As a condition of using this data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.
Anderson, M., A. Bliss, and S. Drobot. 2001, updated 2012. Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I-SSMIS Brightness Temperatures, Version 3. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.
Nimbus-7, DMSP-F8, -F11, -F13, -F17
SMMR, SSM/I, SSMIS
1978 – 2012
Snow melt onset
Data: Flat binary, 1-byte integers
This data set includes yearly snow melt onset dates over Arctic sea ice derived from brightness temperatures from the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I), and the Special Sensor Microwave Imager/Sounder (SSMIS). The introduction of liquid water to snow results in a sharp increase in the emissivity and hence brightness temperature of the snowpack. Snow melt onset is defined as the point in time when microwave brightness temperatures increase sharply due to the presence of liquid water in the snowpack. Data span the years 1979 through 2012 and are in a polar stereographic grid at 25 km resolution. Flat binary, 1-byte integer files and GIF images are accessible via FTP. Several value-added products are also available in flat binary, 4-byte floating point files.
Value-added data sets include the following for each pixel: mean melt onset date, latest (maximum) melt onset date, earliest (minimum) melt onset date, range of melt onset dates (the difference between maximum and minimum -- an index of variability), and standard deviation of melt onset date (another index of variability). Graphical representations of value-added data are also available.
Accurate dates of snow melt onset over sea ice contribute to improved simulations of climate during the Arctic snow melt period. Records of the spatial and temporal variability in snow melt can serve as climate proxies in Arctic sea ice zones (Drobot and Anderson 2001b).
Snow melt onset affects the Arctic energy balance as surface albedo decreases and energy absorption increases in response to the appearance of liquid water (Drobot and Anderson 2001b). Initial locations of sea ice melt vary both spatially and temporally. Melt signatures appear first in lower latitudes and advance northward with time. Along the Asian Arctic coast, snow melt starts in the far eastern (Chukchi Sea) and western (Barents and Kara Seas) regions. Over several weeks, the melt progresses zonally toward the Laptev Sea (Anderson 1987).
Data are in flat binary format, in 304 by 448 pixel grids. A GIF-formatted graphical representation of each data file is also available. Data fields are 1-byte integers with values ranging from 0 to 245
Statistics provided in these files are calculated over the 1979-2012 time period for each pixel. Values in the ancillary files below are calculated only for pixel locations where a melt onset date was calculated for all 34 years of the data record. The fields of the binary (.bin) data files are 4-byte floating point numbers and the browse images (.gif) show a graphical representation of the binary data files. Table 1 describes each file.
|Parameter (unit)||Data File Name
(Browse Image Name)
|Mean melt onset date (1979-2012)|
|Median melt onset date (1979-2012)|
|Latest (maximum) melt onset date observed over the climatology|
|Earliest (minimum) melt onset date observed over the climatology|
|Latest minus earliest melt onset date|
|Standard Deviation (days)||melt_stdev_1979-2012_v3_n.bin
|Standard deviation in melt onset|
Each melt onset data file represents one year of data. The time series is comprised of 34 flat binary files and 34 GIF images. The files range in size from 26 KB to 32 KB for a total data set volume of approximately 2.2 MB.
The file naming convention for the data files and the browse image files is the following and as described in Table 2.
|melt||Indicates the file contains snow melt onset|
|n||Hemisphere (n: Northern)|
|.ext||File extension: .bin = binary, .gif = GIF browse image|
Ancillary, value-added files are named according to the following convention and as described in Table 3.
|melt||Snow melt onset|
|parameter||Valid parameter values: earliest, latest, mean, median, range, and stdev|
|1979-yyyy||Temporal coverage: 1979 through 4-digit year|
|n||Hemisphere (n: Northern)|
|.ext||File extension: .bin = binary, .gif = GIF browse image|
Data cover the Northern Hemisphere, except for circular sectors centered over the pole. Data from the SMMR period (1978-87) have a polar gap 611 km in radius, located poleward of 84.5 degrees North latitude. Data from the SSM/I period (1987 through 2007) and the SSMIS period (2008 through 2012) have a polar gap 311 km in radius, located poleward of 87.2 degrees North latitude. See the Polar Stereographic Projection and Grid spatial coverage map for details. Spatial resolution is 25 km for all sensors.
Data are in a polar stereographic projection with a plane of tangency at 70 degrees north latitude. For a complete description, see the Polar Stereographic Projection and Grid web page.
Grids are the same as those of the DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures; however, only the Northern Hemisphere grid is used in this data set. The grid cell resolution is 25 km true at 70 degrees north latitude. Grid orientation is such that the first data value for the Northern Hemisphere corresponds to 30.98 degrees latitude and 168.35 degrees longitude. The origin of each x, y grid is the pole.
The grids' approximate outer boundaries are defined by corner points in Table 4. Apply values to the polar grids reading clockwise from upper left. Interim rows define boundary midpoints.
|X (km)||Y (km)||Latitude (deg)||Longitude (deg)||Position in Pixel|
Snow melt onset data range from 1978 through 2012, as shown in Table 5. Data are derived from brightness temperatures acquired from multiple platforms. Snow melt onset data are derived once per year for each grid cell.
|Data Type/Sensor||Start Date||End Date|
|Nimbus-7 SMMR||25 October 1978||20 August 1987|
|DMSP F8 SSM/I||9 July 1987||18 December 1991|
|DMSP F11 SSM/I||3 December 1991||31 December 1996|
|DMSP F13 SSM/I||5 May 1995||31 December 2007|
|DMSP F17 SSMIS||01 January 2008||31 December 2012|
Each pixel represents the day of the year that melt first began. The introduction of liquid water to snow results in a sharp increase in the emissivity and hence brightness temperature of the snowpack. Snow melt onset is defined as the point in time when microwave brightness temperatures increase sharply due to the presence of liquid water in the snowpack.
See the Derivation Techniques and Algorithms section of this document for information regarding derivation of snow melt onset values.
|0||Indicates that no melt date was calculated and also includes locations where there is open ocean, land, the pole hole, or locations within the sea ice pack where no melt onset occurred for that year.|
|61-245||Day of melt onset (units: DOY)|
Figure 1. Browse Image for 2010 v3 Data
Brightness temperature data may introduce errors related to pixel averaging, sensor errors, and weather effects. See the following temperature documentation for more information regarding errors in the source data:
Given the short data record and the known errors, users are advised against selecting individual pixels without examining surrounding data points. Also, trend analysis at any given pixel should include a study of nearby pixels to confirm that results are locally consistent.
Data are available via FTP.
Included are tools to display, extract, and export the data; determine geolocation (geocoordinates) of data; and masking tools that limit the influence of non-snow melt data values. Table 7 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.
|Tool Type||Tool File Name|
|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|
Microwave emmissivity of snow increases dramatically as the snow melts and liquid water appears. With the presence of liquid water in the snow pack, surface scattering dominates over volume scattering, resulting in a sharp increase in the brightness temperatures signature. Lower microwave frequencies (19.3 GHz for the SSM/I instrument and 18.0 GHz for the SMMR instrument) are more responsive to melt onset in the firn than are higher frequencies (37.0 GHz for both SSM/I and SMMR), due primarily to the change in emission depth associated with melt. This causes the difference between 19.3 (or 18.0) GHz and 37.0 GHz brightness temperatures to change from positive to near-zero or negative (Kunzi et al. 1982). Furthermore, the increase in brightness temperature associated with melt is frequency- and polarization-dependent. Horizontal channels reflect a stronger dependence on snow conditions during melt (Anderson 1997) due to the change in dielectric properties at the air-snow interface when snow is wet (Abdalati and Steffen 1995).
Brightness temperature data were acquired by the SMMR, SSM/I, and SSMIS instruments. Table 8 provides a specific list of the data sets used and the channels from the instruments.
|Data Set||Description||Channels/Variables Used|
|Nimbus-7 SMMR Polar Gridded Radiances and Sea Ice Concentrations (Gloersen 1994)||Brightness temperatures used to calculate snow melt onset dates (1979-1987)||18GHz and 37GHz|
|DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures, Version 4 (Maslanik and Stroeve 2012)||Brightness temperatures used to calculate snow melt onset dates (1988-2012)||19GHz and 37GHz|
|NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 2 (Meier et al. 2013)||Sea ice concentrations used to create mask of annual sea ice maximum extent. Melt dates calculated for locations in sea ice mask.||goddard_merged_seaice_conc|
Drobot and Anderson calculate snow melt onset dates using daily-averaged brightness temperature data from SMMR, SSM/I (F8, F11, and F13), and SSMIS (F17) satellite radiometers. See Table 8 for a complete list of input data. The investigators record changes in 19H GHz and 37H GHz brightness temperatures for each data point on each day within a 20-day window using the Advanced Horizontal Range Algorithm (AHRA) (Anderson 1997).
Table 9 outlines the processing and algorithm history for this product.
|Version||Date||Description of Changes from Previous Version|
|V01||Dec 2001||Original version of data.|
|Sensor Correction||Source||Overlap Area||Channels||Coefficients||Correction Equation|
|SMMR to F8||Jezek et al. (1991)||---||18H||Slope||0.940||F8=(SMMR-2.62)/0.940|
|F11 to F8||Abdalati et al. (1995)||Greenland||19H||Slope||1.013||F8=1.013*F11-1.890|
|F13 to F11||Stroeve et al. (1998)||NH Sea Ice||19H||Slope||0.986||F11=(F13-2.197)/0.986|
|F17 to F13||Walt Meier (Personal Communication Oct. 2011)||Arctic Mar-Sept 2007||19H||Slope||0.979||F13=(F17-1.646)/0.979|
Abdalati, W. and K. Steffen. 1997. Snowmelt on the Greenland Ice Sheet as Derived from Passive Microwave Satellite Data. Journal of Climate 10(2):165-175.
Abdalati, W., K. Steffen, C. Otto and K. Jezek. 1995. Comparison of brightness temperatures from SSM/I Instruments on the DMSP F8 and F11 Satellites for Antarctica and the Greenland Ice Sheet. International Journal of Remote Sensing 16:1223-1229.
Anderson, M. 1987. The Onset of Spring Melt in First-year Ice Regions of the Arctic as Determined from Scanning Multichannel Microwave Radiometer Data for 1979 and 1980. Journal of Geophysical Research 92(C12):13,153-13,163.
Cavalieri, D., C. Parkinson, P. Gloersen, J. Comiso, and H. J. Zwally. 1999. Deriving Long-term Time Series of Sea Ice Cover from Satellite Passive-microwave Multisensor Data Sets. Journal of Geophysical Research 104(C7):15,803-15,814.
Drobot, S. D. and M. R. Anderson. 2001a. An improved method for determining snowmelt onset dates over Arctic sea ice using scanning multichannel microwave radiometer and Special Sensor Microwave/Imager data. Journal of Geophysical Research 106: 24,033-24,049.
Dubach L. and C. Ng. 1988. NSSDC's Compendium of Meteorological Space Programs, Satellites, and Experiments.
Gloersen, P. 1994. Nimbus-7 SMMR Polar Radiances and Arctic and Antarctic Sea Ice Concentrations. [1979-1987]. Boulder, Colorado USA: National Snow and Ice Data Center.
Gloersen, P. and F. Barath. 1977. A Scanning Multichannel Microwave Radiometer for Nimbus-G and SeaSat-A. IEEE Journal of Oceanic Engineering 2:172-178.
Gloersen, P., W. Campbell, D. Cavalieri, J. Comiso, C. Parkinson, and H. J. Zwally. 1992. Arctic and Antarctic Sea Ice, 1978-1987: Satellite Passive-microwave Observations and Analysis. National Aeronautics and Space Administration Scientific and Technical Information Program. Washington, D.C.
Gloersen, P. and L. Hardis. 1978. The Scanning Multichannel Microwave Radiometer (SMMR) Experiment, in The Nimbus 7 Users' Guide. C. R. Madrid, editor. National Aeronautics and Space Administration. Greenbelt, MD: Goddard Space Flight Center.
Jezek, K., C. Merry, D. Cavalieri, S., Grace, J. Bedner, D. Wilson, and D. Lampkin. 1991. Comparison Between SMMR and SSM/I Passive Microwave Data Collected over the Antarctic Ice Sheet. Byrd Polar Research Center Technical Report No. 91-03, The Ohio State University, Columbus, Ohio, 62 pp.
Kramer, H. 1994. Observation of the Earth and Its Environment - Survey of Missions and Sensors, 2nd Edition. Heidelberg: Springer-Verlag.
Kunzi, K., S. Patil, and H. Rott. 1982. Snow-cover Parameters Derived from Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data. IEEE Transactions on Geosciences and Remote Sensing GE-20:57-66.
Livingstone, C., K. Singh, and L. Gray. 1987. Seasonal and Regional Variations of Active/Passive Microwave Signatures of Sea Ice. IEEE Transactions on Geosciences and Remote Sensing GE-25:159-172.
Maslanik, J. and J. Stroeve. 2004, updated 2012. DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures. Version 4. [1988-2011]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.
Meier, W., F. Fetterer, M. Savoie, S. Mallory, R. Duerr, and J. Stroeve. 2013. NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration. Version 2. [2008-2011]. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N55M63M1.
National Aeronautics and Space Administration. 1978. The Nimbus 7 Users' Guide. C.R. Madrid, editor. Goddard Space Flight Center.
Swanson, P. and A. Riley. 1980. The Seasat Scanning Multichannel Microwave Radiometer (SMMR): Radiometric Calibration Algorithm Development and Performance. IEEE Journal of Oceanic Engineering OE-5:116-124.
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University of Nebraska
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National Center for Atmospheric Research (NCAR)
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form: Contact NSIDC User Services
Table 11 lists acronyms used in this document.
|AHRA||Advanced Horizontal Range Algorithm|
|DOY||Day of Year|
|DSMP||Defense Meteorological Satellite Program|
|ESMR||Electronically Scanning Microwave Radiometer|
|NASA||National Aeronautics and Space Administration|
|NOAA||National Oceanic and Atmospheric Administration|
|NSIDC||National Snow and Ice Data Center|
|SMMR||Scanning Multichannel Microwave Radiometer|
|SSM/I||Special Sensor for Microwave Imaging|
|SSMIS||Special Sensor Microwave Imager/Sounder|