Northern Hemisphere Snow Cover Anomalies

Average values from months 3 and 4 of each year from 1967 to 2011.

During the development of a routine to improve the accuracy of monthly snow areas from the NOAA data, an inconsistency in the cells which NOAA considered to be land, thus potentially snow covered, was discovered. In 1981, NOAA changed their land mask, in the process eliminating 26 cells from consideration of being snow covered (categorizing them as water), while 27 others began to be examined. These grid cells encompass some 1.8 x 106 sq. km, which could amount to a several percent or greater inaccuracy in snow estimates. Neither of the NOAA masks is accurate; both fail to accurately identify all cells, and only those cells, at least half covered by land. Therefore, a definitive land mask has been developed using digital map files analyzed on a geographic information system. The percentage of land in each of the 7921 NMC grid cells was calculated using the National Geophysical Data Center's five minute resolution ETOPO5 file as the primary data source. As this file does not include large interior lakes, the Navy Fleet Numerical Oceanography Center's 10 minute resolution Primary Terrain Cover Types file was used to properly account for these water bodies. Some 48 cells polewards of approximately 30° N which had been considered land in the pre 1981 NOAA and/or the 1981 to present NOAA mask are actually predominantly water covered (< 50% land). Conversely, 54 land cells were found to have been considered water on one or both NOAA masks. Those cells falling under the latter required a first-time analysis to determine whether they might be snow covered. This was accomplished by selecting nearest representative land cells (cells which NOAA has continuously mapped as land) and assigning their snow status to the "new" land cells. Spot checks of a number of hard copy weekly maps proved this to be an adequate approach.

Snow cover between 1966 and 1971 was reanalyzed here at the Rutgers University Climate Lab using daily gridded composites of visible imagery for the eastern and western hemispheres of the Northern Hemisphere. Surface resolution of the imagery is approximately 25 km. The imagery was supplemented with daily reports of snow depth at several thousand stations in the U.S., Canada, China and the former Soviet Union, gridded to 1° x 1° grid cells using all reports from within a given cell. Daily surface weather charts also provided information on cloud cover, precipitation and temperature. Infrared imagery and the above ancillary information were employed in many areas to confirm interpretations made from visible data. The weekly maps were digitized to the National Meteorological Center Limited-Area Fine Mesh grid. This is an 89 x 89 cell Cartesian grid laid over a polar stereographic projection of the Northern Hemisphere. Cell resolution ranges from 16,000 to 42,000 square kilometers (this product has also been regridded to the equal area EASE-grid for November 1966 to June 2007 and is distributed by the National Snow and Ice Data Center). Each grid cell in the digitized product has a binary value. Cells with at least 50% of their surface covered with snow were considered snow covered. All other cells were considered snow free.

Weekly/Monthly GSL Snow Cover Grid Product

In June 1999 NOAA ceased production of weekly snow maps. IMS daily snow charts are now produced at NIC and utilized at Rutgers to create a unique Northern Hemisphere snow cover product. Using Perl software created at the GSL, weekly and monthly 89x89 grid cell charts are generated. In this procedure, weekly areas are calculated from digitized snow files, and monthly values are calculated by weighting the weekly areas according to the number of days of a map week falling in the given month. During this process, the 24 km resolution IMS daily snow product is reduced and subjected to filtering through the corrected land mask created here at the GSL. The result is a Climate Data Record (CDR) that details Northern Hemisphere snow cover over the last 44 years.(from http://climate.rutgers.edu/snowcover/docs.php?target=vis)