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.
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.
COMPREHENSIVE Level of Service
Data: Data integrity and usability verified; data customization services available for select data
Documentation: Key metadata and comprehensive user guide available
User Support: Assistance with data access and usage; guidance on use of data in tools and data customization services
|
Geographic Coverage |
- If you are not currently logged in to Earthdata, you will be prompted to do so.
- You may register for an Earthdata Login if you do not have an account.
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].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.
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 |
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.
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.


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
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
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.
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 |
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.
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.
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.
- 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. - Created sea ice extent mask
Thegoddard_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.
- 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.
- 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.
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).

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:
- DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures
- Nimbus-7 SMMR Polar Radiances and Arctic and Antarctic Sea Ice Concentrations
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 Summary.
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.
Version | Date | Description of Changes from Previous Version |
---|---|---|
V04 | June 2019 |
|
V03 | Mar 2014 |
|
V02 | Nov 2009 |
|
V01 | Dec 2001 | Original version of data. |
Related Data Sets
- MEaSUREs Arctic Sea Ice Characterization Daily 25km EASE-Grid 2.0
- DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures
- Nimbus-7 SMMR Polar Radiances and Arctic and Antarctic Sea Ice Concentrations
- ESMR Polar Gridded Brightness Temperatures and Sea Ice Concentrations
- Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures
- Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations
- DMSP SSM/I-SSMIS Pathfinder Daily EASE-Grid Brightness Temperatures
- Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data
- Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I
- Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS
- NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration
Contacts and Acknowledgments
Mark Anderson
Meteorology/Climatology Program
Department of Geosciences
University of Nebraska
Lincoln, NE 68588-0340 USA
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