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The National Oceanic and Atmospheric Administration/National Environmental Satellite, Data, and Information Service (NOAA/NESDIS) has an extensive history of monitoring snow and ice coverage. Accurate monitoring of global snow/ice cover is a key component in the study of climate and global change as well as daily weather forecasting. By inspecting environmental satellite imagery, analysts from the Office of Satellite Data Processing and Distribution (OSDPD), Satellite Services Division (SSD), Satellite Analysis Branch (SAB), have created a Northern Hemisphere snow/ice map since November 1966. Initially, the product was produced with a spatial resolution of about 16,000 square kilometers to 42,000 square kilometers and temporal resolution of seven days. In 1997, the Interactive Multisensor Snow and Ice Mapping System (IMS) became operational, giving the satellite analysts improved access to imagery and drawing tools. Since February 1997, the IMS product has been produced daily at approximately 24 km resolution (1024 x 1024 grid). Further improvements in computer speed and imagery resolution allowed production of a higher resolution product at approximately 4 km resolution (6144 x 6144 grid), beginning in February 2004. NSIDC archives and distributes both IMS products, as well as GIF image files and latitude and longitude grids for these products. To view the images, please visit the NOAA SSD Snow Products Web site. In June 2006, NSIDC started distributing 4 km GeoTIFF files for use with GIS applications.
NSIDC strongly encourages you to register as a user of this data product. As a registered user, you will be notified of updates and corrections. Please contact NSIDC User Services to register.
NOAA/NESDIS/OSDPD/SSD. 2004, updated 2006. IMS daily Northern Hemisphere snow and ice analysis at 4 km and 24 km resolution. Boulder, CO: National Snow and Ice Data Center. Digital media.
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.
| Category | Description |
|---|---|
| Data format | ASCII and GeoTIFF |
| Spatial coverage and resolution | Bounding coordinates are 180 W to 180 E, and 0 to 90 N. Spatial resolution is 4 km (February 2004 until present) and approximately 24 km (February 1997 through at least 2005). |
| Temporal coverage and resolution | Daily, February 1997 to present (24 km) Daily, February 2004 to present (4 km) |
| Grid type and size | 6144 by 6144 (4 km) and 1024 by 1024 (24 km) I1 single integer |
| File naming convention | Daily files: imsYYYYDDD_Xkm.asc Daily GeoTIFF files: imsYYYYDOY_4kmGIS.zip Monthly files: imsYYYYMM_Xkm.tar Monthly GeoTIFF files: imsYYYYMM_4kmGIS.zip Ancillary files (lat/lon grids): imslat_24km.dat, imslon_24.dat, imslat_4km.bin.gz and imslon_4km.bin.gz |
| File size | 20 KB to 60 KB; 36913 KB for 4 km GeoTIFF files (12 KB for the associated .aux files) |
| Parameters | Snow and ice cover |
| Procedures for obtaining data | FTP |
1. Contacts
2. Overview
3. Detailed Data Description
4. Data Acquisition and Processing
5. Data Access and Related Collections
6. References and Related Publications
7. Document Information
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
The NOAA National Environmental Satellite, Data, and Information Service (NESDIS) has produced a weekly snow and ice analysis since November 1966. This is the longest satellite-derived data set in existence (Ramsey, 1998). These hemispheric snow cover maps are used primarily as initialization fields for numerical weather prediction models (higher resolution, basin-scale snow products are produced by NOAA for hydrological forecasting [see Related NSIDC Data Collections]). Errors in near surface temperature forecasts occurred due to the relatively low temporal resolution of the snow product. Therefore, NOAA began development of the IMS in 1995 in order to make a daily snow map feasible. The IMS has reduced the time required for an analyst to make a chart from about 10 hours to about one hour. See Table 1 for a summary of IMS and pre-IMS snow cover products.
Prior to the introduction of the IMS, analysts displayed satellite data (primarily AVHRR, but increasingly, beginning in 1975, data from geostationary satellites as well) on a workstation screen, identified snow and ice by manual inspection, and transferred boundaries to a paper chart. An electronic version was created by overlaying a grid on this chart and digitizing grid cells. The paper version of this pre-IMS product is archived at the NOAA NESDIS National Climatic Data Center (NCDC). A quality controlled derived version of this product is available in digital form from Rutgers University Global Snow Lab.
The IMS system greatly improved the speed, accuracy and resolution with which the maps could be produced by incorporating additional data sources and allowing the analyst to create digital products directly on a workstation. Passive microwave data are used as well, to improve snow detection under cloudy or nighttime conditions. It is possible to automate the creation of a snow cover product if passive microwave data are used. However snow detection using passive microwave data is subject to error under certain conditions. OSDPD therefore continues to create a manual analysis product from a variety of sources for the greatest possible accuracy (Ramsey 1998).
The OSDPD is planning an ongoing evolution of the IMS product. The first enhancement was the implementation of a 4 km product. Additional enhancements will include expanding to a global product, adding snow water equivalent and snow depth, and increasing the frequency to twice daily. Each new product will be created in parallel with its predecessor for a period of at least 15 months so that product consistency can be evaluated for the entire time series.
By agreement with NOAA, NSIDC archives and makes the IMS products available on a daily basis. It is important to note that the IMS product is produced in near-real time to meet a daily operational need. Therefore changes in product format or production method may occur from time to time, and errors may exist that only a retrospective analysis can identify and correct. Rutgers University Global Snow Lab provides a consistent and quality controlled analysis of snow cover derived from IMS and its predecessor products (Robinson 2000), along with an interface for displaying trends, anomalies, and monthly climatologies. The NSIDC Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 2 includes the Rutgers snow product and sea ice cover from passive microwave for a consistent representation of snow- and ice-covered surfaces with weekly frequency, along with climatologies.
The IMS product is considered operational, but OSDPD does not guarantee availability or timely delivery of data via the OSDPD Web server, and NSIDC does not guarantee availability of this product via the NSIDC Web server. These servers should not be used to support operational observation, forecasting, emergency, or disaster mitigation operations, either public or private. Users with real-time operational needs should contact the Satellite Analysis Branch (SAB) Snow Product Lead within the NOAA/NESDIS/OSDPD/SSD/Interactive Processing Branch at 301-763-8142 or ssdhelpdesk@noaa.gov and request access to the operational server.
Table 1. Summary of NESDIS OSDPD Snow and Ice Map Products
| Product Name | Frequency | Grid Size | Period of Record | Notes |
| Northern Hemisphere Snow and Ice Boundaries | Weekly | Paper format | Produced by the NESDIS Synoptic Analysis Branch, November 1966 to approximately 1993. | This product is not distributed by NSIDC. It is archived by NOAA NCDC. |
| IMS Daily Northern Hemisphere Snow and Ice Analysis, 24 km | Daily | 1024 x 1024 (~24 km) | Began February 1997. Declared operational 1 November 1998 to the present. | GIF browse images are available from NSIDC; to view the images, visit the NOAA SSD Snow Products Web site. |
| IMS Daily Northern Hemisphere Snow and Ice Analysis, 4 km | Daily | 6144 x 6144 (~4 km) | Production began in January 2004. Operational from 23 February 2004 to the present. | GIF browse images are available from NSIDC; to view the images, visit the NOAA SSD Snow Products Web site. GeoTIFF images and corresponding .aux files are also available from NSIDC, beginning in June 2006. |
Data are in a polar stereographic projection with the straight vertical longitude from the Pole (the central meridian) at 80°W, and the standard parallel at 60°N.
Each data file contains header information (ASCII text) about the specific data file, followed by a 6144 x 6144 array (for the 4 km files) or 1024 x 1024 array (for the 24 km files) of format I1 (single integer) data, with (1,1) starting at the lower left corner. The top of the file is South America, the left side is the Pacific Ocean, the right side is Africa, and the bottom is Indonesia. Values in the array are:
Grid points in the southern hemisphere (that is, those which fall off the sphere in the square array) are flagged as "0". There should be no missing values over the mapped hemisphere.
The GeoTIFF files contain the same values (i.e. 1 = sea) as the data files. Each GeoTIFF has an associated .aux file that needs to be in the same directory in order to properly retain the projection information. This is a non-standard type of GeoTIFF.
The GIF images corresponding to these files display ice as yellow, snow as white, land as brown, and water as blue.
Files of grid cell positions are also available as follows.
These are 1024 x 1024 or 6144 x 6144 arrays of 4-byte floating point values (decimal degrees) in Little Endian (PC) byte order. The array values are stored in row-major order (incrementing across each column of the first row, and then each column of the second row, etc). Missing data is marked as NaN.
Longitude ranges from -180°W to 180°E. Latitude ranges from 0°N to 90°N.
Table 2. Summary of the ancillary files (lat/lon grids). Content in this table was supplied by NSIDC scientific programmers.
Data |
Projection |
Latitude of true scale |
Longitude below the pole |
Grid size |
Scale |
Upper left corner of the upper left
cell |
| 24 km | polar stereographic spherical projection with a sphere with radius of 6371200.0 meters | 60° N | 80° W | 1024 x 1024 | 23,684.997 meters per cell in x and y | (x,y) = -12126597.0 meters, 12126840.0 meters) |
| 4 km | polar stereographic ellipsoidal projection with WGS-84 ellipsoid | 60° N | 80° W | 6144 x 6144 | 4,000 meters per cell in x and y | (x,y) = (-12288000.0 meters, 12288000.0 meters) |
Occasionally, a day is missed and no file is available for the archive. This is rare: from the start of the record through 2003 only five files are missing. The file G02156_missing_files.txt tallies missing files and is regularly updated by NSIDC. Note that there are missing GeoTIFF files from February 2004 through April 2006 because the GeoTIFF processing did not begin until mid-2006. There are also missing GeoTIFF data between July 23, 2007 and October 29, 2007. NSIDC did not receive the 4 km GeoTIFF files during this period.
For data users who use NSIDC's mapx software, the two corresponding 4 km and 24 km .gpd files are located here. The .gpd files contain projection and grid parameter definitions used by the mapx software.
Sample header information from the data file ims2004016.asc.
Julian day of IMS data log: 2004016
Processing day: Fri Jan 16 21:12:01 2004
Total # scientific data sets: 1
File description:
This file contains Northern Hemisphere snow and ice coverage produced by the NOAA/NESDIS Interactive Multisensor Snow and Ice Mapping System (IMS) developed under the direction of the Interactive Processing Branch (IPB) of the Satellite Services Division (SSD). For more information, please contact Mr. Bruce Ramsay at bramsay@ssd.wwb.noaa.gov
Map Label: Northern Hemisphere 1024 x 1024 snow and ice coverage
Coordinate System: Polar Stereographic
Data Values: 1 (sea), 2 (land), 3 (sea ice), 4 (snow), Data Values: 0 (outside Northern Hemisphere).
Format: I1
Dimensions: 1024 x 1024
(1,1) starts at: lower left corner
Snow and Ice Chart, United States, Sunday, February 29, 2004 (image courtesy of Satellite Services Division)
NSIDC distributes both data (ASCII) and image (GIF and GeoTIFF) files. The data are available as daily ASCII text files with the following naming format:
imsYYYYDDD_Xkm.asc
where
ims = Interactive Multisensor Snow and Ice Mapping System
YYYY = year
DDD = day of year
_Xkm = 4 km or 24 km for grid cell size of the map (NSIDC adds grid cell size; files downloaded from OSDPD do not have an indication of grid size in the name.)
asc = ASCII text format.
Example filenames: ims2004058_4km.asc and ims1997238_24km.asc.
At the end of each month, all of the daily ASCII files are compressed into two monthly tar files (one for each grid cell size) with the following naming format:
imsYYYYMM_Xkm.tar
Example filenames: ims200410_4km.tar and ims200109_24km.tar.
The daily files for the previous month are then removed from the FTP site.
Data files representing two snow seasons (for example, October 1, 2001, through September 30, 2003) are available on the FTP site. To request files older than two snow seasons, please contact NSIDC User Services.
GIF image files have the following naming format:
imsYYYYDDD.gif
where
ims = Interactive Multisensor Snow and Ice Mapping System
YYYY = year
DDD = day of year using a Julian calendar
GIS = geographic information system
Note: Each zipfile contains a TIFF and an associated .aux file.
GeoTIFF files have the following naming format:
imsYYYYDDD_4kmGIS.zip (daily)
and
imsYYYYMM_4kmGIS.zip (monthly)
where
ims = Interactive Multisensor Snow and Ice Mapping System
YYYY = year
DDD = day of year using a Julian calendar
gif = GIF graphic format.
Example filename: ims2000018.gif.
See the "Ancillary files" section above for other file names.
Some of the compressed ASCII files from 1997 and 1998 have file sizes around 60,000 bytes. All ASCII files after 1998 are about 20,000 bytes. This is the case because a more efficient file creation scheme was implemented at the end of 1998, which eliminated white spaces between data values.
Northern Hemisphere coverage is available at approximately 4 km (February 2004 on) and at approximately 24 km (February 1997 through at least 2005) resolution, with maps drawn daily. The bounding coordinates are –180° West to 180° East, and 0° South to 90° North.
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 analyst. 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.
Sub-grid scale features may not be detected. The documentation for the Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 2 (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 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 2, uses only passive microwave for ice extent.)
SAB analysts draw snow maps on workstations that display data products and satellite imagery from a variety of sources. 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. Geostationary satellites used include Geostationary Operational Environmental Satellites (GOES), Geostationary Meteorological Satellite (GMS), and European Weather Satellite (METEOSAT) data. In addition, ground weather observations and microwave product 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. Microwave products (Special Sensor Microwave/Imager [SSM/I] Environmental Data Records), though at relatively low resolution, 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.
The analyst begins with a previous day’s map as a first guess. Input satellite data and fields are sampled to a standard 6144 km x 6144 km (~4 km per pixel) matrix. The completed map is resampled to a 1024 km x 1024 km (~24 km per pixel) matrix. Both the 4 km and 24 km products are automatically saved in ASCII and GIF formats. The NOAA National Center for Environmental Prediction creates a BUFR format file, but it is not archived.
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% or 60% concentration edge usually is 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.
The ASCII 4 km files that NSIDC distributes are built to NCEP specifications. The 4 km GeoTIFF files were dereived from the ENVI .dat files used to produce the IMS data (Helfrich et al.,, 2007).
Data are available via FTP. Browse images are available on the NOAA SSD Snow Products Web site.
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.
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.
Analyses of a quality controlled and extended version of these data are available from Rutgers University Global Snow Lab.
IMS snow maps may also be viewed on a Web-based GIS Server, provided by the NOAA National Geophysical Data Center.
This data set and documentation were developed at the instigation of NOAA/NESDIS/OSDPD’s Bruce Ramsey, and with the support of Donna McNamara, Team Leader, Environmental Applications Team, OSDPD, and NOAA SDS staff. NSIDC thanks Sean Helfrich, OSDPD Snow and Ice Product Area Lead, for his efforts in making the 4 km GeoTIFF files available for distribution. NSIDC’s Mary Jo Brodzik provided helpful background information and assistance. The product team at NSIDC consisted of Lisa Ballagh, Florence Fetterer, Jonathan Kovarik, and Keri Webster.
Please see the EOSDIS Acronyms list for a general list of Acronyms. The following acronyms are used in this document:
AMSU: POES Advanced Microwave Sounding Unit
AVHRR: Advanced Very High-Resolution Radiometer
DMSP: Defense Meteorological Satellite Program
DOD: Department of Defense
FGDC: Federal Geographic Data Committee
GMS: Geostationary Meteorological Satellite
GOES: Geostationary Operational Environmental Satellite
IMS: Interactive Multisensor Snow and Ice Mapping System
METEOSAT: European Weather Satellite
MODIS: Moderate Resolution Imaging Spectrometer
NCDC: National Climatic Data Center
NCEP: National Centers for Environmental Prediction
NIC: National Ice Center
NESDIS: NOAA National Environmental Satellite, Data, and Information Service
NOAA: National Oceanic and Atmospheric Administration
NOHRSC: National Operational Hydrologic Remote Sensing Center
OSDPD: Office of Satellite Data Processing and Distribution
POES: Polar Operational Environmental Satellite
SAB: OSDPD SSD Satellite Analysis Branch
SNODAS: Snow Data Assimilation System
SSD: OSDPD Satellite Services Division
SSM/I: Special Sensor Microwave/Imager
Florence Fetterer and Keri Webster wrote this documentation based on the cited references, the OSDPD web site, the metadata files supplied by OSDPD, and information from Donna McNamara, Team Leader, Environmental Applications Team, NOAA/NESDIS/OSDPD.
December 2004, edited February 2005 (F. Fetterer removed misleading text from the Sample Data Record), June 2006 (F. Fetterer added information on GeoTIFFs), October 2007 (L. Ballagh updated information about the ancillary grids) and December 2007 (L. Ballagh added information on missing GeoTIFF files).
http://nsidc.org/data/docs/noaa/g02156_ims_snow_ice_analysis/index.html
