Data are available via FTP.

ASCII Data Files

To read the ASCII data files, an NSIDC programmer created an IDL procedure, read_ims.pro, to read the grids. Note: The program was created as a courtesy to users of this data set and to provide one example of how to extract the data, however, the code is not supported or updated.

GeoTIFF Files

The GeoTIFF files can be viewed in GIS applications.

Lat/Lon Grids

To read the lat/lon grid files, an NSIDC programmer created an IDL procedure, read_ims_geolocation.pro, to read the grids. Note: The program was created as a courtesy to users of this data set and to provide one example of how to extract the data, however, the code is not supported or updated. Example plots of IMS data created using NCAR Command Language (NCL) are available from the NCL Web site.

Most input data are acquired, on a daily basis, through the IMS preprocessing system that automatically runs scripts that use FTP to acquire the input files (Helfrich, Li, and Kongoli 2012).

The IMS product is manually created by a NOAA NESDIS Office of Satellite and Product Operations (OSPS) satellite analyst looking at all available satellite imagery, automated snow mapping algorithms, and other ancillary data. OSPS analysts draw snow maps on workstations that display these data products and satellite imagery. The visible imagery of the Polar Operational Environmental Satellites (POES) and geostationary orbiting environmental satellites are primary. Moderate Resolution Imaging Spectrometer (MODIS) imagery is used as well. In addition, ground weather observations from many countries are used. Microwave products from POES Advanced Microwave Sounding Unit (AMSU) and the Department of Defense (DOD) Defense Meteorological Satellite Program (DMSP) are incorporated into the daily snow and ice chart because even though they are at relatively low resolution, they allow a view through clouds. A weekly sea ice analysis from the National Ice Center (NIC), the United States Air Force Snow and Ice Analysis Product, and snow products from the National Operational Hydrologic Remote Sensing Center (NOHRSC) are made available to the analyst, as well as several automated snow detection layers developed by NESDIS and the National Centers for Environmental Prediction (NCEP). A complete list of input data products is provided in Table 12.

The OSPS analyst begins with a previous day's map as a first guess. Input satellite data and fields are resampled to a the three IMS grids: 24576 x 24576 (~1 km/pixel) , 6144 x 6144 (~4 km/pixel) matrix, and 1024 x 1024 (~24 km per pixel). All resolutions are saved in ASCII, GeoTIFF, and GIF formats. The ASCII files are built to NCEP specifications because they are the primary users of this product. The NOAA National Center for Environmental Prediction (NCEP) creates a Binary Universal Form for the Representation of meteorological data (BUFR) format file and a GRidded Information in Binary (GRIB) format file, but they are not archived at NSIDC.

For snow extent, analysts rely primarily on visible band satellite imagery. For sea ice, analysts rely first on visible imagery, then on passive microwave data, followed by the NIC analysis product, depending on the timeliness of the data, the resolution of the data, and the time of year. When analysts use passive microwave imagery, the 40 percent or 60 percent concentration edge is most often used to correlate with about 7/10+ coverage on NIC ice charts. Analysts are conservative in adding ice. The IMS ice edge will generally depict an ice edge that is less advanced than the NIC ice edge.

Processing Steps

IMS processing can be broken down into four generalized steps (Helfrich, Li, and Kongoli 2012):

1. A preprocessing system takes all input products and imagery from their native formats and resolutions and converts them into the IMS formats and resolutions and places them on a internal server.
2. An IMS GUI system picks up the processed data from the sever at intervals throughout the day and displays the data on the IMS projection.
3. The OSPS analysts tag locations as snow covered and ice covered over the entire Northern Hemisphere. The GUI system also generates snow depth, ice thickness, and time since last observation via code that analysts can also alter before the analysis is exported.
4. Scripts produce final products and distribute final products to proper destinations.

Quality Assessment

The quality of the snow- and ice-cover charts will depend on the availability of clear sky imagery, the georegistration of that imagery, the quality of other input data sources, and the experience of the OSPS analyst. This is a manually created product which uses multiple images to map the snow/ice regions. Surface data is also made available to the analysts to aid with real-time quality control. Regions covered by cloud during the 24-hour analysis period are generally mapped as persistence, taking lower resolution passive microwave data and surface observations into account where possible. Other than grid points in the square array which, from a hemispheric view, fall off the sphere and are flagged as 0 (outside the northern hemisphere), there should be no missing values over the mapped hemisphere.

Sub-grid scale features may not be detected. The documentation for the Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 3 (Armstrong and Brodzik 2002) and for snow products at Rutgers University Global Snow Lab includes more information on quality assessment, including the following from the Global Snow Lab:

"Despite the shortwave limitations [ ...], the NOAA maps are quite reliable at many times and in many regions. These include regions where, 1) skies are frequently clear, commonly in Spring near the snowline, 2) solar zenith angles are relatively low and illumination is high, 3) the snow cover is reasonably stable or changes slowly, and 4) pronounced local and regional signatures are present owing to the distribution of vegetation, lakes and rivers. Under these conditions, the satellite-derived product will be superior to maps of snow extent gleaned from station data, particularly in mountainous and sparsely inhabited regions. Another advantage of the NOAA snow maps is their portrayal of regionally-representative snow extent, whereas maps based on ground station reports may be biased, due to the preferred position of weather stations in valleys and in places affected by urban heat islands, such as airports."

See the NSIDC Sea Ice Index:Interpretation Resources for Sea Ice Trends and Anomalies for a general discussion of passive microwave imagery for sea ice extent. Note that while the NOAA IMS product makes use of both passive microwave and visible band imagery to map ice extent, the NSIDC product, Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 3, uses only passive microwave for ice extent.

Table 12 provides a list of all of the sensors and instruments used as input to this data set.

Table 13 provides the version history of this product. Note that NSIDC has used minor versions (v1.1, v1.2) to distinguish between IMS versions. The files we download from NIC are labeled with major versions (v1, v2). NSIDC versions align with the NIC versions in the following way: NSIDC v1.1 corresponds to NIC v1, NSIDC v1.2 corresponds to NIC v2, and v1.3 corresponds to v3.

Armstrong, R.L. and M.J. Brodzik. 2002. Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 2. Boulder, CO: National Snow and Ice Data Center. CD-ROM.

Dye, D. 2005. Timing and statistics of autumn and spring annual snow cover for the Northern Hemisphere. Boulder, CO: National Snow and Ice Data Center. Digital media.

Helfrich, S. R., M. Li, and C. Kongoli. 2012. Interactive Multisensor Snow and Ice Mapping System Version 3 (IMS V3) Algorithm theoretical basis document version 2.0 Draft 4.1. Noaa NESDIS Center for Satellite Applications and Research (STAR). 59 pp.

Helfrich, S. R., D. McNamara, B. H. Ramsay, T. Baldwin, and T. Kasheta. 2007. Enhancements to, and forthcoming developments to the Interactive Multisensor Snow and Ice Mapping System (IMS), Hydrological Processes 21: 12, 1576-1586.

National Operational Hydrologic Remote Sensing Center. 2004. SNODAS Data Products at NSIDC. Boulder, CO: National Snow and Ice Data Center. Digital media.

Ramsay, B. H. 2000. Prospects for the Interactive Multisensor Snow and Ice Mapping System (IMS). 57th Eastern Snow Conference, Syracuse, NY, 2000: ramsayECS57.

Ramsay, B. H. 1998. The Interactive Multisensor Snow and Ice Mapping System. Hydrological Processes 12: 1537-1546.

Ramsay, B. H. 1995. An Overview of NOAA/NESDIS’s Interactive Multisensor Snow and Ice Mapping System. Proceedings of 1995 First Moderate Resolution Imaging Spectrometer (MODIS) Workshop, Greenbelt, MD, National Aeronautics and Space Administration Conference Publication 3318: 23-27.

Robinson, D.A. 2000. Weekly Northern Hemisphere snow maps: 1966-1999. 12th Conference on Applied Climatology, Asheville, NC, American Meteorological Society. 12-15.

Robinson, D.A. and A. Frei. 2000. Seasonal variability of northern hemisphere snow extent using visible satellite data. Professional Geographer 51: 307-314.

Robinson, D.A., J.D. Tarpley, and B. Ramsay. 1999. Transition from NOAA weekly to daily hemispheric snow charts. Proceedings of the 10th Symposium on Global Change, Dallas, TX, American Meteorological Society. 487-490.

NSIDC has many snow cover products. Three are closely related to the IMS: Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 2 (Armstrong and Brodzik 2002), and Timing and Statistics of Autumn and Spring Annual Snow Cover for the Northern Hemisphere (Dye, 2005), which both make use of the quality controlled NOAA snow cover record beginning in 1966 (provided by Robinson), and Snow Data Assimilation System (SNODAS) Data Products at NSIDC (National Operational Hydrologic Remote Sensing Center 2004). The IMS product is one of several data sources for the NOHRSC SNODAS grids.

The Multisensor Analyzed Sea Ice Extent - Northern Hemisphere (MASIE-NH) product is based on this IMS product and provides measurements of daily sea ice extent and sea ice edge boundary for the Northern Hemisphere.

Additionally, NSIDC has many sea ice data products. Please refer to Sea Ice Products at NSIDC for a comparison of the products. The IMS product sea ice extent will be similar to that from passive microwave ice extent products, but the IMS also uses operational ice chart data, like those in Environmental Working Group Joint U.S.-Russian Arctic Sea Ice Atlas.

Other Related Data Collections

Analyses of a quality controlled and extended version of these data are available from Rutgers University Global Snow Lab.

NOAA SSD Snow and Ice Products Web site also provides browse images of the data on their IMS Products Web page.

• IMS Products Web page for the NIC IMS product.