Multisensor Analyzed Sea Ice Extent – Northern Hemisphere (MASIE-NH)

The Multisensor Analyzed Sea Ice Extent – Northern Hemisphere (MASIE-NH) products provide measurements of daily sea ice extent and sea ice edge boundary for the Northern Hemisphere and 16 Arctic regions in a polar stereographic projection.

Table of Contents

  1. Summary
  2. Background Information
  3. Detailed Data Description
  4. Data Acquisition and Processing
  5. Related Collections and Resources
  6. References and Related Publications
  7. Contacts and Acknowledgments
  8. Document Information

Citing These Data

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

National Ice Center (NIC) and NSIDC. 2010, updated daily. Multisensor Analyzed Sea Ice Extent – Northern Hemisphere. [indicate subset used]. Developed by F. Fetterer, M. Savoie, S. Helfrich, and P. Clemente-Colón. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N5GT5K3K

Overview

Parameters

Sea Ice Extent
Sea Ice Edge/Boundary

Spatial Coverage
and Resolution

Southernmost latitude: 0° N
Northernmost latitude: 90° N
Westernmost longitude: 180° W
Easternmost longitude: 180° E

Resolution: 4 km

The MASIE product is also divided up into 16 smaller regions located
in and around the Arctic. See Table 1 for a list of these 16 regions.

Temporal Coverage
and Resolution

01 October 2006 to present, daily resolution

Platform

Satellite and in-situ

Data Format

Data Files: Comma Separated Value (CSV) ASCII Text (.csv)
Daily Georeferenced Images: GeoTIFF (.tif)
Daily Browse Images: PNG (.png)
Daily Sea Ice Extent Boundary: Shapefiles (.shp)
Time Series Plots: PNG (.png)
Google Earth Files: KMZ (.kmz)
Ancillary files: NetCDF (.nc), GeoTIFF (.tif), PNG (.png), and Excel (.xls)

Metadata Access

View Metadata Record

Data Access

Most Recent: MASIE Web site
Complete Archive: FTP

1.Summary

MASIE products include an ASCII text file of sea ice extent values in square km over the entire Northern Hemisphere with 16 separate Arctic regions identified, time series plots of the 16 regions, and image files that visually show where the sea ice is. The MASIE-NH imagery are provided at a nominal 4 km resolution. The input data comes from the 4 km Interactive Multisensor Snow and Ice Mapping System (IMS) snow and ice product produced by the National Ice Center (NIC). NIC utilizes visible imagery, passive microwave data, and NIC weekly analysis products to create their data product. The MASIE-NH products are distributed in a number of formats including ASCII text, GeoTIFF, PNG, shapefiles, and Google Earth files and are available from 01 October 2006 to the present. The most recent days worth of imagery plus the ASCII text data file are provided via the MASIE Web site. A complete archive of the imagery can be obtained via FTP.

Note: MASIE may look like several other sea ice products distributed at NSIDC and elsewhere, but its source data and intended uses are different. If intended and appropriate uses of the data are not clear after reading the documentation, please contact NSIDC User Services.

2. Background Information

The input data set for the MASIE-NH product is the 4 km IMS snow and ice product produced by NIC. This product is archived at NSIDC in the IMS Daily Northern Hemisphere Snow and Ice Analysis at 4 km and 24 km Resolution data set.

NIC constructs the sea ice component of this data set using satellite data, but they also draw on information from NIC operational charts and other sources. NIC operational charts are produced on a weekly to biweekly basis for Arctic and Antarctic waters. These charts use a wide variety of data sources and are constructed by analysts trained in remote sensing imagery interpretation and sea ice climatology. NIC is a multi-agency operational center directed by the United States Navy, the National Oceanic and Atmospheric Administration (NOAA), and the United States Coast Guard (USCG).

The IMS product was designed to aid numerical weather prediction by providing a proxy boundary layer albedo field. The IMS ice and snow extent fields are produced with fixed standards and quantify areal coverage with set metrics. In contrast, operational ice charts meet the needs of those going into the ice and provide general situational awareness, such as the extent of fast ice or of ice of any concentration greater than zero percent. Chart production is more flexible than is IMS production in order to meet changing user needs and source data availability.

Both NIC IMS and NIC chart products usually represent sea ice more accurately than do products based on single-sensor satellite data alone. For any given region or day, a user who wants the most accurate analysis of ice edge position and concentration should use products from an operational ice service such as NIC.

While operational analyses are usually the most accurate and timely representation of sea ice, they have errors and biases that change over time. If one is interested in long-term trends in sea ice or how it responds to changing climate forcing, generally, it is best not to use an operational product, but rather one that is consistently produced and retroactively quality controlled. The NSIDC Sea Ice Index monthly ice extent, and the satellite passive microwave data sets upon which it is based, is one example. The Sea Ice Index gives a daily image of extent as well as monthly products. However, these daily images are not meant to be used for climate studies or for inferring anything longer than seasonal trends. Satellite data are not quality controlled quickly enough; and for reasons explained in the Sea Ice Index documentation, the daily ice edge position can be off by tens of kilometers or more from the ice edge that an analyst would draw. Reasons include known errors in thin ice detection, bias in summertime concentration estimates, and the relative compactness of the marginal ice zone. See Partington et al. (2003) for an assessment of operational versus satellite-derived ice concentration.

MASIE was developed by NSIDC and NIC to fill a need for an intermediate type of product; one between operational charts and the passive microwave based Sea Ice Index. MASIE is based on the NIC IMS product and gives a daily picture of ice extent that is easy to use and available in several formats. Daily values for hemisphere-wide and regional ice extent are made available in an Excel file that is a rolling archive of the most recent four weeks of extent values and archived imagery is available for the complete data set. MASIE gives a quick picture of ice extent that is more accurate than the daily Sea Ice Index product and allows users to compare day-to-day changes in extent values. However, in general, it would not be appropriate to compare a recent MASIE extent value to one more than a few weeks old because the data sources and analysts NIC uses may have changed.

In addition to IMS, NIC has two other operational products that were considered as potential source data for MASIE. They are the weekly or biweekly ice charts and a daily ice edge product. The ice charts usually show the ice edge region in great detail but are unsuitable for MASIE because they are not a daily product. The daily ice edge product is used to warn navigators and others in arctic seas where ice exists or is likely to form at any concentration. The daily ice edge product edge is always more conservative, or outboard, of the IMS edge. Ice charts and ice edge products have marine transportation interests as primary users, while the IMS product is designed primarily for modelers.

The IMS product is an intermediate product. It is produced relatively consistently when compared to chart and edge products, but also benefits from the same careful manual analysis that is used for those products, which is why it was chosen as the input for the MASIE product. This article, written in 2006, gives a brief history of NIC's operations that will help you understand the setting in which its products are created: The Evolution of Operations at the U. S. National Ice Center: From Paper to Pixel.

Note: From publication of MASIE in November 2010 until June 2014, only the last four weeks of data were made available. This was because MASIE is based on an operational product that may not be consistently produced and may not be appropriate for looking at changes in ice over time periods longer than a few weeks. In June 2014, we decided to make the MASIE product available back to 2006. This was done in response to user requests, and because the IMS product output, upon which MASIE is based, appeared to be reasonably consistent. Prior to 2006, the IMS product is different in format and cannot be used for MASIE.

3. Detailed Data Description

Data Access

The most recent data (generally the previous day) can be obtained from the MASIE Web site. Archives of the data from 01 October 2006 are available via FTP. Gaps in the data record may occur.

Note: MAISE is based on the IMS product. 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 the IMS product or of the MASIE 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 NIC Liaison to request access to the operational server.

Parameters

Sea ice extent, that is, the area covered by sea ice in square km, and sea ice edge are the parameters of this product.

The input IMS gridded product contains cells that are binary; they are either set to ice or not ice. MASIE ice extent values are obtained by counting IMS product ice cells and multiplying by their area. Sea ice extent is synonymous with sea ice area for this product, but NSIDC uses the term extent. The All about Sea Ice: Terminology Web page explains how the terms sea ice area, sea ice concentration, and sea ice extent are related and used in many NSIDC data sets.

Spatial and Temporal Coverage and Resolution

This data product covers the entire Northern Hemisphere with a nominal spatial resolution of 4 km and the following spatial coordinates:

Southernmost latitude: 0° N
Northernmost latitude: 90° N
Westernmost longitude: 180° W
Easternmost longitude: 180° E

The MASIE product is also divided up into 16 smaller regions located in and around the Arctic. See Table 1 for a list of these 16 regions. For specific lat/lon vertices of the regions, see the file MASIE_regions_polygon_vertices.xls.

Temporal Coverage and Resolution

The temporal coverage of this product spans 01 October 2006 to the present (usually yesterday) at a daily resolution.

June 2014: Until now, the MASIE product has been a rolling archive showing only the most recent four weeks. Users wanting a longer view of changes in sea ice were directed to products like the Sea Ice Index. This was because operational products, like that upon which MASIE is based, can change suddenly and unpredictably in response to changing needs, data sources, and capabilities at an operational center such as the National Ice Center. However, in response to user demand and because the product appears to be consistent back through 2006, we are releasing the entire archive of MASIE products as of June 2014 with the exception of the masie_extent_sqkm.csv file which still only contains that last 28 days of data. However, this file will be updated with the entire record in the future.

Projection and Grid Description

The MASIE products are provided in a polar stereographic projection with the WGS 1984 datum and a standard parallel at 60° N with a nominal grid size of 4 km. For complete details on the specifics of this projection, see the polar_stereographic variable in the netCDF file masiemask_ims4km.nc.

Format

The MASIE-NH product is distributed in a number of formats. See each section below for the format of that specific product:

Data Files

The data values are provided in one single CSV ASCII text file (.csv) named masie_extent_sqkm.csv. This file contains daily sea ice extent values in sq km going back to 01 October 2006 and contains 18 columns. The first column is the date of the data value in the 4-digit year, 3-digit day of year format (yyyyddd); the last 17 columns are the regions. See Table 1 for region names. The file is 16 KB in size and is updated daily with the oldest day's record being removed and the newest record added.

Sample Data File

The following is the first seven rows and seven columns of masie_extent_sqkm.csv created on 09 November 2010.

MASIE NSIDC/NIC Sea Ice Product G02186 - Daily Ice Extent by Region in Square Kilometers
 yyyyddd,(0) Northern_Hemisphere, (1) Beaufort_Sea, (2) Chukchi_Sea, (3) East_Siberian_Sea, (4) Laptev_Sea, (5) Kara_Sea, 
 2010283, 6097384.75,             725802.23,         276512.82,       535245.53,             325942.76,      160667.28,    
 2010284, 6377051.74,             754403.69,         303362.38,       572381.44,             366616.07,      185168.46,    
 2010285, 6442208.46,             776393.09,         316489.89,       573868.90,             372137.54,      185168.46,    
 2010286, 6663079.49,             789444.60,         320156.15,       585474.24,             419904.30,      336664.85,    
 2010287, 6732130.10,             800888.26,         300392.82,       574780.20,             477135.95,      342397.58,    
  

 

Daily Georeferenced Images

The daily georeferenced images are provided in GeoTIFF format. There are two different types of files. One version contains all surface type classifications for sea ice, land, coastline, lake, ocean, and missing; the other version is a binary sea ice/not sea ice file. The binary sea ice/not sea ice GeoTIFF image files are provided to aid users who may want to layer ice in other applications. See Table 2 for byte values and RGB color values of the classifications. There is a daily file for each of the 17 regions: Northern Hemisphere-wide plus 16 regional files. See Table 1 for a list of regions, their size, and their byte values. The size of the files ranges from 128 KB to 37 MB depending on the region. The files are updated daily if data are available. For specific lat/lon vertices of the 16 regions, see the file MASIE_regions_polygon_vertices.xls.

Table 1. Description of Regions
Region No. Region Name Size (grid cells) Byte Value
0 Northern Hemisphere 6144 x 6144 N/A
1 Beaufort Sea 497 x 449 1
2 Chukchi Sea 416 x 297 2
3 East Siberian Sea 476 x 457 3
4 Laptev Sea 524 x 513 4
5 Kara Sea 415 x 523 5
6 Barents Sea 612 x 533 6
7 Greenland Sea 582 x 850 7
8 Baffin Bay/Gulf of St. Lawrence 807 x 1257 8
9 Canadian Archipelago 531 x 564 9
10 Hudson Bay 591 x 665 10
11 Central Arctic 573 x 631 11
12 Bering Sea 423 x 932 12
13 Baltic Sea 530 x 420 13
14 Sea of Okhotsk 527 x 1079 14
15 Yellow Sea 339 x 373 15
16 Cook Inlet 382 x 304 16

 

Table 2. Surface Classification Byte Values and Colors
Classification Byte Value Color (name/[RGB])
missing/not sea ice 0 light grey [193, 190, 207]
ocean 1 light blue [145, 215, 249]
land 2 slate green [162, 186, 164]
sea ice 3 off white [254,254,254]
coast line 4 dark forest green [41, 77, 48]
lake 5 dark blue [50, 80, 120]
border of region images 6 dark grey [71, 68, 68]

 

The GeoTIFF files are named according to the following convention and as described in Table 5:

masie_type_rxx_vzz_yyyyddd_4km.tif

Sample Images

Figures 1a and 1b show an example of the GeoTIFF files.

Region 2 GeoTiff all surfaces sample Region 2 GeoTiff ice/not ice sample
Figure 1a. Example of GeoTIFF for All Surface Types for 08 November 2010 for Region 2 - Chukchi Sea (masie_all_r02_v01_2010312_4km.tif) Figure 1b. Example of GeoTIFF for Binary Sea Ice/Not Sea Ice Surfaces for 08 November 2010 for Region 2 - Chukchi Sea (masie_ice_r02_v01_2010312_4km.tif)

 

Daily Browse Images

The daily browse image files are provided in PNG format. The browse images are distributed to give a quick view of ice conditions and have a lat/lon graticule on them unlike the GeoTIFF files. They are provided for the all surfaces classification (sea ice, land, coastline, lake, ocean, and missing) for the entire Northern Hemisphere, the 16 MASIE regions, and a zoomed in image of the Northern Hemisphere focused on the Arctic. See Table 1 for a list of regions. The size of the files ranges from 16 KB to 768 KB depending on the region. The files are updated daily.

The browse images are named according to the following convention and as described in Table 5:

masie_all_rxx_vzz_yyyyddd_4km.png

Sample Image

Figure 2 shows an example of a PNG file.

Region 2 Sample Browse image
Figure 2. Sample PNG image for All Surface Types for 08 November 2010 for Region 2 - Chukchi Sea (masie_all_r02_v01_2010312_4km.png)

 

Daily Sea Ice Extent Boundary

The daily sea ice extent boundaries are provided as polygons in shapefiles (.shp). These are distributed for the entire Northern Hemisphere only; no region shapefiles are provided. Each file shows the outline of the sea ice edge for a particular day; no land or other surfaces are identified. Each daily shapefile and associated ancillary files (.dbf, .prj, and .shx) are zipped together in one file for easier downloading. The size of each zipped file is 32 KB, and the files are updated daily.

The shapefiles are named according to the following convention and as described in Table 5:

masie_ice_r00_vzz_yyyyddd_4km.zip

Sample Image

Figure 3 shows an example of the sea ice boundary in a shapefile.

Region 0 (Northern Hemisphere)
Figure 3. Example of Shapefile for 08 November 2010 for the Entire Arctic Region (masie_ice_r00_v01_2010312_4km.shp)

 

Time Series Plots

The time series plots are provided in PNG format and are available for the 16 MASIE regions (a hemisphere wide plot is not provided). For a list of regions, see Table 1. These plots show the sea ice extent for the previous four weeks for each year since 2006. The files range in size from 53 KB to 77 KB depending on the region.

The plots are named according to the following convention and as described in Table 5:

rxx_region_name_ts.png

Sample Image

Figure 4 shows an example of the region time series plots.

Region 2 Time Series Plot
Figure 4. Example of a time series plot for region 2, Chukchi Sea, from 13 November to 12 December for 2010 and 2009 (r02_Chukchi_Sea_ts.png)

 

Google Earth Files

The Google Earth files are provided as KMZ files (zipped KML files). These files show the ice extent on a virtual globe for the entire Northern Hemisphere (regional files are not provided). The files range in size from 98 KB to 130 KB depending on the date.

The files are named according to the following convention and as described in Table 5:

masie_ice_rxx_vzz_yyyyddd.kmz

Sample Image

Figure 5 shows an example of the MASIE data in Google Earth.

Google Earth Example for 12 December 2010
Figure 5. Example of MASIE data in Google Earth for 12 December 2010 (masie_ice_r00_v01_2010346.kmz)

 

Ancillary Files

The ancillary files provide specific information on the grid and projection, region masks, and region lat/lon vertices of the MASIE products.

Region Masks

The region masks are provided in three formats: NetCDF, GeoTIFF, and PNG.

NetCDF

The netCDF version, named masiemask_ims4km.nc, contains static, ancillary information about the data set. The netCDF file contains a number of variables that are described in Table 3. For detailed information on the netCDF file format, see the NSIDC NetCDF Software Tools Web page and the UCAR Unidata NetCDF Web site. The file is 504 MB in size.

Table 3. Description of Variables in masiemask_ims4km.nc
Variable Description
mask Land mask in a 2-dimensional byte array that identifies static surface types (missing, ocean, land, ice, coast, and lake) with the byte values described in Table 2.
regions Raster description of each region in a 2-dimensional byte array that contains the classification of the regions where each region is labeled with the byte value given in Table 1.
lat Latitude at the grid cell center in degrees North in a 2-dimensional float array
lon Longitude at the grid cell center in degrees East in a 2-dimensional float array
area Grid cell area in sq km in a 2-dimensional float array
x X coordinate of projection in meters in a 1-dimensional double array
y Y coordinate of projection in meters in a 1-dimensional double array
polar_stereographic Projection information in a 1-dimensional long array

 

GeoTIFF and PNG

The GeoTIFF and PNG versions of the masks for land, lake, ocean, coastline, ice, and missing for the entire Northern Hemisphere as well as the 16 MASIE Arctic regions provide visual representations of the masks. The GeoTIFF versions contain georeferencing information embedded in the files, and the PNG files are suitable for quickly browsing the masks. The GeoTIFF files range in size from 120 KB - 37 MB and the PNG files range in size from 7 KB - 670 KB depending on the region.

These files are named according to the following convention and as described in Table 5:

landmask_rxx.tif
landmask_rxx.png

Region Latitude/Longitude Vertices

Information about the spatial coverage and lat/lon vertices of each of the 16 regions can be found in the file, MASIE_regions_polygon_vertices.xls. The file is 57 KB is size.

File and Directory Structure

The data and image files are organized on the FTP site in six main directories: ancillary, geotiff, kmz, plots, png, and shapefiles. See Figure 6a and 6b for images of the directory structure. Table 4 describes the contents of the directories.

Table 4. Directory Structure
Directory Description
ancillary Contains the ancillary data files.

masiemask_ims4km.nc and MASIE_regions_polygon_vertices.xls reside in this directory.

The subdirectory landmask contains two additional directories:
geotiff - contains masks for the entire Northern Hemisphere and the 16 MASIE Arctic regions in GeoTIFF format
png - contains masks for the entire Northern Hemisphere and the 16 MASIE Arctic regions in PNG format
geotiff Contains the GeoTIFF image product files. It contains two subdirectories:
all_surface - contains the GeoTIFFs with all of the surface classifications. Within this directory are year directories containing each year of data except for the most recent year which reside directly within the all_surface directory.
ice_only - contains the GeoTIFFs with the binary sea ice/not sea ice classifications. Within this directory are year directories containing each year of data except for the most recent year which reside directly within the ice_only directory.
kmz Contains the KMZ Google Earth files within year directories containing each year of data except for the most recent year which reside directly within the kmz directory.
plots Contains the 16 regional time-series plots.
png Contains the PNG browse images within year directories containing each year of data except for the most recent year which reside directly within the png directory.
shapefiles Contains the zipped shapefiles within year directories containing each year of data except for the most recent year which reside directly within the shapefile directory.
masie_extent_sqkm.csv CSV file containing the last 28 days of data values.

 

Top level directory structure Expanded directory structure
Figure 6a. Top Level Directory Structure Figure 6b. Expanded Directory Structure

File Naming Convention

Table 5 describes the variables used in the file naming convention.

Table 5. File Naming Convention
Variable Description
type Surfaces classified in this file (all: all surfaces, ice: sea ice/not sea ice only)
landmask Identifies this file as containing a mask
rxx Region (r00 - r16 and zoom). See Table 1 for region names
vzz Version (v01: version 1)
yyyy 4-digit year
ddd 3-digit day of year
4km Indicates 4 km nominal resolution
ts Indicates this file contains a time series
region_names Region name, see Table 1 for a list
ext Extension that identifies the format of the file (tif: GeoTIFF, png: PNG, shp: shapefile, nc: netCDF, kmz: zipped kml files, and zip: zip file)

Error Sources

The main source of data for the input IMS sea ice information is visible band data interpreted as described in Section 4 of this document. However, passive microwave data and some analysis chart information are also used. Error can be introduced in all of these data sources.

For a general discussion of the errors possible when using passive microwave imagery for sea ice extent, see the NSIDC Sea Ice Index Interpretation Resources for Sea Ice Trends and Anomalies. Note: The Sea Ice Index input data comes from the SSM/I instrument on the DMSP satellites, but IMS uses the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) instrument on the Aqua satellite. AMSR-E has a much higher resolution than does SSM/I. For this reason, as well as others explained in Section 4, the IMS product is more accurate on a daily basis than is the sea ice extent from the Sea Ice Index.

IMS analysts also reference operational charts in addition to the visible band, passive microwave, and other sources of data. Operational charts are assembled at NIC as described in the National Ice Center Arctic Sea Ice Charts and Climatologies in Gridded Format documentation. Since these charts are not daily products, using their information for the IMS product at times requires some extrapolation by the NIC analyst charged with IMS production. Errors in operational charts and their quality will vary based on the available input information, that is, what data are available at the time the product is created.

The IMS product is an operational product and is largely manually produced. These two characteristics are both strengths and potential sources of error or inconsistency. For example, error can be introduced if an analyst misinterprets some satellite imagery. Inconsistency can be introduced when a new analyst interprets data sources slightly differently than does the previous analyst or when the operational nature of the product calls for some sudden adjustment to processing steps. The known inconsistency of operational products means that they are not suitable for long-term trends. At the same time, however, manual analysis results in a product that is more accurate than automatically generated products.

Quality Assessment

A quality assessment of this product has not been made.

 

4. Data Acquisition and Processing

MASIE is based on the NOAA 4 km IMS snow and ice product. The IMS product has been produced in some form since 1966, but has been produced at NIC since 03 March 2008. NSIDC downloads this product daily from NIC. The product is archived and distributed from NSIDC as well as from NIC.

The following section describes all of the processing required to create the MASIE product. It is broken down into two categories:

General Derivation of IMS Sea Ice Coverage at NIC

This section concentrates on how the sea ice component of the IMS product is produced at NIC.

Method

The IMS product is mostly manually generated by an analyst looking at all available satellite imagery, at output from a satellite passive microwave ice mapping algorithm, and at other ancillary data. The analyst begins with a map from the previous day 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 analysts integrate all data sources for the best estimate of areal coverage of ice cover. A cell is considered ice covered if more than 40 percent of the 4 km cell is covered with ice. This is regardless of the ice thickness or ice type. The 4 km product is automatically saved in ASCII, GeoTIFF, and GIF formats.

For further information, see the documentation for the IMS Daily Northern Hemisphere Snow and Ice Analysis at 4 km and 24 km Resolution product at NSIDC or contact NIC.

Data Sources

For determining sea ice areas, IMS analysts combine information from derived ice charts, modeled ice conditions, and surface observations, as well as from visible band, passive microwave, and active microwave data from polar orbiting and geostationary satellite platforms. The use of data sources varies by the timeliness of the data, the resolution of the data, weather conditions, and the time of year. While there is no hierarchy in data sources used for determining ice conditions, analysts tend to prefer visible band imagery. Other satellite sources, such as passive microwave imagers and sounders, Synthetic Aperture Radar (SAR), and scatterometry, are favored when visible light at the surface is either absent or obscured by clouds. In the Arctic, this is quite common.

Ice charts, modeled ice data, ship observations, oceanographic data, and atmospheric conditions are also considered when satellite sources are analyzed in order to provide context and to support the analyst's interpretation regarding the presence or absence of ice.

A cell is considered ice covered if more than 40 percent of the 4 km cell is covered with ice. The 40 percent threshold on ice demarcation for the IMS product is not directly related to 40 percent ice coverage estimated from derived passive microwave sources. At a 4 km2 resolution, several IMS cells fit within each coarser passive microwave derived sea ice concentration cell that are 12.5 km2 or greater. These IMS cells can reflect a mixture of areas with and without ice. Analysts often use data that are of sufficient resolution to allow that mixture to be determined. Furthermore, the use of visible and SAR imagery during summer melt seasons means that IMS product accuracy does not degrade as much as does the accuracy of products based solely on passive microwave data, when surface melt water attenuates the passive microwave signal. This microwave attenuation leads to underestimates of the ice concentrations, particularly along the marginal ice zone during the summer. For more information and references on this error source, see the Error Sources section of the Sea Ice Index documentation.

Data sources for the IMS product have changed since the 4 km resolution was introduced in February 2004; before 2004, the resolution was 24 km. New satellites and other sources have been introduced to replace those that are no longer available. Metadata that records which imagery was used to generate the snow and ice maps are not kept at this time. Helfrich et al. (2007) have an estimate of the percentage of imagery used from each source made before production was moved from the NOAA Satellite Analysis Branch to NIC in 2008.

As of 2010, the primary visible band imagery that analysts use comes from the Moderate Resolution Imaging Spectrometer (MODIS). Other visible satellite data sources include Advanced Very High Resolution Radiometer visible band (AVHRR-VIS), Geostationary Operational Environmental Satellite (GOES) Imager, Spinning Enhanced Visible and Infrared Imager (SEVIRI), and the Multi-functional Transport Satellite (MTSAT) Imager. AMSR-E 89 GHz brightness temperature at 6.25 m resolution is an important passive microwave data source. Analysts directly interpret areas with high 89 GHz brightness temperatures as areas covered by ice when this interpretation is supported by information from other sources. Other passive microwave sources include AMSR-E derived ice concentrations, Special Sensor Microwave Imager (SSM/I) derived ice concentrations, and Advanced Microwave Sounding Unit (AMSU) derived ice concentrations. Automated NOAA ice cover output (Sean Helfrich, citing the work of Peter Romanov, personal communication 08 November 2010) applying AVHRR, SSM/IS, GOES imager, and SEVIRI at the same IMS resolution are also examined as an objective evaluation of ice conditions. SAR imagery from RADARSAT-2, European Remote Sensing Satellite-2 (ERS-2), Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR), and Envisat Advanced Synthetic Aperture Radar (ASAR) are used but are not analyzed on the same screen with other IMS data sources. SAR data are examined in adjoining NIC Sea Ice Prediction and Analysis System (SIPAS) workstations and referenced as the IMS analysis is produced.

Derived ice conditions from ice charts and ice edge products from the US, Canadian, Norwegian, Danish, Russian, German, Swedish, and Japanese ice charting agencies also serve as data sources in the absence of direct satellite data or in areas where passive microwave derived ice data has inherit flaws. The use of ice charts is limited due to the infrequency of analyzed ice chart production. Ice edge products are examined thoroughly but are not used directly for the IMS product due to differences in mission ice identification requirements for each product. The NIC and Canadian Ice Service (CIS) ice edge products attempt to delineate not only where ice is present, but also where any ice is likely, regardless of the concentrations, for safety of navigation. The IMS product attempts to demarcate each 4 km x 4 km raster cell that appears to have more than 40 percent ice concentration. This means that the ice edge products encompass many open water areas that the IMS does not. Helfrich et al. (2007) have information on how NIC operational charts tend to differ from NIC operational IMS products.

Modeled ice conditions from the National Centers for Environmental Protection (NCEP) Marine Modeling and Analysis Branch (MMAB) and coupled Numerical Weather Prediction (NWP) models are also available for analysis, though these are generally used only for context and to understand where areas favor ice formation. Ship reports and ice buoys also enhance the analysis by providing limited ground truth, boundary layer weather conditions, and ice motion information.

Further data sources are likely to be introduced in 2012 with the availability of other advanced visible and passive microwave satellite data, coupled ice models, and more SAR imagery. There are also plans to incorporate ancillary information about data quality in future versions of the IMS. Ideas for this include a days since last observed variable so that users know on a cell-by-cell basis if the surface type assignment for a given cell was updated or simply carried over from the last analysis.

NSIDC Derivation of MASIE Products

This section concentrates on how the MASIE product is created at NSIDC from the input IMS Daily Northern Hemisphere Snow and Ice Analysis at 4 km and 24 km Resolution product.

The following steps are used to create the products:

  1. One time creation of the static ancillary files: netCDF ancillary file and the GeoTIFF and PNG landmasks.
  2. Obtain 4 km GeoTIFF IMS file.
  3. Apply mask to the IMS file to create the daily georeferenced file (GeoTIFF) for all surface types and for the binary sea ice/not sea ice and the daily browse image (PNG) for all surface types.
  4. Create the CSV file containing the extent values.
  5. Create the hemisphere-wide shapefiles.

Each of these steps is explained in detail below.

Creation of Static Ancillary Files

NetCDF File: masiemask_ims4km.nc

The static mask file masiemask_ims4km.nc is a netCDF file that identifies static surface types (land, ocean, coastline, sea ice, lakes, and missing) and the areas covered by the 16 regions along with other geographic information. Note: Cell locations identified as ocean in the mask file may be identified as ice in later processing steps that create the daily product files.

To create the file, NSIDC uses the following steps:

  • Begin with an IMS product mask, IMS4kmmask.nc, obtained from S. Helfrich of NIC in January 2010. This file masks only ocean, land, and off-earth.
  • Identify lakes in IMS4kmask.nc using an IDL region labeling function to first number all contiguous objects. If an object is within the area identified as land in the IMS4kmmask.nc, it is given the value for lake in the masiemask_ims4km.nc file. The Black Sea is manually identified as ocean.
  • Exclude lake and land ice in the input IMS product and label them just lake or land, respectively.
  • Identify coastline using a separate processing step to create a binary ocean/not ocean mask. The ocean region is expanded (dilated) with a 3 x 3 cell structuring element. This grows ocean over land by one cell; where the two now overlap is identified as coastline.
  • Identify regions using a shapefile created at NSIDC and based loosely on the areas used by some NSIDC passive microwave products as a starting point. The shapefile (a vector file) is converted to a raster file where each grid cell belonging to a region is assigned a common identifying value. See Table 1 for a list of the regions and their byte values.
Hemisphere and Regional Masks in GeoTIFF and PNG Formats

The static land mask files in GeoTIFF and PNG format with names like landmask_r05.png are derived from masiemask_ims4km.nc. They identify static surface types. Values for the surface types in the GeoTIFF files are given in Table 2.

Production of Daily Hemisphere-wide and Regional Image Files

The daily georeferenced (GeoTIFF) and browse images (PNG) are created with the following steps:

  • Nightly, the MASIE processing script obtains the 4 km GeoTIFF IMS file that originates at NIC and that is archived at NSIDC in the IMS Daily Northern Hemisphere Snow and Ice Analysis at 4 km and 24 km Resolution data set. The IMS GeoTIFF files have values for sea, land, ice (this includes land ice), snow, and off-earth. Note: Occasionally the daily file is not received from NIC, so MASIE processing cannot take place. The most recent MASIE product may then be two or more days old. When data flow from NIC resumes, production at NSIDC resumes, leaving a gap in the daily record.
  • To create the hemisphere-wide binary sea ice/not sea ice GeoTIFF files, the static masiemask_ims4km.nc file is used to convert the surface values in the IMS file to binary sea ice/not sea ice data. All cells labeled as surfaces that are not sea ice (land, lake, etc.) are given the not sea ice value and sea ice cells are given the sea ice value. See Table 2 for values.
  • To create the hemisphere-wide all surfaces GeoTIFF files, masiemask_ims4km.nc is again used to identify and convert land ice areas to the land surface type value. The other surface types (lake, sea ice, and ocean) are kept the same and given values. See Table 2 for values. Sea ice, then, in the resulting MASIE regional and hemisphere-wide GeoTIFF will be sea ice that has been identified by NIC in the IMS product.
  • To create the regional files for both types of GeoTIFFs, the regions variable in the static masiemask_ims4km.nc file is used to mask the out the regions in the hemisphere-wide file and then saved to a separate region file. See Table 1 for a list of regions.
  • The all surfaces PNG files are created from the all surfaces GeoTIFF files.

Example filenames for the products produced in this step are masie_all_r01_v01_2010290_4km.tif for the all surfaces GeoTIFF files and masie_ice_r01_v01_2010290_4km.tif for the binary sea ice/not ice data GeoTIFF files. The PNG files have names like masie_all_r01_v01_2010290_4km.png.

Production of Daily Sea Ice Extent Values File

To create the data file, masie_extent_sqkm.csv, and obtain the extent data values in square km, we use the following steps:

  • Total the area of each grid cell labeled as sea ice from the input IMS file excluding any ice on land or lakes. Regional values are computed by including only data that fall within regions defined by the variable regions in masiemask_ims4km.nc.
  • The newest values are computed daily and added to the data file, masie_extent_sqkm.csv, then the data from oldest day are removed so that the file always contains the most recent 28 days of data.

Production of Daily Hemisphere-wide Sea Ice Boundary Shapefile

Unlike the GeoTIFF data products, the processing to create the shapefiles, with names like masie_ice_r00_v01_2010256_4km.zip, includes some additional steps to make the sea ice layer, and particularly the edge, appear more like where an analyst would draw the ice edge if given only the IMS product as input. Therefore, the sea ice labeled in the shapefiles is slightly different from the sea ice labeled in the GeoTIFF files.

We perform the following steps to create the files:

  • Apply a morphological closing operation to source IMS data. The closing operation is a sequential application of a dilation operation followed by an erosion operation on cells labeled as sea ice with a structuring element that is a circle with a radius of two grid cells. The nominal resolution of the grid cells is 4 km. This step fills small holes and smooths the ice edge to a point that is subjectively judged to be close to where an analyst would draw the edge on the IMS image.
  • Discard sea ice polygons smaller than 16 cells (roughly 256 km2) by converting them into ocean.
  • Contour exterior edge of sea ice grid cells to create the shapefile.

The closing operation step is illustrated in Figure 7.

Processing Steps Visualized
Figure 7. Illustration of Closing Operation
The medium green on the left side is land; the dark green surrounding it is coastline. The area of the lightest green is the sea ice area prior to the closing operation. The light green within the lightest green is sea ice added during the closing operation. The single stand-alone white cell was marked as sea ice in the input IMS product, but is eliminated when sea ice polygons smaller than 16 cells, or roughly 256 sq km, are discarded. The grey line around the light and lightest green is the smoothed ice edge.

5. Related Collections and Resources

Related NSIDC Data Collections

The following related data collections are available from NSIDC:

Other Related Data Collections

The following related data collections are available from other data centers and universities:

6. References and Related Publications

Arctic Climatology Project. 2000. Environmental Working Group joint U.S.-Russian Sea Ice Atlas. Edited by F. Tanis and V. Smolyanitsky. Ann Arbor, MI: Environmental Research Institute of Michigan in association with the National Snow and Ice Data Center. CD-ROM.

Fetterer, F., compiler. 2006. A Selection of Documentation Related to National Ice Center Sea Ice Charts in Digital Format. NSIDC Special Report 13. Boulder, CO, USA: National Snow and Ice Data Center.

Fetterer, F., K. Knowles, W. Meier, and M. Savoie. 2002, updated 2009. Sea Ice Index. Boulder, Colorado USA: 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.

McKenna, P., and W. N. Meier. 2002. SSM/I Sea Ice Algorithm Inter-comparison: Operational Case Studies from the National Ice Center. IGARSS Proceedings, INT_A32_04, Toronto, 24-28 June 2002.

Meier, W. N., M. L. van Woert, and C. Bertoia. 2001. Evaluation of Operational SSM/I Ice Concentration Algorithms. Annals of Glaciology 33: 102-108.

Meier, W. 2005. Comparison of Passive Microwave Ice Concentration Algorithm Retrievals With AVHRR Imagery in Arctic Peripheral Seas. IEEE Transactions on Geoscience and Remote Sensing 40(6): 1324-1334.

Meier, W. N., F. Fetterer, C. Fowler, P. Clemente-Colón, T. Street. 2006. Operational Sea Ice Charts: An Integrated Data Product Suitable for Observing Long-term Changes in Arctic Sea Ice? Poster presented at the AGU Fall Meeting, 2006 and available on the NSIDC Posters and Presentations page.

Meier, W. N., T. Maksym, and M. L. Van Woert. 2002. Evaluation of Arctic Operational Passive Microwave Products: A Case Study in the Barents Sea During October 2001. Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice, Dunedin, New Zealand, 2nd-6th December 2002, International Association of Hydraulic Engineering and Research.

National Ice Center. 2006. National Ice Center Arctic Sea Ice Charts and Climatologies in Gridded Format. Edited and compiled by F. Fetterer and C. Fowler. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Partington, K., T. Flynn, D. Lamb, C. Bertoia, and K. Dedrick. 2003. Late Twentieth Century Northern Hemisphere Sea-ice Record from U.S. National Ice Center Ice Charts. Journal of Geophysical Research 108(C11), doi:10.1029/2002JC001623.

Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan. 2003. Global Analysis of Sea Surface Temperature, Sea Ice, and Night Marine Air Temperature Since the Late Nineteenth Century. Journal of Geophysical Research. 108 (D14), doi:10.1029/2002JD002670.

Willis Z., M. Foster, C. Bertoia, and K. Dedrick. 2000. National Ice Center/Naval Ice Center Support to Submarine Operations. Undersea Warfare 8: 18-21.

7. Contacts and Acknowledgements

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

Acknowledgements

Following visits in 2009 to NSIDC and NIC by Rear Admiral David Titley, at the time Commander Naval Meteorology and Oceanography Command (CNMOC), the concept for a collaborative MASIE product was developed through further discussions between NIC's Chief Scientist Pablo Clemente-Colón and NSIDC's NOAA Liaison Florence Fetterer. Product development objectives included wider dissemination of information from NIC products and more accurate daily sea ice edge position views for NSIDC's user base. Commander Denise M. Kruse, NIC's Director at the time, endorsed the collaboration that made MAISE possible. Sean Helfrich of NIC leads work on the IMS and helped shaped MASIE development.

The MASIE product team at NSIDC included Matt Savoie for algorithm and code development, Stephen Truex and Ann Windnagel for Web site development, Lisa Ballagh for assistance with shapefiles, and Walter Meier as science advisor.

Funding for MASIE development was provided by NIC and the US Naval Oceanographic Office (NAVO). We are currently seeking funding for its ongoing maintenance and further development. Distribution of the data set from NSIDC is supported by the NOAA@NSIDC Team with funding from NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) and the National Geophysical Data Center (NGDC).

8. Document Information

Acronyms

Table 6 lists the acronyms used in this document.

Table 6. Acronyms
Acronym Description
ALOS Advanced Land Observing Satellite
AMSR-E Advanced Microwave Scanning Radiometer - Earth Observing System
AMSU Advanced Microwave Sounding Unit
ASAR Advanced Synthetic Aperture Radar
ASCII American Standard Code for Information Interchange
AVHRR-VIS Advanced Very High Resolution Radiometer - Visible Band
CIS Canadian Ice Service
DMSP Defense Meteorological Satellite Program
FTP File Transfer Protocol
GeoTIFF Georeferenced Tagged Image File Format
GOES Geostationary Operational Environmental Satellite
IDL Interactive Data Language
IMS Interactive Multisensor Snow and Ice Mapping System
MASIE-NH Multisensor Analyzed Sea Ice Extent - Northern Hemisphere
MMAB Marine Modeling and Analysis Branch
MODIS Moderate Resolution Imaging Spectroradiometer
MTSAT Multi-functional Transport Satellite
NAVO Naval Oceanographic Office
NESDIS National Environmental Satellite, Data, and Information Service
NetCDF Network Common Data Format
NGDC National Geophysical Data Center
NIC National Ice Center
NOAA National Oceanic and Atmospheric Administration
NSIDC National Snow and Ice Data Center
NWP Numerical Weather Prediction
PALSAR Phased Array type L-band Synthetic Aperture Radar
PNG Portable Network Graphics
RGB Red, Green, Blue
SAR Synthetic Aperture Radar
SEVIRI Spinning Enhanced Visible and Infrared Imager
SSM/I Special Sensor Microwave Imager
URL Uniform Resource Locator

Document Authors

Florence Fetterer and Ann Windnagel wrote this documentation in November 2010 based primarily on information from Matt Savoie (NSIDC) and Sean Helfrich (NOAA NIC), as well as on documentation for related data sets.

Document Creation Date

November 2010

Document Revision Date

June 2014: A. Windnagel updated the document to reflect the release of all of the archive data to FTP and the document was put into the new NSIDC gd template.

Document URL

http://nsidc.org/data/docs/noaa/g02186_masie/index.html