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IMS Daily Northern Hemisphere Snow and Ice Analysis at 4 km and 24 km Resolution

Summary

The National Environmental Satellite, Data, and Information Service (NESDIS), part of the National Oceanic and Atmospheric Administration (NOAA), has an extensive history of monitoring snow and ice coverage. Accurate monitoring of global snow and 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 Satellite Analysis Branch (SAB) at the Office of Satellite Data Processing and Distribution (OSDPD), Satellite Services Division (SSD), created a Northern Hemisphere snow and ice map from November 1966 until the National Ice Center (NIC) took over production in 2008. Initially, the product was produced with a nominal spatial resolution of 190 km and a 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 the inception of IMS, the charts have been produced daily at a nominal resolution of 24 km (1024 x 1024 grid). Beginning in February 2004, further improvements in computer speed and imagery resolution allowed for the production of a higher resolution daily product with a nominal resolution of 4 km (6144 x 6144 grid). NSIDC archives and distributes the 24-km and the 4-km IMS product in ASCII text format from February 1997 to present and February 2004 to present, respectively. NSIDC also distributes browse images in GIF format and latitude and longitude grids for these products. In June 2006, NSIDC started distributing 4-km GeoTIFF files for use with GIS applications.

Note: The IMS product is considered operational, but NIC does not guarantee availability or timely delivery of data via the NIC 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 visit the NIC Web site and contact the National Ice Center Liaison to request access to the operational server.

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.

Citing These Data

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

National Ice Center. 2008, updated daily. IMS daily Northern Hemisphere snow and ice analysis at 4 km and 24 km resolution. Boulder, CO: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N52R3PMC

Overview Table
 

Category Description
Data format Data:
ASCII text (.asc) one-digit integer (I1) (Except as noted in the ASCII Data Files section of the Detailed Data Description)
GeoTIFF (4 km only)
Lat/Lon Grids: Flat binary (.bin or .dat), 4-byte floating point values (little endian)
Browse images: GIF format (.gif)
Ancillary Data: See the Ancillary Data Format section of this document
Spatial coverage and resolution Northern Hemisphere:

Southernmost Latitude: 0° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Resolution: 4-km and 24-km nominal resolution
Temporal coverage and resolution 4-km ASCII data: 23 February 2004 to present, daily resolution
24-km ASCII data: 04 February 1997 to present, daily resolution
4-km GeoTIFF data: 01 January 2006 to present, daily resolution
Grid type and size 4 km: 6144 by 6144
24 km: 1024 by 1024
Projection Information Polar Stereographic projection centered at 90° N with the vertical longitude from the Pole at 80° W and the standard parallel at 60° N
File naming convention ASCII data files: imsYYYYDDD_Xkm.asc
GeoTIFF data files: imsYYYYDOY_4kmGIS.zip
Lat/Lon grids: imslat_24km.dat, imslon_24.dat, imslat_4km.bin.gz, and imslon_4km.bin.gz
Browse images: imsYYYYDDD.gif
Ancillary data: See the Ancillary Data Format section of this document
File size ASCII Data files: 1 MB to 36.9 MB
GeoTIFF Data files: 37 MB (plus 12 KB for the associated .aux files)
Lat/Lon grids: 4 MB - 123 MB
Browse Images: 11 KB - 24 KB
Parameters Snow cover
Ice cover
Tools for accessing data See the Tools section of this document for information.
Metadata access View metadata
Data access Data are available via FTP

Table of Contents

  1. Contacts
  2. Overview
  3. Detailed Data Description
  4. Data Access and Tools
  5. Data Acquisition and Processing
  6. Related Collections
  7. References and Related Publications
  8. Document Information

1. Contacts

Technical Contact

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

2. Overview

History of Product Development

From November 1966 through 2008, NESDIS produced weekly snow and ice charts until NIC took over their production. These charts are the longest satellite-derived data set in existence (Ramsay 1998) and are primarily used as initialization fields for numerical weather prediction models but also provide an excellent, high-quality climate record of snow and ice. For information on higher resolution, basin-scale snow products produced by NOAA for hydrological forecasting, see the Related NSIDC Data Collections section of this document.

In 1995, due to errors seen in near surface temperature forecasts caused by the relatively low temporal resolution of the snow product, NOAA began development of the Interactive Multisensor Snow and Ice Mapping System (IMS) in order to make a daily snow map feasible. According to Ramsay (2000), the IMS is a geographic information system that "was developed to permit meteorologists to interactively prepare daily NH [Northern Hemisphere] snow and ice maps and to take advantage of additional remotely sensed imagery such as that based on DMSP passive microwave data from SSM/I. The previously used manual method in production of the weekly chart took eight to ten hours depending upon the season. Meteorologists using IMS routinely produce daily maps in under 90 minutes." See Table 1 for a summary of IMS and pre-IMS snow cover products.

The purpose of the IMS is to provide SAB analysts with the ability to interactively create, save, and distribute highly accurate maps depicting the extent of daily hemispheric snow and ice coverage. 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. Then they identified snow and ice by manual inspection and transferred boundaries to a paper chart. An electronic version was created by overlaying a grid onto 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 using passive microwave data. However, snow detection via this method is subject to error under certain conditions. Therefore, for the greatest possible accuracy, creation of a manual analysis product from a variety of sources continues (Ramsay 1998).

Ongoing evolution of the IMS product is planned. The first enhancement was the implementation of a 4-km product in 2004. 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.

Note: When NIC took over production of the charts in 2008 the citation was changed to reflect this. When NESDIS was producing the charts the citation was as follows:

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.

Role of NSIDC

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 3 includes the Rutgers snow product and sea ice cover from passive microwave data for a consistent representation of snow- and ice-covered surfaces with weekly frequency, along with climatologies.

The IMS product is considered operational, but NIC does not guarantee availability or timely delivery of data via the NIC 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 visit the NIC Web site and contact the National Ice Center Liaison to 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. This product is distributed by NSIDC along with GIF browse images.
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. This product is distributed by NSIDC along with GIF browse images. Also, GeoTIFF images and corresponding .aux files are also available from NSIDC, beginning in June 2006.

 

3. Detailed Data Description

Format

The data and their associated ancillary files come in a variety of formats. The data are provided in ASCII text (.asc) and GeoTIFF (.tif) formats. Latitude/longitude grids in binary (.bin and .dat) format accompany the ASCII text data and an associated metadata file (.aux) accompany the GeoTIFF files. Other ancillary files such as metadata information are provided in ASCII text. Quick view browse images are also provided in GIF (.gif) format. For full details, see each section below:

ASCII Data Files

Both the 4-km and 24-km resolution data are provided as ASCII (.asc) text files (Note that the files are gzipped (.gz) on the FTP site). The top of each data file contains header information pertinent to that specific file. Note: The headers are not the same size across all files. For information on reading these files, see the Tools section of this document. The header is followed by a 6144 x 6144 array of data values for the 4-km files or a 1024 x 1024 array of data values for the 24-km files. Most of the values in the arrays in the ASCII data files are 1-digit integers (I1); this is known as the packed form. However, a small portion of the 24-km ASCII data files contain values that are 3-digit integers instead of 1-digit; this is known as the unpacked form. For a list of dates of these files, see the Dates of 24-km IMS Data Files in Unpacked Format text file. For more information on this, see the Note on Packed Versus Unpacked Form. Note: In the ASCII data files, the index (1,1) starts at the lower left corner of the grid where 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. Data values in the files are described in Table 2.
 

Table 2. Values in the ASCII Data Files
Value Description
0 Represents pixels that are south of the equator. This data set is for the Northern Hemisphere only, and each file contains the circle of the Northern Hemisphere properly enclosed in the square grid of data, so grid points which fall off the sphere in the square array (that is, in the southern hemisphere) are flagged.
1 Sea
2 Land
3, 164 Sea Ice (Note: The value 164 only pertains to ASCII files listed in Files in Unpacked Format)
4, 165 Snow (Note: The value 165 only pertains to ASCII files listed in Files in Unpacked Format)

There are no missing values over the mapped hemisphere.

Note on Packed Versus Unpacked Format

Over time, NOAA used two standard formats for the 24-km IMS data files.  One format was a packed form in which the data values are stored as one data row per file row, with each ASCII character representing the data value for the corresponding data column. The other format was an expanded form in which a data row spans several rows in the file, with data values stored in 4-column fields, separated by a single space. For a list of dates of the unpacked files, see the Dates of 24-km IMS Data Files in Unpacked Format text file.

GIS/GeoTIFF Data Files

The GeoTIFF files (.tif) are compatible with GIS applications and contain the same values as the ASCII data files. See Table 2 for these values. Each GeoTIFF file has an associated metadata file ( .aux) containing geographic and projection information. This file must reside in the same directory as the GeoTIFF file in order to properly retain the projection information.

Lat/Lon Grids

The ASCII data files provided by NESDIS do not come with lat/lon grids, so NSIDC created these file and added them to the data set as a courtesy to the data community. The lat/lon grids are provided for both the 4 km and 24 km data and are provided in flat binary 4-byte, floating point values (decimal degrees) in little-endian 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, and so on). Missing data is marked as NaN. Latitude/longitude pairs represent the location of the lower left corner of the corresponding grid cell. See Table 3 for a full description of the lat/lon grids.

Note: With respect to the ASCII text data files, the lat/lon grids are flipped in orientation. Specifically, the binary arrays are stored beginning with the upper left corner, whereas the ASCII text data are stored beginning with the lower left corner. Please be aware of this when working with these files.

Note: Although the data files that these lat/lon grids correspond to are in ASCII text format, the lat/lon grids are provided in binary format. Therefore, the lat/lon grids cannot be viewed in a text editor like the data files. An NSIDC programmer created an IDL procedure to read the grids. See the Tools section of this document for more information.

 

Table 3. Description of Lat/Lon Grids
Spatial Resolution
4 km 24 km
File Names
Latitude File: imslat_4km.bin.gz
Longitude File: imslon_4km.bin.gz
Latitude File: imslat_24km.dat
Longitude File: imslon_24km.dat
Format
Flat binary 4-byte floating point values (decimal degrees) in little-endian byte order Flat binary 4-byte floating point values (decimal degrees) in little-endian byte order
Grid Size
6144 x 6144 1024 x 1024
Longitude Values
Range from 0° to 360° (with values greater than 180° representing west longitudes and values less than 180° representing east longitudes) Range from -180° to 180° (with values less than 0° representing west longitudes and values greater than 0° representing east longitudes)
Latitude Values
Range from 0° to 90° representing north latitudes Range from 0° to 90° representing north latitudes
Projection
Polar stereographic ellipsoidal projection with WGS-84 ellipsoid Polar stereographic spherical projection with a sphere with radius of 6371200.0 meters
Latitude of true scale
60° N 60° N
Longitude below the pole
80° W 80° W
Scale
4,000 meters per cell in x and y 23,684.997 meters per cell in x and y
Upper left corner of the upper left cell
(x,y) = (-12288000.0 meters, 12288000.0 meters) (x,y) = (-12126597.0 meters, 12126840.0 meters)
Missing Data Value
NaN NaN

 

Other Ancillary Files

Missing Data File

On rare occasions, a day is not processed for various reasons resulting in no data file being available for the archive. Although this is rare, the ASCII text file, G02156_missing_files.txt, tallies missing files and is regularly updated by NSIDC. It provides the missing dates for both the 4-km and 24-km ASCII text data as well as for the 4-km GIS (GeoTIFF) files. Note: For the GIS files, the last day of every month since April 2009 is missing.

Mapx Software Files

For data users who use Mapx software, the two corresponding 4-km and 24-km .gpd files are located on the FTP site: Ims4km.gpd and Ims24km.gpd, respectively. The .gpd files contain projection and grid parameter definitions used by the mapx software.

Browse Images

The quick view browse images are provided in GIF format (.gif). The GIF images display ice as yellow, snow as white, land as green, and water as blue.

Sample Data Record

Sample header information from the data file ims2004016.asc. Note: The headers are not the same size across all files.

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

Sample Browse Image

Browse image for the 24 km resolution data for 29 February 2004
Figure 1. 24 km Northern Hemisphere Snow and Ice Chart Browse Image for 29 February 2004 (ims2004060.gif). Image courtesy of Satellite Services Division.

 

File and Directory Structure

Data are located on the FTP site in the G02156 directory. Within this directory are five subdirectories as described in Table 4 and Figure 2.

Table 4. Directory Descriptions
Directory Description
24km Contains the 24-km resolution ASCII text data files (Note that the files are gzipped (.gz) on the FTP site). This directory is further broken down into subdirectories, one for each year that data was collected (1997 - current year) labeled as the 4-digit year (YYYY). The data files reside in their respective year directory.
4km Contains the 4-km resolution ASCII text data files (Note that the files are gzipped (.gz) on the FTP site). This directory is further broken down into subdirectories, one for each year that data was collected (2004 - current year) labeled as the 4-digit year (YYYY). The data files reside in their respective year directory.
GIS Contains the 4-km resolution GeoTIFF files. This directory is further broken down into subdirectories, one for each year that GeoTIFFs have been produced (2006 - current year) labeled as the 4-digit year (YYYY). The GeoTIFF files reside in their respective year directory.
images Contains the GIF browse images. This directory is further broken down into subdirectories, one for each year that GIFs have been produced (1997 - current year) labeled as the 4-digit year (YYYY). The GIF files reside in their respective year directory.
metadata Contains the lat/lon grid files, missing data file, mapx software files, and metadata files.

 

Graphic of Directory Structure
Figure 2. Directory Structure

 

File Naming Convention

ASCII Data Files

Daily ASCII text files are named according to the following convention and as described in Table 5. Note that the files are gzipped (.gz) on the FTP site.

imsYYYYDDD_Xkm.asc

Where:

Table 5. Naming Convention Description for Daily ASCII Files
Variable Description
ims Identifies this as data coming from Interactive Multisensor Snow and Ice Mapping System
YYYY 4-digit year
DDD 3-digit day of year
Xkm Cell size: 4 or 24
asc Identifies this as an ASCII text file

GeoTIFF Files

The GeoTIFF files come with an associated .aux file, so they are zipped up in pairs.

The zip files and the data files have a similar naming convention as described below and in Table 6:

imsYYYYDDD_4kmGIS.zzz

Where:

Table 6. Naming Convention Description for Zipped GeoTIFF Files
Variable Description
ims Identifies this as data coming from Interactive Multisensor Snow and Ice Mapping System
YYYY 4-digit year
DDD 3-digit day of year
4km Identifies this as being a 4-km resolution file
zzz File extension (tif: GeoTIFF files, aux: auxiliary files, zip: zip file)

Lat/Lon Grids

There are four lat/lon grid files. See the Lat/Lon Grids Format section of this document for a complete description of the files.

Other Ancillary Files

See the Other Ancillary Files Format section of this document for a complete description.

Browse Image Files

GIF browse image files are named according to the following convention and as described in Table 7:

imsYYYYDDD.gif

Where:

Table 7. Naming Convention Description for GIF Image Files
Variable Description
ims Identifies this as data coming from Interactive Multisensor Snow and Ice Mapping System
YYYY 4-digit year
DDD 3-digit day of year
gif Identifies this as an GIF image file

 

File Size

Table 8 lists the sizes of the files that comprise this data set.

Table 8. File Size
File Type Size
4-km ASCII ~36.9 MB unzipped (~450 KB gzipped)
24-km ASCII 1 MB (packed form) or 5 MB (unpacked form) unzipped (25-30 KB gzipped).
See Note on Packed Versus Unpacked Format
GeoTIFF 37 MB (plus 12 KB for the associated .aux files)
Lat/Lon grids 4 MB - 121 MB
Browse images 11 KB - 24 KB

Spatial Coverage and Resolution

Northern Hemisphere coverage is available at approximately 4-km and at approximately 24-km resolution. The bounding coordinates are as follows:

Southernmost Latitude: 0° N
Northernmost Latitude: 90° N
Westernmost Longitude: 180° W
Easternmost Longitude: 180° E

Projection

Data are in a polar stereographic projection centered at 90° N with the vertical longitude from the Pole at 80° W and the standard parallel at 60° N.

Grid Description

The ASCII text data are available in two grid sizes based on their spatial resolution.

4 km: 6144 by 6144
24 km: 1024 by 1024

Temporal Coverage and Resolution

ASCII Data Files

The 4-km ASCII data files start on 23 February 2004 and continue to the present with a daily resolution. The 24-km ASCII data files starts on 04 February 1997 and continue to the present with a daily resolution.

GeoTIFF Files

The 4-km GeoTIFF data files begin on 01 January 2006 and continue to the present with a daily resolution.

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 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.

4. Data Access and Tools

Data Access

Data are available via FTP. Browse images are also available on the IMS Products Web page from the NOAA SSD Snow Products Web site.

Tools

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.

5. Data Acquisition and Processing

The IMS product is manually created by a satellite analyst looking at all available satellite imagery, several automated snow mapping algorithms, and other ancillary data. SAB 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. 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 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. 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 (NCEP) creates a Binary Universal Form for the Representation of meteorological data (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 percent or 60 percent 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 derived from the ENVI .dat files used to produce

6. Related Collections

Related NSIDC Data Collections

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.

7. References and Related Publications

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., 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.

Acknowledgments

Beginning in 2006, IMS production transitioned to the National Ice Center. NSIDC thanks Sean Helfrich, Office of Satellite Products and Operations (OSPO) Lead, for his contributions to this product. These include making the 4 km GeoTIFF files available for distribution.

This data set and documentation were developed in 2004 at the instigation of NOAA/NESDIS/OSDPD's Bruce Ramsay and with the support of OSDPD, NOAA SDS Staff, and Donna McNamara, Team Leader of the Environmental Applications Team. NSIDC's Mary Jo Brodzik provided helpful background information and assistance. At the time of the creation of this document, the product team at NSIDC consisted of Lisa Ballagh, Florence Fetterer, Jonathan Kovarik, and Keri Webster.

8. Document Information

List of Acronyms

Table 9 lists the acronyms used in this document.

Table 9. Acronyms Used in this Document
Acronym Description
AMSU POES Advanced Microwave Sounding Unit
ASCII American Standard Code For Information Interchange
AVHRR Advanced Very High-Resolution Radiometer
BUFR Binary Universal Form for the Representation of Meteorological Data
DMSP Defense Meteorological Satellite Program
DOD Department of Defense
FGDC Federal Geographic Data Committee
FTP File Transfer Protocol
GeoTIFF Geospatial Tagged Image File Format
GIF Graphics Interchange Format
GMS Geostationary Meteorological Satellite
GOES Geostationary Operational Environmental Satellite
IDL Interactive Data Language
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
NH Northern Hemisphere
NOAA National Oceanic and Atmospheric Administration
NOHRSC National Operational Hydrologic Remote Sensing Center
NSIDC National Snow and Ice Data Center
OSDPD Office of Satellite Data Processing and Distribution
OSPO Office of Satellite Products and Operations
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

Document Authors

Florence Fetterer and Keri Webster wrote the original product documentation based on the cited references, the OSDPD web site, the metadata files supplied by OSDPD, and information from Donna McNamara, Team Leader of the Environmental Applications Team, NOAA/NESDIS/OSDPD in 2004.

Document Creation Date

December 2004

Document Revisions

December 2011, A. Windnagel: Updated documentation to reflect the fact that the ASCII data files are now being gzipped.
December 2010, A. Windnagel: Minor changes to route users to G02186 for more information on how sea ice is mapped in the IMS product. Changed the citation which was originally "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." This was changed to reflect the production at NIC beginning in 2008.
January 2010, A. Windnagel: Did a full edit/revision of this document including major additions to the File Naming and Convention section to make the document current.
June 2009, A. Windnagel: Updated the lat/lon information for the ancillary files
July 2008, A. Windnagel: Added information about the two different formats of the ASCII text files. Also added some formatting and styling.
December 2007, L. Ballagh: Added information on missing GeoTIFF files.
October 2007, L. Ballagh: Updated information about the ancillary grids.
June 2006, F. Fetterer: Added information on GeoTIFFs.
February 2005, F. Fetterer: Removed misleading text from the Sample Data Record.

Document URL

http://nsidc.org/data/docs/noaa/g02156_ims_snow_ice_analysis/index.html