Date of Last Ice Cover Metadata ================================ File Name last_ice.txt File contents: dates of last ice cover, winters: 1898-2002 Dates are based on the following data sources -------------------------------------------------------------------------------- 1. modeled data for winters 1898-1972 (Assel 1990) 2. Canadian Ice Service data for winters 1973-1988 calculated from data in Assel (2003) 3. National Ice Center data for winters 1996-2002 calculated from data in Assel (2003) 4. An average of CIS and NIC data, based on daily data, for winters 1989-1995 -------------------------------------------------------------------------------- If the user wants the original cis and nice files for winters 1989-1995 they are available as well in cis-last.txt and nic-last.txt. ----------------------------- Record FORMAT: (4(I5,2I3,I4,2X)) ------------------------------------------------------ Date of spatial average first ice > 10% concentration ------------------------------------------------------ field 1. Lake averaged year field 2. Lake averaged month field 3. Lake averaged day field 4. Lake averaged julian date field 5. West basin averaged year field 6. West basin averaged month field 7. West basin averaged day field 8. West basin averaged julian date field 9. Center basin averaged year field 10. Center basin averaged month field 11. Center basin averaged day field 12. Center basin averaged julian date field 13. East basin averaged year field 14. East basin averaged month field 15. East basin averaged day field 16. East basin averaged julian date ------------------------------ Number of records on each file ------------------------------ last_ice.txt 105 nic-last.txt 7 cis-last.txt 7 ------------------------------------------------------------------------------------- Definition of last ice cover, code for winters without ice cover ------------------------------------------------------------------------------------- The last date the spatial average ice cover concentration is greater or equal to 10%. Basins that did not have ice cover greater or equal to 10% include: the center basin in winters: 1953 and 1998; the east basin in winters: 1932, 1933, 1949, and 1953. A code of -1 was entered for the month, day, and julian date for these basins for these winters. A 10% threshold is used because the original observations are only considered accurate to the nearest 10%. In addition, the term "open water" was used on many ice charts to indicate that less then 10% of the area was covered by ice. In some cases the lake was virtually ice free. Therefore, the 10% threshold value is a conservative estimate of the date of last reported ice. In addition, the standard error of estimate of the modeled ice cover data for each of the three lake basins was greater than 10% (Assel 1990). Assel, R.A., 1990. An ice-cover climatology for Lake Erie and Lake Superior for the winter seasons 1897-98 to 1982-83, International Journal of Climatology, 10:731-748. Assel, R.A., 2003. NOAA Atlas, An Electronic Atlas of Great Lakes Ice Cover winters 1972-2003. NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan 48105. Data Set Created December 2003 ------------------------------ Raymond Assel NOAA/GLERL 4840 S. State Rd. Ann Arbor, MI email: Ray.Assel@noaa.gov