Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record


The Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record is produced by Thomas Mote at the University of Georgia as part of the “Northern Hemisphere Snow and Ice Climate Data Records” project, led by Rutgers University and supported by NASA’s Making Earth System Data Records for Use in Research Environments (MEaSUREs) program.

Please note that this is a candidate release of the Climate Data Record (CDR), which is currently being vetted by the National Snow and Ice Data Center (NSIDC) (http://nsidc.org).

1. Introduction

The purpose of this document is to describe the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record (CDR). This CDR uses the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR), as well as the Special Sensor Microwave/Imager (SSM/I) and Special Sensor Microwave Imager and Sounder (SSMIS) sensors on U.S. Department of Defense Meteorological Satellite Program (DMSP) platforms. The goal of the CDR is to provide a consistent, reliable, and well-documented product that meets CDR guidelines as defined in Climate Data Records from Environmental Satellites interim report to the National Academy of Sciences (NAS 2004).

2. Products Generated

The product described here is the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record. The product is generated from gridded brightness temperatures (TBs) from the Nimbus-7 SMMR and DMSP series of SSM/I and SSMIS passive microwave radiometers. The surface melt CDR is a binary estimate of melt or no melt that is produced by identifying grid cells in the daily TBs that exceed threshold TBs associated with 1% liquid water by volume in the snow. The threshold TBs are calculated for each 25km grid cell, each year, and the process is described in more detail in section 4.

A near-real-time (NRT) version of the CDR product using brightness temperatures from the NOAA Comprehensive Large Array-data Stewardship System (CLASS) (http://www.class.noaa.gov), with standard on-board calibrations, is produced daily at and is available from NSIDC. The NSIDC NRT TBs are used to produce the NRT Greenland Daily Surface Melt grids. The NRT fields are interim fields that are not part of the permanent CDR archive and are removed when brightness temperatures are obtained from NSIDC.

3. Instrument Characteristics

The Scanning Multichannel Microwave Radiometer (SMMR) was a five-frequency instrument on the Nimbus-7 satellite. It had dual-polarized, horizontal (H) and vertical (V), channels at 6.63, 10.69, 18.0, 21.0, and 37.0 GHz (Gloersen et al. 1984). The first SSM/I sensor was launched aboard the DMSP F-8 mission in 1987 (Hollinger et al., 1987). A series of SSM/I sensors on subsequent DMSP satellites has provided a continuous data stream since then. Only sensors on the F-8, F-11, F-13, and F-17 platforms are used in the generation of the CDR. The SSM/I sensor has seven channels at four frequencies. The 19.4, 37.0, and 85.5 GHz frequencies are dual polarized; the 22.2 GHz frequency has only a single vertically polarized channel. For simplicity, the channels are sometimes denoted as simply 19H, 19V, 22V, 37H, 37V, 85V and 85H. Only the 37H channel is used in the melt algorithm.

Beginning with the launch of F-16 in 2003, the SSM/I sensor was replaced by the SSMIS sensor. The SSMIS sensor has the same 19.4, 22.2, and 37.0 GHz channels; however, the 85.5 GHz channels on SSM/I are replaced with 91.0 GHz channels on SSMIS. The SSMIS sensor also includes several higher frequency sounding channels that are not used. Depending on the platform, the satellites altitudes are 830 to 860 km and sensor (Earth incidence) angles are 52.8 to 53.4 degrees.

SMMR, SSM/I and SSMIS TBs are available from separate data sets distributed through NSIDC. NSIDC processes and combines swath brightness temperature data from Remote Sensing Systems, Inc. (RSS) (http://www.ssmi.com). The RSS brightness temperatures use enhanced processing methods to correct errors and improve calibration and geolocation.

Calibrated and gridded brightness temperatures from SMMR, SSM/I and SSMIS passive microwave sensors are used as the primary input data for this Greenland Daily Surface Melt CDR. These gridded brightness temperatures are produced from the swath data obtained from RSS. NSIDC puts the input swath data for SSM/I and SSMIS onto a 25 km polar stereographic grid for both Arctic and Antarctic regions. NSIDC also makes these data publicly available on the DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures (http://nsidc.org/data/nsidc-0001.html) data set web pages. Nimbus-7 SMMR Polar Gridded Radiances and Sea Ice Concentrations are also distributed by NSIDC (http://nsidc.org/data/nsidc-0007.html). Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Brightness Temperatures are also available through NSIDC (http://nsidc.org/data/nsidc-0080.html). Specific processing information on the SSM/I-SSMIS input swath data is available from RSS, Inc.

4. Algorithm

Passive microwave sensors may be used to estimate the frequency with which melt occurs due to the increase in emissivity as liquid water forms in previously dry snow. The emissivity of snow changes rapidly as melt occurs. Because of the high dielectric constant of liquid water, wet snow results in an increase in absorption relative to volume scattering, which reduces scattering and enhances emission. This change in TB with liquid water is frequency dependent. Below approximately 10 GHz, a spectral region where volume scattering is small compared to surface scattering, the TB decreases with increasing liquid water content; near 10 GHz, the TB is nearly insensitive to liquid water content (Ulaby et al 1986). Although snow density, temperature, crystal structure and crystal size all contribute to the snowpack emissivity, changes in the liquid water content produce the most prominent changes in TB. The TB becomes increasingly sensitive to water content at higher frequencies.

Changes in melt duration and extent on the Greenland ice sheet have been mapped using the seasonal change in emissivity (Mote et al 1993, Mote and Anderson 1995), the frequency dependence of emissivity, such as the cross polarized gradient ratio (XPGR) of Abdalati and Steffen (1997) and Steffen et al. (2004), and the diurnal change in emissivity (Tedesco, 2007). The melt algorithm here follows Mote and Anderson (1995) and Mote (2007).

A microwave emission model was employed to determine the 37GHz, horizontally polarized (37H) TBs associated with 1% volumetric water content for each grid cell, each year, on the Greenland ice sheet. The 1% value was chosen because it is near the lower limit of the radiometer's sensitivity to changes in water content (Stiles and Ulaby 1980) and corresponds to other measures of surface melt. The use of annual thresholds allows this method to account for the influence of changes in annual snow accumulation.

The emission model is based on the work of Ulaby and Stiles (1980), who give the TB produced by individual snow layers as a function of snow temperature, layer depth, density, propagation angle, and mass scattering and absorption coefficients. This can be solved analytically and expanded for multiple snow layers, ignoring diffuse scatter and multiple reflections at boundary layers, to determine the TB at the surface of the snowpack (Abdelrazik et al 1981). The TB detected by the satellite sensor is the sum of the surface TB, upwelling sky radiation, reflected downwelling sky radiation, and the reflected emission from free space. The absorption coefficient is a function of the complex dielectric constant of snow, which can be calculated using a dielectric mixing model from Tinga et al (1973). That model has been used to determine the dielectric properties of snow undergoing melt (Tiuri and Schultz 1980). Smooth boundaries between snow layers and at the snow-air boundary are assumed for calculation of transmission coefficients at each layer and the surface boundary. Observed 37H TBs were averaged for a period from each year when the temperature profile is assumed to have been nearly isothermal and prior to the onset of melt. The emission model uses scattering coefficients that are empirically derived by inverting the model during the spring. Mote and Anderson (1995) describe the modeling process in greater detail.

The emission model is used to simulate the brightness temperature associated with melt for each grid cell, each year. These TBs are used as threshold values to distinguish melt from non-melt.

The three sensors (SMMR, SSM/I, and SSMIS) differ slightly in view angle, radiometric resolution and calibration and swath width. However, because this approach derives a new set of threshold TBs each year, and because the model explicitly accounts for the sensor frequency and view angle, the impact of using different sensors and satellite platforms is minimized.

5. Data Format

A 61x111 subset centered over Greenland is extracted from the 304x448 25km polar stereographic grid of brightness temperatures. Comparisons between the daily 37H TBs and the threshold values of TBs are made on the 61x111 subset. A binary value of melt or no-melt is then resampled from the polar stereographic grid to the EASE-Grid 2.0 (http://nsidc.org/data/ease/ease_grid2.html) projection using the MAPX utilities available from NSIDC (http://nsidc.org/data/tools) and saved as 720x720 flat binary files.

A land-ocean-ice mask based on the Boston University MOD12Q1 V004 Land Cover Product (BU-MODIS) data, converted to EASE-Grid 2.0 was applied to mask water and ice-free land (http://nsidc.org/data/ease/ancillary.html#bumodis). The daily 25km EASE-Grid 2.0 720x720 flat binary files are converted to netCDF format using a custom script in NCL. Monthly netCDF files are aggregated using the netCDF record concatenator (ncrcat). Daily browse images are also created in NCL.

6. Future Efforts

Efforts have begun to merge the CDR with other visible and microwave satellite and station-observed estimates of extent and depth over Northern Hemisphere lands as well as with CDRs being developed for snow melt atop Arctic sea ice as part of NASA’s Making Earth System Data Records for Use in Research Environments (MEaSUREs) program.

7. Contact Information

Requests for additional information or questions about the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record should be directed to Thomas L. Mote, Department of Geography, University of Georgia, Athens GA 30602-2502 (tmote at uga.edu).

8. Data Citation

Please include a citation in the references section of your publication. While we appreciate acknowledgement in the text of the publication, we are better able to track the use of our data sets if citations are included as references. Here is a current citation for the candidate release of the Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record data set:

Mote, T.L. 2012, updated current year. Greenland Daily Surface Melt 25km EASE-Grid 2.0 Climate Data Record, [list dates of temporal coverage used]. Athens, Georgia, USA: University of Georgia. Digital media.

9. References

Abdalati, W., and K. Steffen, 1997: Snowmelt on the Greenland Ice Sheet as derived from passive microwave satellite data. Journal of Climate, 10, 241-51, 1997.

Abdelrazik, M., F. Ulaby and H. Stiles, 1981: A model describing the microwave emission from a multi-layer snowpack at 37 GHz. Greenbelt, MD, Goddard Space Flight Center. (NASA CR-16708.)

Brodzik, M. J., B. Billingsley, T. Haran, B. Raup, M. H. Savoie. 2012. EASE-Grid 2.0: Incremental but Significant Improvements for Earth-Gridded Data Sets. ISPRS Int. J. Geo-Inf. 1(1):32-45, doi:10.3390/ijgi1010032.

Gloersen, P., D.J. Cavalieri, A.T.C. Chang, T.T. Wilheit, W.J. Campbell, O.M. Johannessen, K.B. Katsaros, K.F. Kunzi, D.B. Ross, D. Staelin, E.P.L. Windsor, F.T. Barth, P. Gudmandsen, E. Langham and R.O. Ramseier, 1984: A summary of results from the frist NIMBUS-7 SMMR observations, Journal of Geophysical Research, 89, 5335-5344.

Hollinger, J., R. Lo, G. Poe, R. Savage, and J. Pierce, 1987: Special Sensor Microwave/Imager User’s Guide. Naval Research Laboratory Report, Washington, DC.

Mote, T.L., 2007: Greenland surface melt trends 1973-2007: Evidence of a large increase in 2007. Geophysical Research Letters, 34, L22507, doi:10.1029/2007GL031976.

Mote, T.L., and M.R. Anderson, 1995: Variations in melt on the Greenland ice sheet based on passive microwave measurements. Journal of Glaciology, 41, 51-60.

Mote, T.L., M.R. Anderson, K.C. Kuivinen, and C.M. Rowe, 1993: Passive microwave-derived spatial and temporal variations of summer melt on the Greenland ice sheet. Annals of Glaciology, 17, 233-238.

NAS, 2004: Climate data records from environmental satellites: Interim report, National Academies of Science (NAS), National Academies Press, Washington, D.C., 150 pp.

Steffen, K., S. V. Nghiem, R. Huff, and G. Neumann, 2004: The melt anomaly of 2002 on the Greenland Ice Sheet from active and passive microwave satellite observations. Geophysical Research Letters, 31, L20402, doi:10.1029/2004GL020444.

Tedesco, M., 2007: Snowmelt detection over the Greenland ice sheet from SSM/I brightness temperature daily variations. Geophysical Research Letters, 34, L02504, doi:10.1029/2006GL028466.

Tinga, W.R., W.A.G. Voss, and D.F. Blossey, 1973: Generalized approach to multiphase dielectric mixture theory. Journal of Applied Physics, 44, 3897-3902.

Tiuri, M., and H. Schultz, 1980: Theoretical and experimental studies of microwave radiation from a natural snowfield. In Rango, A., ed., Microwave remote sensing of snowpack properties. Proceedings of a workshop sponsored by the National Aeronautics and Space Administration and held at Fort Collins, Colorado, May 20-22, 1980. Washington, DC, NASA Goddard Space Flight Center. (Conference Publication No. 2153.)

Ulaby, F.T., and W.H. Stiles, 1980: The active and passive microwave response to dry snow parameters, 2. water equivalent of dry snow. Journal of Geophysical Research, 85, 1045-1049.

Ulaby, F.T., R.K. Moore, and A.K. Fung, 1986: Microwave Remote Sensing, Active and Passive, Vol. 3., Reading, Mass., Addison-Wesley.