the ARCSS Ocean-Atmosphere-Ice Interactions (OAII), Paleoclimates of Arctic Lakes and Estuaries (PALE) and Greenland Ice Sheet Project Two (GISP2) providing overview presentations as well. The major objectives of the meeting were to plan for the coming field season and future work, to conduct an extended meeting of the LAII Steering Committee, and to encourage integration among the ARCSS components through an exchange of ideas. Matt Cross and Claire Hanson outlined plans for the long-term ARCSS data archive and for data access and distribution strategies for LAII.

Discussion during the Saturday morning data session revolved around issues of data quality control and data submittal procedures. We proposed an "evolutionary" approach to data set quality improvement based on the NSIDC passive microwave data product development and distribution project for NASA EOSDIS. In this model, the PI provides data sets or products that are as correct and accurate as possible, based on careful checking and often following publication of science results. NSIDC archives and distributes an accurate copy of the data sets or products as provided by the PI, or works with the PI to arrive at such a product. Users of a data set or product may find errors or inaccuracies in the data, based on use of the data and through the peer- review process of the original and subsequent papers. NSIDC then participates with the original PI to revise the data set or product. This improvement could consist of changing the format or delivery method, providing corrected algorithms or re-processing data using such algorithms (developed by or with the PI), enhancing or augmenting documentation, or any other action resulting in a "revision 1" or "version 2" of the data set or product. The new version is distributed and the cycle continues. Since no one can "guarantee" there are no errors in a given data set, this model distributes the responsibility for quality among producers, distributors and users of data, and usually has the clear benefit of getting data out to the community where they can be used for further and broader research. It does put some burden on the end user to understand the data's limitations and use a data product appropriately. One possible solution would be to encourage data users to collaborate with data producers to ensure appropriate use of the data. As ARCSS is all about integration, getting the data out to the community is vital.

While there are no "deadlines" or specific paperwork involved in submitting data to the ARCSS archive at NSIDC, we encourage each ARCSS-funded investigator to begin now to identify data you could make available to your colleagues through the archive. Contact NSIDC User Services. You will be asked to provide a brief description of each data set as the basis for developing an archiving and documentation plan.

PRODUCTS AND SERVICES

NOAA/NASA SSM/I Pathfinder Benchmark Data Set Distribution Underway

NSIDC, in collaboration with the SSM/I Products Working Team (SPWT), distributed the Equal Area SSM/I Earth Grid (EASE-Grid) Northern Hemisphere Volumes 1.0 and 2.0 Brightness Temperature CDS in February 1995.

The released CDS contain earth-gridded SSM/I Brightness Temperatures for the full Northern Hemisphere for the period 1 August 1987 to 28 November 1987. These are the first of approximately 36 CDS being produced for the NOAA/NASA Pathfinder Benchmark Period, from August 1987 through November 1988. The complete EASE-Grid Brightness Temperature Benchmark data set will consist of three series of CDS, one series each for the EASE-Grid Northern Hemisphere (approx. 10 CDS), Southern Hemisphere (approx. 10 CDS) and Cylindrical (i.e. "global") projections (approx. 16 CDS).

The intent of the EASE-Grid is to provide a global coverage earth-gridded data format to the general user that is easier to use than swath format, yet maximizes the radiometric integrity of the original TB values, maintains high spatial and temporal precision, and involves no averaging of original swath data. The final EASE-Grid format is based on user feedback received from the distribution of two prototype versions during 1993 and 1994.

The cell size is 25.0 km for all channels and 12.5 km for 85 GHz. The gridding method involves an interpolation which, artificially increases swath sampling density (16 times) using antenna pattern coefficients, and assigns a TB to an earth grid cell by applying the value of nearest neighbor within the oversampled array. There are two files (arrays) for each day (ascending and descending orbits) for each projection and for each frequency and polarization. At higher latitudes (>55 degrees) where coverage by more than one ascending or descending orbit per day is possible, the data which are closest in local time to the equator crossing time are selected. Data are in flat binary files, one image per file, and data compression is applied.

Users with access to the World Wide Web via NCSA's Mosaic or other visual display tools may be interested in browsing the NOAA/NASA Pathfinder Equinox Sampler, at URL http://pathfinder.arc.nasa.gov. This Sampler contains data from all of the Pathfinder projects, for the Vernal Equinox (March 20) 1988, and includes EASE-Gridded data in displayable .GIF formats. The NSIDC Home Page is accessible from the Sampler page by clicking on the National Snow and Ice Data Center hyperlink.

For more information on data availability, please contact NSIDC User Services.

Historical Soviet Daily Snow Depth (HSDSD) CD-ROM

The Historical Soviet Daily Snow Depth (HSDSD) CD-ROM is now available from NSIDC. Production of this CD-ROM was funded by NOAA's Earth System Data and Information Management (ESDIM) initiative through the National Geophysical Data Center (NGDC).

HSDSD is based on observations at a series of 284 World Meteorological Organization (WMO) stations throughout the former Soviet Union. The earliest operational stations began recording snow depth in 1881 and the data continue until 1985. Geographic distribution of stations is primarily in the mid-latitudes of Eurasia and corresponds to inhabited areas. Stations range from 35 to 75 degrees N latitude and from 20 to 180 degrees W longitude, and from -15 meters to 2,100 meters in altitude.

Daily data, as well as NSIDC-generated monthly means, are available on a single CD-ROM containing ASCII data files, extraction software, and data documentation. The source of the data used is the State Hydrometeorological Service in Obninsk, Russia. Data were provided to NSIDC by the National Climatic Data Center via the Bilateral US-USSR WG-8 Exchange. The cost of the CD-ROM is $50.

For further information please contact NSIDC User Services.

Figure 1 shows Historical Soviet Daily Snow cover sites from 1881 to 1985.

Historical Soviet Daily Snow Cover Sites, 1881-1985

Figure 1. Historical Soviet Daily Snow Cover Sites, 1881 to 1985.

News About Polar 1-km AVHRR Data Set

NSIDC has recently completed setting up the required software and hardware to receive polar LAC and HRPT data from the University of Colorado's DOMSAT receiver. This new source of NOAA satellite data will allow NSIDC to cover both poles more evenly and completely, and will provide better access to NOAA-12 satellite data. Because of the high data volumes available using the DOMSAT link, our earlier policy of archiving all available data will revised. NSIDC now intends to archive only NOAA-12 and NOAA-14 data, and to limit temporal frequency of coverage to two to three scenes per day over most areas. The daily spatial coverage objective for the data set will continue to be the complete northern and southern SSM/I sea-ice mapping grid areas.

For further information, contact NSIDC User Services. For information on scientific applications of the data set contact: Ted Scambos at (303) 492-1113 or fax: 492-2468, or Internet: teds@nsidc.org.

SEASAT and GEOSAT Altimetry Data Available on CD-ROM

Radar altimetry data, both gridded digital elevation models (DEM) and retracked orbital profiles, are now available on a limited edition CD-ROM from NSIDC. The CD-ROM was created under the direction of Dr. Jay Zwally of NASA's Goddard Space Flight Center (GSFC) by a team of contractors working for Hughes STX Corporation. The data on the CD-ROM include the entire SEASAT mission (July 1978 to October 1978) and the first phase of the GEOSAT mission (April 1985 to September 1986). Both the Greenland and Antarctic ice sheet areas are covered to a latitude of ± 72 .

The CD-ROMs include software for viewing and manipulating the data, and instructions on installing this software on a variety of platforms, including several workstation types and PCS. The CD-ROM also includes text files describing the organization of the data and the structure and content of the output files when subsets of the data are extracted. Output files of gridded DEM data consist of ASCII records with five fields: gridpoint latitude, gridpoint longitude, grid cell X coordinate, grid cell Y coordinate and elevation (cell coordinates are related to an arbitrarily selected grid for both ice sheets). Output files of retracked orbit elevation profiles have four fields: orbit number, latitude, longitude, and elevation. In both types of output files, decimal points are removed, so latitude and longitude are reported in decimal degrees x 106 and elevation is reported in cm above sea level. Sea level is based on the OSU91A geoid model.

With the CD-ROM now available, NSIDC will generally distribute the entire data set to requestors who are interested in altimetry data, unless they ask for assistance in subsetting a small, specific study area. Waveform data for these missions are still available from NSIDC through our computer access to a Goddard archive. However, the very large volume of this waveform data (>10 Gbytes) requires that we ask that all requestors have very specific, focused requests for a limited, well bounded portion of the archive.

NSIDC is preparing to send the CD-ROM to all users who have previously contacted us regarding alitmetry data. A questionnaire will be included with the CD-ROM to provide us with feedback on the layout of the data and the retrieval software, so that future Altimetry CD-ROMs, such as for the ERS-1 data, may incorporate any suggested changes.

A limited number of the radar altimetry CDs will be available to other NASA-approved researchers. If you are interested in receiving a copy of this CD-ROM, please contact NSIDC User Services.

For information on scientific applications of the data set contact: Ted Scambos at (303) 492-1113 or fax: (303) 492-2468, or Internet: teds@icehouse.colorado.edu.

DMSP SSM/I Sea Ice Concentration Grids: Updates

DMSP F-11 SSM/I Sea Ice Concentration grids for the Polar Regions are now available. These files were generated using the NASA Team ice concentration algorithm.

To obtain F-11 ice concentration data from NSIDC: ftp sidads.colorado.edu, (login as 'anonymous'), (type your e-mail address as password), cd pub/DATA_SETS/DATA/SSMI/F11

The F-11 ice concentration data are in Hierarchical Data Format (HDF). Software to read and manipulate HDF data files is available via ftp from NCSA (helphdf@ncsa.uiuc.edu). IDL, other commercial or "shareware" packages can also be used to read and manipulate HDF files.

The DMSP F8 SSM/I Monthly-averaged Sea Ice Concentration data, available through anonymous ftp, have been moved to a new directory.

ftp sidads.colorado.edu, (login as 'anonymous'), (type your e-mail address as password), cd pub/DATA_SETS/DATA/SSMI/F8.

LEAP YEARS

Which of the following years is NOT a leap year: 1700, 1800 or 1900?

Answer: None of the above is a leap year.

Most of us remember the rule that leap years are divisible by four. That would, at first glance, tell us that 1700, 1800 and 1900 are all leap years. However, there is an additional clause to that rule which says if the year is divisible by 100, it must also be divisible by 400 to be a leap year. This means that 1600 and 2000 are leap years, but the century years in between are not, even though they are indeed divisible by 4.

As the DBA for NSIDC, I came across this problem while processing the Historical Soviet Daily Snow Depth dataset for advertisement in the V0 IMS. I found data files dated 29 February 1900! Those files were actually empty, but my processing was temporarily thrown off when the INGRES database would not accept a timetag of 29-Feb.-1900.

The explanation is, of course, that the actual length of a year is not just 365.25, but more like 365.2524 (plus some change). We all know the .25 of a day is "saved up" and added to the Gregorian calendar every four years, on February 29th. But the additional .0024 of a day is handled by holding out the leap day on all but every fourth century year. This makes 1600, 2000, 2400, etc., leap years, but the century years in between, are not.

Karen Robinson, Data Base Administrator, NSIDC.

CORRECTIONS

In the last issue of NSIDC Notes (no. 11, Fall 1994), the telephone number for the GSFC DAAC User Support Office was incorrect. The correct number is (301) 286-3209.

AARI Sea Ice Data (no. 11, Fall 1994)

NSIDC has not received data from AARI for 1991 and 1992.

The data NSIDC has received from AARI is in the SIGRID format, not the SIGRID-2 format. AARI plans to use the SIGRID-2 format adopted by the Global Digital Sea Ice Data Base (GDSIDB) Working Group for the exchange of digitized sea ice charts.