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Data Set ID: 
NSIDC-0105

Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I-SSMIS Brightness Temperatures, Version 4

This data set includes yearly snow melt onset dates over Arctic sea ice derived from Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), and the Special Sensor Microwave Imager/Sounder (SSMIS) brightness temperature measurements. The data are gridded to the 25 km Northern Hemisphere Polar Stereographic projection and available from 1979 through 2017. One browse image is available for each year.

This data set also contains value-added statistics for each grid cell, including: 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 onset dates), and the standard deviation of melt onset dates. One browse image is also provided for each statistical field.

This is the most recent version of these data.

Version Summary: 

Changes to this version include:
Reprocessing the entire data set using an newer version of the source data used to produce sea ice extent masks within the snow melt onset algorithm. No other changes were made to the algorithm.
Extending the data record through 2017.
Adding quality flags to indicate where snow melt dates were not derived. The flags discriminate between sea ice where the snow melt onset date was not found and pixels included in the land mask, the pole hole, and areas of open water.
Updating data fields that summarize basic statistics over the length of the data record.
Distributing the data as a netCDF file.

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Parameter(s):
  • SEA ICE > SNOW MELT
Data Format(s):
  • PNG
  • NetCDF
Spatial Coverage:
N: 90, 
S: 30, 
E: 180, 
W: -180
Platform(s):DMSP, DMSP 5D-2/F11, DMSP 5D-2/F13, DMSP 5D-2/F8, DMSP 5D-3/F17, Nimbus-7
Spatial Resolution:
  • 25 km x 25 km
Sensor(s):SMMR, SSM/I, SSMIS
Temporal Coverage:
  • 1 January 1979 to 31 December 2017
Version(s):V4
Temporal Resolution1 yearMetadata XML:View Metadata Record
Data Contributor(s):Mark Anderson, Angela Bliss, Sheldon Drobot

Geographic Coverage

Download Via HTTPS: An Earthdata Login account is required to access these data.

Once you have logged in, you will be able to click and download files via a Web browser. There are also options for downloading via a command line or client. For more detailed instructions, please see Options Available for Bulk Downloading Data from HTTPS with Earthdata Login.

As a condition of using these 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. C. Bliss, and S. Drobot. 2019. Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I-SSMIS Brightness Temperatures, Version 4. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/A9YK15H5EBHK. [Date Accessed].
Created: 
20 May 2019
Last modified: 
18 June 2019

Data Description

Parameters

The main parameter for this data set is the Snow Melt Onset Date (SMOD) over Arctic sea ice, or the Day of Year (DOY) when microwave brightness temperatures increase sharply due to the presence of liquid water in the snowpack. This data set also includes statistical analyses of each grid cell's SMOD over the period between 1979 through 2017. Table 1 includes a description of valid SMOD values, while Table 2 contains a description of valid statistical fields.

Table 1. SMOD Values and Descriptions
Value Description
5 Pole hole; no melt date was calculated
10 Water (ocean, lakes); no melt date was calculated
15 Land; no melt date was calculated
61-245 Day of snow melt onset, recorded as the Day of Year (DOY)
255 Sea ice did not melt; no melt data was calculated
Table 2. Valid Parameter Range for Statistics Fields
Value
Description
-150
No data; open water or missing melt date
-100
Pole hole; no melt dates were calculated
-50
Land mask; no melt dates were calculated
-30 to 255
Valid data

File Information

Format

Data are provided in netCDF (.nc) file format. 

PNG (.png) browse images and Extensible Markup Language (.xml) files with associated metadata are also provided.

File Contents

NetCDF file contents are described in Table 3.

Table 3. Description of netCDF Variables
Variable Name Description Units
SMOD Annual snow melt onset date Day of Year (DOY); see Table 1 for more details
mean Mean snow melt onset date  DOY
median Median snow melt onset date  DOY
latest Latest (maximum) snow melt onset date  DOY
earliest Earliest (minimum) snow melt onset date  DOY
range Latest minus earliest snow melt onset date  Days
stdev Standard deviation of snow melt onset dates  Days
trend Decadal trend in snow melt onset based on a least squares linear regression Days per Decade
latitude Latitude Degrees North
longitude Longitude Degrees East
projection Description of projected coordinate system N/A
time Time Days since 1970-01-01
x Projected x coordinate Meters
y Projected y coordinate Meters

One browse image is provided for each year of data showing the annual SMOD; Figure 1 contains a sample image. One browse image is also provided for each statistical field, as exemplified by Figure 2. 

Figure 1. Sample browse image showing snow melt onset dates for 1979.
Figure 2. Sample statistical-field browse image showing the earliest snow melt onset date recorded between 1979 and 2017.

Directory Structure

Data are available for download via HTTPS; the link is accessible through the "Download Data" tab on the data set landing page. Within the file directory, browse images are available in the subfolder labeled "Browse."

Naming Convention

The netCDF file name is:
SMOD_1979-2017_v04r00.nc

Browse images showing the annual snow melt onset date are named according to the following convention and as described in Table 4:
melt_<year>_v04r00_n.png

Table 4. Snow Melt Onset Date Browse Images
Variable Description
melt Indicates the file is part of the Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I-SSMIS Brightness Temperatures data set
<year> Year of data represented by the image
v04 Data set version number (e.g. 04)
r00 Data set revision number (e.g. 00)
n Hemisphere (n = Northern)

Browse images for statistical fields are named according to the following convention and as described in Table 5:
melt_<field>_1979-2017_v04r00.png

Table 5. Statistical Browse Images
Variable Description
melt Indicates the file is part of the Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I-SSMIS Brightness Temperatures data set
<field> Statistical field represented in the image (e.g. range, earliest, latest)
1979-2017 Data set temporal coverage
v04 Data set version number (e.g. 04)
r00 Data set revision number (e.g. 00)

File Size

The netCDF file is approximately 1.9 MB.

Browse images are approximately 345 KB.

Spatial Information

Coverage

Data cover the Northern Hemisphere, except for three circular gaps centered over the pole that correspond to the three satellite records. Data from the SMMR period (1978-87) have a gap with a radius of 611 km, located poleward of 84.5 degrees North. Data from the SSM/I period (1987 through 2007) have a polar gap with a radius of 311 km, located poleward of 87.2 degrees North. Lastly, data from the SSMIS period (2008 through 2017) have a gap poleward of 89.2° North. See the Polar Stereographic Projection and Grid spatial coverage map for details. 

Resolution

25 km

Geolocation

The data are provided on the 25 km Northern Hemisphere Polar Stereographic Grids, as described in Tables 6 and 7. For more information, see the Polar Stereographic Projections and Grid web page.

Table 6. Geolocation Details
Geographic coordinate system Unspecified datum based upon the Hughes 1980 ellipsoid
Projected coordinate system NSIDC Sea Ice Polar Stereographic North
Longitude of true origin -45
Latitude of true origin 70
Scale factor at longitude of true origin 1
Datum Not_specified_based_on_Hughes_1980_ellipsoid
Ellipsoid/spheroid Hughes 1980
Units meter
False easting 0
False northing 0
EPSG code 3411
PROJ4 string +proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs 
Reference https://epsg.io/3411

Table 7. Grid Details
Grid cell size (x, y pixel dimensions) 25.0 km
Number of rows 448
Number of columns 304
Geolocated lower left point in grid 33.92° N, 279.26° W
Nominal gridded resolution 25 km x 25 km
Grid rotation N/A
ulxmap – x-axis map coordinate of the center of the upper-left pixel (XLLCORNER for ASCII data) -3850 projected km
ulymap – y-axis map coordinate of the center of the upper-left pixel (YLLCORNER for ASCII data) 5850 projected km

Temporal Information

Coverage

This data set extends from 1979 through 2017. Snow melt onset dates were derived from brightness temperatures acquired from multiple platforms, as described in Table 8. For all years, only brightness temperatures from DOY 61 (early March) through 245 (early September) were used to calculate snow melt onset dates.

Table 8. Temporal Coverage by Input Sensor
Platform / Sensor Start Date End Date
Nimbus-7 SMMR 01 Jan 1979 20 August 19871
DMSP F8 SSM/I 01 Jan 1988 18 December 1991
DMSP F11 SSM/I 01 Jan 1992 31 December 1995
DMSP F13 SSM/I 01 Jan 1996 31 December 2007
DMSP F17 SSMIS 01 Jan 2008 31 December 2017
1Other data was not substituted for the missing SMMR data to cover the last 11 days of the melt season in August 1987. Those days are flagged as "no data."

Resolution

Snow melt onset dates were derived once per year for each grid cell.

Data Acquisition and Processing

Background

Accurate snow melt onset dates 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 also serve as climate proxies in Arctic sea ice zones. 

Snow melt onset dates are estimated based on changes in brightness temperature measurements. Microwave emissivity 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 (e.g. 18.0 GHz and 19.3 GHz) are more responsive to melt onset in ice than are higher frequencies (e.g. 37.0 GHz), primarily due to the change in emission depth associated with melt. Melt therefore causes the difference between low-frequency and high-frequency brightness temperatures to change from positive to near-zero or negative. Furthermore, the increase in brightness temperature associated with melt is polarization-dependent. Horizontal channels reflect a stronger dependence on snow conditions during melt due to the change in dielectric properties at the air-snow interface when snow is wet.

Acquisition

Snow melt onset dates were derived from brightness temperature (Tb) measurements acquired by the Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), and Special Sensor Microwave Imager/Sounder (SSMIS) instruments. Sea ice extent masks were also used to constrain the melt algorithm. Table 9 describes the input data sources in more detail.

Table 9. Input Data Sets
Data Set Description Channels/Variables Used
Nimbus-7 SMMR Polar Gridded Radiances and Sea Ice Concentrations, Version 1 (Gloersen 2006) Tb used to calculate snow melt onset dates from 1979 to 1987. 18.0 GHz and 37.0 GHz
DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures, Version 4 (Maslanik and Stroeve 2004) Tb used to calculate snow melt onset dates from 1988 to 2017. 19.3 GHz and 37.0 GHz
NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 3 (Meier et al. 2017) Sea ice concentrations used to a create mask of the annual sea ice maximum extent. Snow melt dates were only calculated for locations inside the sea ice mask. goddard_merged_seaice_conc

Processing

Snow melt onset dates were estimated using daily average brightness temperature (Tb) data from SMMR, SSM/I (F8, F11, and F13), and SSMIS (F17) satellite radiometers. Changes in 18.0 or 19.3 H-polarization GHz and 37.0 H-polarization GHz Tb were recorded for each grid cell using the Advanced Horizontal Range Algorithm (AHRA). For more details on AHRA, please refer to Anderson 1997. 

  1. Ensure consistent input data
    For this data set, SSM/I F8 was used as the standard input sensor. Regression analysis was used to convert SMMR, SSM/I F11 and F13, and SSMIS F17 Tb to SSM/I F8 Tb (after Abdalati et al. 1995). Table 10 provides an overview of the correction coefficients used. These conversions ensure a consistent data record for determining temporal trends in the snow melt onset dates. If data are not consistent, snow melt trends could be attributable to instrument characteristics rather than climate conditions.
  2. Created sea ice extent mask
    The goddard_merged_seaice_conc variable from the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 3 data set was used as a measure of daily Sea Ice Concentration (SIC). Beginning on Day of Year (DOY) 61 (early March), these SIC estimates were used to determine which pixels had a SIC ≥ 50%. Snow melt onset dates were only calculated at pixel locations that met this criterion.
    • DOY 61 was used because this date roughly corresponds to the time period of maximum annual sea ice extent.
    • Since the SMMR data were collected every other day, in the event of data outages (i.e. a missing swath), a SIC ≥ 50% for any day between DOY 61 and DOY 65 was used to define the sea ice extent.
    • A unique sea ice extent mask was created for each year of data.
  3. Run AHRA
    The AHRA calculates the difference between low-frequency (18.0 GHz or 19.3 GHz) and high-frequency (37.0 GHz) Tb measurements. When conditions are dry and frozen, low-frequency Tb measurements are larger than high-frequency Tb measurements. When low-frequency Tb drop below high-frequency Tb measurements, melt has started. Turn points are described below; refer to Drobot and Anderson 2001 for more details.
    • If (18.0 / 19.3 GHz - 37.0 GHz) > 4 K, the AHRA assumes winter conditions and proceeded to the next day with data for that pixel.
    • If (18.0 / 19.3 GHz - 37.0 GHz) ≤ -10 K, the AHRA assumes liquid water was present in the snowpack and classifies that day as the snow melt onset date.
    • If 4 K > (18.0 / 19.3 GHz - 37.0 GHz)  > -10 K, the AHRA determines if snow melt onset occurred based on a 20-day time series of Tb. The algorithm subtracts the minimum and maximum Tb values for the ten days prior to the potential melt onset date, and again for the period from the potential melt onset date to nine days later. The former number is then subtracted from the latter number. If the difference is greater than 7.5 K, the algorithm assigns a snow melt onset date to that particular grid cell because a large difference indicates variability in the 18.0 / 19.3 GHz - 37.0 GHz range after the potential melt onset date. If the difference was less than 7.5 K, then liquid water is unlikely to be in the snowpack, and the algorithm moves on to the next day.
  4. Assigned quality flags​
    • Assigned a value of 5 to all pixels that were part of the pole hole.
    • Assigned a value of 10 to all pixels that were over open ocean.
    • Assigned a value of 15 to all pixels that were over land.
    • Assigned a value of 255 to all pixels where melt was not calculated.
Table 10. Linear Regression Coefficients and Equations Used to Calibrate TBs Between SMMR, SSMI, and SSMIS Sensors using F8 as the Standard
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
Int. (K) 2.62
37H Slope 0.954 F8=(SMMR-2.85)/0.954
Int. (K) 2.85
F11 to F8 Abdalati et al. (1995) Greenland 19H Slope 1.013 F8=1.013*F11-1.890
Int. (K) -1.89
37H Slope 1.024 F8=1.024*F11-4.220
Int. (K) -4.22
F13 to F11 Stroeve et al. (1998) NH Sea Ice 19H Slope 0.986 F11=(F13-2.197)/0.986
Int. (K) 2.179
37H Slope 0.966 F11=(F13-6.110)/0.966
Int. (K) 6.11
F17 to F13 Meier et al. (2011) Arctic Mar - Sept 2007 19H Slope 0.979 F13=(F17-1.646)/0.979
Int. (K) 1.646
37H Slope 0.999 F13=(F17-0.649)/0.999
Int. (K) 0.649

Quality, Errors, and Limitations

Differences between V3 and V4 Data

The differences between the Version 3 (V3) and Version 4 (V4) snow melt onset dates (SMOD) are illustrated for two years in Figure 3. Differences are limited to the sea ice edge and around the pole hole. Along the ice edge, blue pixels indicate grid cells where SMOD was computed in V4 but not V3. Conversely, red pixels along the ice edge indicate grid cells where SMOD was computed in V3 but not in V4. These differences are due to slight variations in the V2 and V3 NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration data sets, which were used to create the sea ice masks in V3 and V4 of this data set, respectively.

The biggest differences between V3 and V4 SMOD occur at the pole hole in the years 2008 - 2012. In the previous SIC data version, a larger pole hole was used to mask SICs obtained using SSMIS; thus, no SMOD could be computed within this region. However, the updated SIC data now use a smaller pole hole mask for these years, which allows SMOD to be computed. These new SMOD are shown as the dark blue ring in Figure 3 (right panel) surrounding the North Pole. Note that almost differences exist between V3 and V4 SMOD for the bulk of the sea ice area because no changes have bene made to the AHRA algorithm since publication by Bliss and Anderson (2014).

Figure 3. Visual comparison of snow melt onset dates from Version 3 and Version 4 of this data set.

Error Sources

Brightness temperature data may have errors related to pixel averaging, sensor errors, and weather effects. See the following brightness temperature documentation for more information regarding errors in the source data:

Limitations

Given 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.

Instrumentation

Brightness temperature input data were acquired from the Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), and Special Sensor Microwave Imager/Sounder (SSMIS) instruments. For more details, refer to the SMMR, SSM/I, and SSMIS Sensors page.

Software and Tools

For a comprehensive list of all polar stereographic tools, see the Polar Stereographic Data Tools Web page.

Version History

Table 11 outlines the processing and algorithm history for this product.

Table 11. Description of Version Changes
Version Date Description of Changes from Previous Version
V04 June 2019
  • Reprocessed the entire data set using a newer version of the sea ice extent masks within the snow melt onset algorithm. No other changes were made to the algorithm.
  • Extended the data record through 2017.
  • After snow melt dates were computed, flags were added to the data to indicate where snow melt dates were not derived. The flags discriminate between sea ice where the snow melt onset date was not found and pixels included in the land mask, the pole hole, and areas of open water.
  • Updated data fields that summarize basic statistics over the length of the data record.
  • Data are now distributed as a netCDF file.
V03 Mar 2014
  • Extended data record through the 2012 melt season which includes the use of the SSMIS instrument on the DMSP F17 satellite.
  • Removed the two-pixel buffer surrounding the coastlines.
  • Included the use of an annual sea ice extent mask to indicate sea ice locations where a melt onset date was calculated.
  • Changed the regression coefficients used to convert the F11 brightness temperatures to ones from Stroeve et al. (1998).
  • Changed parameter values from 0 to 255 (inclusive) to 0 to 245 (inclusive). 
  • Trend files are no longer being generated.
  • Used the gsfc_25n.msk land/coast mask.
V02 Nov 2009
  • Removed 9-point median filter that corrected for spurious melt dates in V01
  • Added flags to input brightness temperatures to correct for bad scanlines, then reprocessed input brightness temperatures.
V01 Dec 2001 Original version of data.

Related Data Sets

Contacts and Acknowledgments

Mark Anderson
Meteorology/Climatology Program
Department of Geosciences
University of Nebraska
Lincoln, NE 68588-0340 USA

Angela C. Bliss
Oregon State University
College of Earth, Ocean and Atmospheric Sciences
104 CEOAS Admin Building

Corvallis, OR, 97331-5503 USA

Sheldon Drobot
Research Applications Lab
National Center for Atmospheric Research (NCAR)
University Corporation for Atmospheric Research (UCAR)
P.O. Box 3000
Boulder, CO 80307-3000 USA

References

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. DOI: https://doi.org/10.1080/01431169508954473

Anderson, M. 1997. Determination of a Melt Onset Date for Arctic Sea Ice Regions Using Passive Microwave Data. Annals of Glaciology 25:382-387. DOI: https://doi.org/10.3189/s0260305500014324

Bliss, A. C. and M. R. Anderson. 2014. Arctic sea ice melt onset from passive microwave satellite data: 1979 - 2012. The Crysophere 8:2089-2100. DOI: https://doi.org/10.5194/tc-8-2089-2014

Drobot, S. and M. Anderson. 2001b. Comparison of Interannual Snowmelt Onset Dates with Atmospheric Conditions. Annals of Glaciology 33: 79-84. DOI: https://doi.org/10.3189/172756401781818851

Gloersen, P. 2006. Nimbus-7 SMMR Polar Gridded Radiances and Sea Ice Concentrations, Version 1. [1979-1987]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. DOI: https://doi.org/10.5067/QOZIVYV3V9JP.

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.

Maslanik, J. and J. Stroeve. 2004. DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures, Version 4. [1988-2017]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. DOI: https://doi.org/10.5067/AN9AI8EO7PX0.

Meier, W. N., Khalsa, S. J. S., and M. H. Savoie. 2011. Intersensor calibration between F-13 SSM/I and F-17 SSMIs near-real-time sea ice estimates. IEEE Trans. Geosci. Remote Sens. 49:3343–3349. DOI: 10.1109/TGRS.2011.2117433

Meier, W., F. Fetterer, M. Savoie, S. Mallory, R. Duerr, and J. Stroeve. 2017. NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 3. [1979-2017]. Boulder, Colorado USA: National Snow and Ice Data Center. DOI: https://doi.org/10.7265/N59P2ZTG.

Stroeve, J., L. Xiaoming, and J. Maslanik. 1998. An Intercomparison of DMSP F11- and F13-derived Sea Ice Products. Remote Sensing of the Environment 64:132-152. DOI: https://doi.org/10.1175/1520-0442(2004)017<0067:DOTASI>2.0.CO;2

No technical references available for this data set.

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