Nimbus-5 ESMR Polar Gridded Sea Ice Concentrations

Summary

Daily and monthly averaged sea ice concentrations from the Nimbus-5 Electrically Scanning Microwave Radiometer (ESMR) are available for the Arctic and Antarctic from 12 December 1972 through 31 December 1976 at 25 km gridded resolution. Raw data were reprocessed to include ocean masks that reduce weather effects and coastal contamination, and to include a 15 percent ice threshold. Reprocessed data are in Hierarchical Data Format (HDF) and are available via FTP.

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Parkinson, C. L., J. C. Comiso, and H. J. Zwally. 1999, updated 2004. Nimbus-5 ESMR Polar Gridded Sea Ice Concentrations. Edited by W. Meier and J. Stroeve. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Overview Table
 

Category	Description
Data format	Reprocessed data are in Hierarchical Data Format (HDF).
Spatial coverage and resolution	Coverage includes the Arctic and Antarctic at 25 km gridded resolution.
Temporal coverage and resolution	Daily and monthly averages span from December 1972 through December 1976.
Tools for accessing data	Because ESMR sea ice data are in the same polar stereographic grid as SSM/I data, the SSM/I geolocation tools and masks can be used to read and display the ESMR data. Tools are available via FTP.
Data range	Pixel values for sea ice concentration range from 0 to 100 (0-100%). Other pixel values are as follows: Missing = 157 Land = 168 Coast = 178 Ocean = 125 (from the ocean mask) Lakes = 120 Low ice concentration = 200-215 (corresponding to concentration values of 0-15%)
Grid type and size	North: 304 columns, 448 rows South: 316 columns, 332 rows
File naming convention	Example daily file: ESMR-1974329.tne.15
File size	Arctic: 138 KB Antarctic: 107 KB
Parameter(s)	Sea ice concentration (%)
Procedures for obtaining data	Data are available via FTP.

Table of Contents

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

1. Contacts and Acknowledgments

Investigator(s) Name and Title

Claire L. Parkinson, Joey C. Comiso, and H. Jay Zwally
NASA Goddard Space Flight Center
Greenbelt, MD, USA 20771

Technical Contact

2. Detailed Data Description

The ESMR data set provides the earliest all-weather, all-season imagery of polar sea ice. Some satellite data of sea ice in the visible and infrared wavelengths were available in the late 1960s and early 1970s, but since the polar regions are either dark or cloud-covered for much of the year, the generation of consistent, long-term data records from visible and infrared sensing was not practical.

The passive microwave data collected by the Nimbus-5 ESMR introduced a major advance in the usefulness of satellite sea ice imaging. The value of the ESMR data for sea ice studies derives from the large contrast in microwave emissivities between sea ice and open water. At the 19 GHz frequency of the ESMR, open water has an emissivity of approximately 0.44, whereas various sea ice types have emissivities ranging from approximately 0.8 to 0.97. The resulting contrast in microwave brightness temperatures (T_bs) allows conversions to approximate sea ice concentrations (percentages of ocean area covered by sea ice) and hence identification of sea ice distributions throughout the region of observation, as well as temporal variations of these distributions throughout the time period of observation.

Format

Data are in HDF format. The size of the arrays are 304 columns x 448 rows for the Arctic and 316 columns x 332 rows for the Antarctic. Pixel values for ice concentration range from 0 to 100 (0-100% ice concentration). Flag values for land and missing pixels are consistent with other NSIDC SSM/I ice concentration products; however, additional flags are included for ESMR:

Missing = 157
Land = 168
Coast = 178
Ocean = 125 (from the ocean mask)
Lakes = 120
Low ice concentration = 200-215 (corresponding to concentration values of 0-15%)

Usage

Users wishing to create custom sea ice concentrations should refer to Calculating Sea Ice Concentration from ESMR for information on calculating ice concentration from brightness temperatures. Please note, however, that it may be difficult to compare these sea ice concentrations with the SMMR and SSM/I sea ice concentrations, because no overlap period exists between the SMMR and ESMR instruments. Additionally, because ESMR has just one channel of data, the sea ice concentrations derived from ESMR may not be as accurate as those derived from the SMMR and SSM/I instruments.

File and Directory Structure

Data are available via FTP. The north and south directories each contain the following subdirectories:

bad:	Data with bad scans and other errors that were not used in the monthly averages or climatological means. See Processing Steps in this document for a list of dates for which data were removed.
daily00:	Daily fields with ice concentration threshold of 0%
daily15:	Daily fields with ice concentration threshold of 15%
monthly:	Monthly averages and files that list the number of samples at each pixel location
means:	Climatological means for each month, 1973-1976

File Naming Convention

Daily files: ESMR-yyyyddd.the
Monthly files: ESMR-yyyymm.the.15
Monthly climatologies: ESMR-1973-1976-mm.the.15
Files containing the number of samples at each pixel in the monthly mean field: ESMR-yyyymm.count.the.15

Where:

yyyy is the 4-digit year
mm is the 2-digit month
ddd is the 3-digit day of year
t is the total ice concentration
h is the hemisphere (n: Northern; s: Southern)
e = ESMR single-channel algorithm
15 = 15% ice concentration threshold

File Size

Arctic files are 138 KB, and Antarctic files are 107 KB.

Spatial Coverage

Instrument coverage is global except for circular sectors centered over the poles, 280 km in radius, poleward of 87°N and 87°S, which are never measured due to orbit inclination. Data set coverage includes the polar regions defined by the spatial coverage map below.

Spatial Coverage Map

ESMR data are gridded to a polar stereographic projection with the following coverage:

North

South

Spatial Resolution

The ESMR footprint size varies from approximately 32 km x 32 km to about 28 km x 28 km at 50° latitude. Gridded resolution is 25 km.

Projection

ESMR sea ice grids are in a polar stereographic projection, which specifies a projection plane (i.e., the grid) tangent to the earth at 70°. The planar grid is designed so that the grid cells at 70° latitude are 25 km x 25 km. For more information on this topic please refer to Pearson (1990) and Snyder (1987).

The polar stereographic projection often assumes that the plane (grid) is tangent to the Earth at the pole. Thus, there is a one-to-one mapping between the Earth's surface and grid (with no distortion) at the pole. Distortion in the grid increases as the latitude decreases because more of the Earth's surface falls into any given grid cell, which can be quite significant at the edge of the northern polar grid where distortion reaches 31 percent. The southern polar grid has a maximum distortion of 22 percent. To minimize the distortion, the projection is true at 70° rather than at the poles. This increases the distortion at the poles by three percent and decreases the distortion at the grid boundaries by the same amount. The latitude of 70° was selected so that little or no distortion would occur in the marginal ice zone. Another result of this assumption is that fewer grid cells are required as the Earth's surface is more accurately represented.

The polar stereographic formulae for converting between latitude/longitude and X-Y grid coordinates are taken from Snyder (1982). This projection assumes a Hughes ellipsoid with a radius of 3443.992 nautical mi or 6378.273 km and an eccentricity (e) of 0.081816153 (or e**2 = 0.006693883).

Grid Description

North: 304 columns, 448 rows
South: 316 columns, 332 rows

Polar Stereographic Grid Coordinates

The origin of each x, y grid is the pole. The grids' approximate outer boundaries are defined in the following table. Corner points are listed; apply values to the polar grids reading clockwise from upper left. Interim rows define boundary midpoints.

North Polar:

X (km)	Y (km)	Latitude (deg)	Longitude (deg)
-3850	5850	30.98	168.35	corner
0	5850	39.43	135.00	midpoint
3750	5850	31.37	102.34	corner
3750	0	56.35	45.00	midpoint
3750	-5350	34.35	350.03	corner
0	-5350	43.28	315.00	midpoint
-3850	-5350	33.92	279.26	corner
-3850	0	55.50	225.00	midpoint

South Polar:

X (km)	Y (km)	Latitude (deg)	Longitude (deg)
-3950	4350	-39.23	317.76	corner
0	4350	-51.32	0.00	midpoint
3950	4350	-39.23	42.24	corner
3950	0	-54.66	90.00	midpoint
3950	-3950	-41.45	135.00	corner
0	-3950	-54.66	180.00	midpoint
-3950	-3950	-41.45	225.00	corner
-3950	0	-54.66	270.00	midpoint

Temporal Coverage

Data range from 12 December 1972 to 31 December 1976. The following dates have bad data:

Northern Hemisphere:
1972: Day 346
1973: Days 23, 29, 38, 44, 150
1974: Days 31, 66, 310, 315, 359, 362
1975: Days 28, 288, 300-302
1976: Days 40, 98
1977: Day 131

Southern Hemisphere:
1973: Days 63-147, 217-241, 266, 280, 284, 287, 297, 334, 335
1974: Days 13-15, 84-95, 129, 136, 184, 212-220, 252, 295-298, 305
1975: Days 62, 76, 91-97, 106, 108-115, 117, 150
1976: Days 217, 272, 314, 334, 346
1977: Days 72, 88-100

Several days may also have missing data.

Temporal Resolution

Daily and monthly averaged data are provided.

Parameter or Variable

Parameter Description

Sea ice concentration is the fraction of a given area covered by sea ice irrespective of ice type, or the ratio describing the areal density of ice in a given area.

Unit of Measurement

Ice concentration values are given as percentages.

Parameter Source

These data are derived from Nimbus-5 ESMR brightness temperatures, originally processed by the Goddard Space Flight Center (GSFC). NSIDC obtained the data from GSFC and reproccesed them to remove coastal and weather contamination, and to include a 15 percent ice threshold.

Parameter Range

Pixel values range from 0 to 255.

Error Sources

The Processing Steps section of this document discusses removal of some errors from the original data.

Quality Assessment

The Processing Steps section of this document discusses quality control steps taken.

3. Data Access and Tools

Data Access

Data are available via FTP.

Software and Tools

Because ESMR brightness temperatures are in the same polar stereographic grid as SSM/I brightness temperatures, some SSM/I tools can be used to read and display the ESMR data. Included are IDL display programs to extract and display the data, geolocation (geocoordinate) tools, and pixel-area grids. Table 1 lists the tools that can be used with this data set. For a comprehensive list of all polar stereographic tools and for more information, see the Polar Stereographic Data Tools Web page.

Table 1. Tools for this Data Set

Tool Type	Tool File Name(s)
Data Extraction	extract_ice.pro
Geocoordinate	locate.for
	mapll.for and mapxy.for
	psn25lats_v3.dat and pss25lats_v3.dat
	psn25lons_v3.dat and pss25lons_v3.dat
Pixel-Area	psn25area_v3.dat and pss25area_v3.dat

Related Data Collections

Sea Ice Data

4. Data Acquisition and Processing

Theory of Measurements

See Calculating Sea Ice Concentration from ESMR for calculations of sea ice concentration from brightness temperatures.

Source/Platform

The Nimbus-5 flew in a circular sun-synchronous orbit at 1112 km (600 nautical miles), had a local noon (ascending) and midnight (descending) equator crossing, and an 81° retrograde inclination. Successive orbits crossed the equator at 27° longitude separation. The orbital period was about 107 minutes. Nimbus-5 used an attitude control system which stabilized the spacecraft with respect to the earth and orbital plane, such that the yaw axis pointed normal to the earth and the roll axis aligned with the spacecraft velocity vector, and which also maintained the solar paddles' orientation to the sun. The system permitted fine control of ± 1° in pitch and ± 0.5° in roll and yaw.

Source or Platform Mission Objectives

The Nimbus-5 mission had two major goals:

1. To help meet the objectives of NASA's expanding meteorological program (in 1972, the Nimbus-5 played a central role in this program).
2. To initiate satellite studies in the applications areas, particularly the development of advanced sensors for the exploration of natural resources and geophysical phenomena.

The meteorological program called for the application of space technology to increase understanding of the atmosphere and efficiency in making global meteorological observations. The Nimbus-5 provided a versatile orbital platform for a variety of experiments designed to

• Observe atmospheric conditions and processes that influence weather prediction.
• Develop techniques for measuring the global parameters required for modeling atmospheric circulation.

Sensor or Instrument Description

ESMR consisted of four major components:

• A phased array microwave antenna consisting of 103 waveguide elements each having its associated electrical phase shifter. The aperture area is 83.3 cm X 85.5 cm. The polarization is linear, parallel to the spacecraft velocity vector.
• A beam steering computer that determines the coil current for each of the phase shifters for each beam position.
• A microwave receiver with a center frequency of 19.35 GHz and an IF bandpass of from 5 to 125 MHz; thus it is sensitive to radiation from 19.225 to 19.475 GHz, except for a 10 MHz gap in the center of the band.
• Timing, control and power circuits.

Principles of Operation

Unlike conical scan instruments such as the SMMR and SSM/I, the ESMR was a cross-scan instrument, with a resolution of approximately 30 km, which measured primarily the intensity of electromagnetic radiation thermally emitted from the Earth's surface at a wavelength of 1.55 cm (19.35 GHz). The instrument recorded radiation from 78 scan positions, and all observations were first converted to equivalent nadir observations.

Sensor/Instrument Measurement Geometry

The Nimbus-5 ESMR recorded radiation from 78 scan positions varying ± 50° from the satellite track every four seconds (Wilheit 1972). The beam width is 1.4° x 1.4° near nadir and degrades to 2.2° crosstrack by 1.4° downtrack at the 50° extremes. For a nominal orbit of 1100 km altitude, the resolution is 25 km x 25 km near nadir, degrading to 160 km crosstrack by 45 km downtrack at the ends of the scan. Full coverage of the entire polar area could be obtained from a sequence of six satellite orbits, or one-half day of good data, if all 78 beam positions were used; however, because of the large disparity in the radiometer field of view from the outer beam position to the middle beam position (70 km x 140 km compared with 25 km x 25 km), only the middle 52 beam positions were used for a swath-angle coverage of ± 30.5° and a minimum resolution of 29 km by 42 km. This swath angle corresponds to a spatial coverage of about 1280 km on the Earth's surface.

Calibration

The radiometer was originally calibrated using hot and cold reference sources. A sky horn measuring the 3 K cosmic background provided the cold-load temperature reference (T_C). The hot-load temperature was provided by reference to a floating ambient termination in the spacecraft. Calibration parameters were gathered from eight scans of data. Calibration temperatures (T_C and T_H) were calculated from multiplex data, and values of four ambient and four cold calibration voltages averaged through the set of eight scans. For each beam position the brightness temperature (T_in) corresponding to voltage (V) was then calculated by:

T_IN = T_H + [(T_C - T_H)/(V_C - V_H)] (V - V_H)

See the Additional Calibration in the ESMR brightness temperature documentation for information about further adjustment in the source brightness temperature.

Data Acquisition Methods

Telemetry data from the Nimbus-5 satellite were transmitted to two spaceflight tracking and data network stations located near Fairbanks, Alaska, and Rosman, North Carolina. The data were relayed from these stations to the NASA Goddard Space Flight Center (GSFC). At GSFC, the telemetry data were unpacked, decommutated, supplemented with flags and ends of files, and stored on magnetic tapes called experimental tapes (ETs). For data processing convenience, the data from the ESMR instrument were combined from several ETs to form stacked experimental tapes (SETs). The 10-bit telemetry data on the ETs were converted to 32-bit format on the SETs for use on the GSFC computers. The SETs were used with ephemeris tapes to generate Earth-located calibrated brightness temperature (CBT) tapes.

Data Source

These data are derived from Nimbus-5 ESMR brightness temperatures, originally processed by the Goddard Space Flight Center (GSFC). NSIDC obtained the data from GSFC and reproccesed them to remove coastal and weather contamination, and to include a 15 percent ice threshold.

Derivation Techniques and Algorithms

See Calculating Sea Ice Concentration from ESMR.

Processing Steps

GSFC regridded the original ESMR data (see the north and south directories on the FTP site) to a polar stereographic projection. Data gaps in the original data were filled in using a temporal average of the day before and day after, over the entire field. These data do not include ocean masks and weather filters, and days with bad scans still remain. In Autumn 2003, NSIDC reprocessed these data with the following steps:

1. An ocean mask was applied to remove most weather effects and coastal contamination. (See Ocean Masks from NSIDC for more information.) The masks are created from SMMR and SSM/I Bootstrap sea ice concentrations. Climatological masks for each month are produced from the length of the time series, November 1978 through September 2002. Any pixel that did not include ice during the time series is classified as ocean. An additional buffer of two pixels (~50 km) is added to the ice edge to make the mask more conservative; thus, the mask is unlikely to mask out legitimate ice retrievals in the ESMR fields. But weather effects in the remaining open water regions, between the ESMR ice edge and the ocean pixels in the mask, are not removed.
    
2. To remove some of the remaining weather effects, fields with a 15 percent threshold of ice concentration are included. Thus, any pixel with less than 15 percent ice is considered ice-free. A value of 200 is added to low ice concentrations (0-15%); thus, values of 200-215 indicate low ice concentration. Low ice concentrations can be recovered by simply subtracting 200 from values greater than or equal to 200.
   
   Using 15 percent to define the ice edge is consistent with previous studies showing that the 15 percent isopleth in passive microwave ice concentrations corresponds best with the true ice edge. However, some low ice concentrations may be eliminated with this method. In addition, some weather effects leading to false concentrations greater than 15 percent are not removed.
    
3. Remaining ice concentration errors from bad scans were removed. Errors ranged from a few scan lines to entire fields with bad data, the latter occurring primarily in the Southern Hemisphere. These fields are saved in the bad directory for each hemisphere, on the FTP site. Following is a list of dates for which bad data were removed:
   
   Northern Hemisphere:
   1972: Day 346
   1973: Days 23, 29, 38, 44, 150
   1974: Days 31, 66, 310, 315, 359, 362
   1975: Days 28, 288, 300-302
   1976: Days 40, 98
   1977: Day 131
   
   Southern Hemisphere:
   1973: Days 63-147, 217-241, 266, 280, 284, 287, 297, 334, 335
   1974: Days 13-15, 84-95, 129, 136, 184, 212-220, 252, 295-298, 305
   1975: Days 62, 76, 91-97, 106, 108-115, 117, 150
   1976: Days 217, 272, 314, 334, 346
   1977: Days 72, 88-100
    
4. Monthly means were created where sufficient data were available, i.e., a minimum of 10 samples for a given pixel over an entire month. A disadvantage of this approach, however, is that a small number of samples may not represent the true monthly mean ice conditions. To assist users in judging ice conditions, files containing the number of samples at each pixel in the monthly field are included in the monthly directory on the FTP site.
   
   Low ice concentration values in the daily fields were included in the monthly means. After computing the monthly means, ice concentration values below 15 percent were set to 0. The flag values are the same as those for daily grids (see Format above), except without an ocean mask; ocean pixels have a 0 percent concentration. Several months during the ESMR time series did not have sufficient data to create a monthly mean. The following table indicates which means were produced for each hemisphere (N and S):
    

   	1972	1973	1974	1975	1976	1977
   January		N,S	N,S	N,S	N,S	S
   February		N,S	N,S	N,S	N,S	S
   March			N,S	N,S	N,S	N,S
   April			N,S		N,S
   May			N,S	N,S	N,S
   June		N,S	N,S		N,S
   July		N,S	N,S		N,S
   August			N,S		N,S
   September		N,S	N,S	N,S	N,S
   October		N,S	N,S	N,S	N,S
   November		N,S	N,S	N,S	S
   December	N,S	N,S	N,S	N,S	S

   
   From January 1973 through December 1976, where ESMR provided coverage throughout the year, a climatology was produced for each month by averaging all available years of monthly means for that month. Up to four fields are averaged for each month, one for each year; however, some monthly mean fields were not created due to insufficient data. In these cases, the climatology may be based on more than four years. The flag values in the fields are the same as in the monthly means. These fields are also in the monthly subdirectories within the north and south directories.

5. References and Related Publications

Comiso, J. C., and H. J. Zwally. 1980. Corrections for Anomalous Time Dependent Shifts in Brightness Temperature from Nimbus-5 ESMR. NASA TM-82055. Greenbelt, MD.

Parkinson, C., J. Comiso, H. J. Zwally, D. Cavalieri, P. Gloersen, and W. Campbell. 1987. Arctic Sea Ice, 1973-1976: Satellite Passive-Microwave Observations. NASA SP-489.

Parkinson, C., J. Comiso, and H. J. Zwally. 1987. Satellite-Derived Ice Data Sets No. 2: Arctic Monthly Average Microwave Brightness Temperatures and Sea Ice Concentrations, 1973-1976. NASA Technical Memorandum 87825.

Pearson, F. 1990. Map Projections: Theory and Applications. CRC Press. Boca Raton, Florida. 372 pages.

Snyder, J. P. 1987. Map Projections - A Working Manual. U.S. Geological Survey Professional Paper 1395. U.S. Government Printing Office. Washington, D.C. 383 pages.

Snyder, J. P. 1982. Map Projections Used by the U.S. Geological Survey. U.S. Geological Survey Bulletin 1532.

Wilheit, T. 1972. The Electrically Scanning Microwave Radiometer (ESMR) Experiment. Nimbus-5 User's Guide. NASA/Goddard Space Flight Center. p. 59-105.

Zwally, H. J., J. Comiso, C. Parkinson, W. Campbell, F. Carsey, and P. Gloersen. 1983. Antarctic Sea Ice, 1973-1976: Satellite Passive-Microwave Observations. NASA SP-459.

Zwally, H. J., J. Comiso, and C. Parkinson. 1981. Satellite-Derived Ice Data Sets No. 1: Antarctic Monthly Average Microwave Brightness Temperatures and Sea Ice Concentrations 1973-1976. NASA Technical Memorandum 83812.

6. Document Information

Acronyms

The following acronyms are used in this document.

CBT	Calibrated Brightness Temperature Tape
CIRES	Cooperative Institute for Research in Environmental Sciences
ET	Experimental Tape
ESMR	Electrically Scanning Microwave Radiometer
FTP	File Transfer Protocol
GSFC	Goddard Space Flight Center
NASA	National Aeronautics and Space Administration
NSIDC	National Snow and Ice Data Center
SET	Stacked Experimental Tape
SSM/I	Special Sensor Microwave/Imager
SMMR	Scanning Multichannel Microwave Radiometer
Tb	Brightness Temperature

Document Creation Date

August 1999

Document Revision Date

May 2008
February 2004

Document URL

http://nsidc.org/data/docs/daac/nsidc0009_esmr_seaice.gd.html