Southern Greenland ice sheet elevation change estimates are derived from SEASAT and GEOSAT radar altimetry data from 1978 to 1988. Data are confined to 61-72 deg N, 30-50 deg W, above 1700 m elevation. The addition of GEOSAT Geodetic Mission (GM) data results in twice as many crossover points and 50% greater coverage than previous studies. Coverage above 2000 m elevation is improved to 90%, and about 75% of the area between 1700 m and 2000 m is now covered. Data are in ASCII text format, available via FTP, and consist of elevation change rate (dH/dt, cm/year) and corresponding error estimates in 50 km grid cells.

The Greenland Climate Network (GC-NET), established in spring 1994, currently consists of 18 stations with a distributed coverage over the Greenland ice sheet. The GC-Net emphasis is to monitor climatological and glaciological parameters at various locations on the ice sheet over a 10 year time period. The GC-Net Automatic Weather Stations (AWS) are equipped with instruments to measure surface energy and mass balance. Currently, the GC-NET archive contains 50 plus station years of measurements. These data have been quality controlled and calibrated. The objectives of the GC-Net AWS network are: (1) Assess daily, annual and interannual variability in accumulation rate, surface climatology and surface energy balance at selected locations on the ice sheet where high sensitivity of the ice sheet mass balance to climate anomalies is predicted from modeling results; (2) Assess accurate surface elevation, location, near-surface density at the AWS location with the option to revisit the locations in order to get temporal information for dynamic ice sheet modeling. The statistics are based on 12 critical parameters such as: incoming shortwave radiation, reflected shortwave radiation, net radiation, temperature-humidity-wind profile at two levels, wind direction, pressure and surface height. The possible data point count for all AWS and channels is 5.04574 106, whereas 4.72291 106 data were retrieved, resulting in an overall success rate of 93.6 percent. Recent six days of hourly surface meteorological ASCII data and plots are updated hourly on the WWW at: http://cires.colorado.edu/steffen/aws/current_GC-Net_plots.html. The GC-Net sites that are using GOES and ARGOS satellite data links are operationally updating automatically as of September 16 19

The Greenland ice sheet melt extent data, acquired as part of the NASA Program for Arctic Regional Climate Assessment (PARCA), is a daily (or every other day, prior to August 1987) estimate of the spatial extent of wet snow on the Greenland ice sheet since 1979. It is derived from passive microwave satellite brightness temperature characteristics using the Cross-Polarized Gradient Ratio (XPGR) of Abdalati and Steffen (1997). It is physically based on the changes in microwave emission characteristics observable in data from the Scanning Multi-channel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) instruments when surface snow melts. It is not a direct measure of the snow wetness but rather is a binary indicator of the state of melt of each SMMR and SSM/I pixel on the ice sheet for each day of observation. It is, however, a useful proxy for the amount of melt that occurs on the Greenland ice sheet. The data are provided in a variety of formats including raw data in ASCII format, gridded daily data in binary format, and annual and complete time series climatologies in gridded binary and GeoTIFF format. All data are in a 60 x 109 pixel subset of the standard Northern Hemisphere polar stereographic grid with a 25 km resolution and are available via FTP.

Two 150-MHz coherent radar depth sounders were developed and flown over the Greenland ice sheet to obtain ice thickness measurements in support of PARCA investigations and the North Greenland Ice Core Project. These radar depth sounders are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice. These systems are used to collect radar data over the interior and margins of the ice sheet and several outlet glaciers. Both radar systems are operated on the NASA P-3B aircraft equipped with GPS receivers. Radar data are tagged with GPS-derived location information and are collected in conjunction with laser altimeter measurements. All data collected from 1993 to 2014 have been processed, and ice thickness was derived with an accuracy of ± 10 m over 90 percent of the flight lines flown as a part of the PARCA initiative. Radar echo grams and derived ice thickness data are available via FTP.

The Hydrologic Sub-basins of Greenland data set contains Geographic Information System (GIS) polygon shapefiles that include 293 hydrologic sub-basins of the Greenland Ice Sheet, as created by Lewis and Smith (2009). Five km Digital Elevation Models (DEMs) of ice-surface and bedrock topography (Bamber et al. 2001) were used to estimate the hydraulic potentiometric surface for Greenland, which accounts for the effects of ice overburden pressure, such as hydrostatic pressure, surface topography, and underlying bedrock topography (Paterson 1994). Using the 5 km potentiometric grid, a modeled basin network for Greenland's ice sheet was created using the hydrological tools within ESRI ArcGIS (ArcInfo 9.2). Although the GIS initially generated more than 293 basins, basins less than 100 km2 in size, as well as basins located completely outside of the ice sheet extent, were removed from the data set. Data are available via FTP, and can be read in any GIS software. The following journal article provides documentation for this data set: Lewis, Sarah and Laurence Smith. 2009. Hydrologic Drainage of the Greenland Ice Sheet. Hydrological Processes DOI: 10.1002/hyp.7343. This journal article can be accessed from the Documentation link at the top of this Web page.

Ice-sheet mass balance can be inferred from comparison between total net accumulation and total ice discharge, and this data set contains results from such a comparison for higher-elevation parts of the Greenland ice sheet. During 1993-1997, velocities were measured at a line of stakes about 30 km apart completely around the ice sheet approximately along the 2000 m elevation contour except in the south west, where it was higher because of high mountains, nunataks, and crevasses [Thomas et al., 2000]. More information on these velocities can be found on our web site. Ice thickness was also measured along the traverse by a low-frequency ice-sounding radar [Gogineni et al., 1998]. Snow accumulation was from existing estimates [Ohmura and Reeh, 1991], supplemented by recent German and PARCA measurements and with estimates of total precipitation at weather stations for coastal accumulation rates. Each estimate refers to the time period over which measurements were made, ranging from years to centuries, but the overall set of results is heavily influenced by results from recent PARCA cores, most of which refer to the past two or three decades. Total snow accumulation within catchment areas corresponding to selected groups of stakes, expressed as a volume flux of ice, was compared to ice discharge between the stake locations. This was calculated as the product of ice thickness and velocity integrated between the stakes, after correcting for the ratio (R) between surface and column-averaged velocity, based on model simulations of the velocity/depth profile [Huybrechts, 1996]. Errors for small catchment areas are quite large because of uncertainties in local accumulation rates, the size of the catchment area, and R [Thomas et al., 1998]. For areas larger than about 100,000 sq km, the random nature of these errors reduces their overall effect to less than seven percent of the snow-accumulation rate, and considerably less for the entire region under study [Thomas et al., 2000]. Snow-accumulation rates range from less than 0.1 m of water equivalent per year in the northeast to more than one meter per year in parts of the southeast, with an average value for the entire ice sheet of about 0.3 m/yr [Bales et al., this issue]. The data set presented here is in tabular form, with a listing for individual gates of: station locations forming the gates; velocities, ice thicknesses and R estimates at the gates; total upstream accumulation; and resulting estimates of average ice-thickening rates (dH/dt) for the catchment areas appropriate to the gates. Because errors in dH/dt are large for single gates, additional Tables are provided giving:

1. Values for larger ice-sheet catchment areas (approximately 30,000 sq km) formed by including several adjacent gates.
2. Values for 12 larger gates (up to >100,000 sq km) that represent regions of the ice sheet with distinctive patterns of dH/dt. These estimates are compared to independent estimates from altimetry measurements in Thomas et al.

Errors in accumulation rates are the largest source of uncertainty in these mass-balance estimates, but they are progressively reduced as more information is acquired. The data presented here were compiled before all PARCA accumulation measurements had been analyzed, so they include different estimates of dH/dt based on accumulation estimates ranging from those in Ohmura and Reeh [1991] to our own attempts to improve these using then available PARCA data, and details of these can be found on our web site. Consequently, the dH/dt estimates provided here will be progressively improved as our estimates of snow accumulation also improve. Ongoing work with these data include:

• Investigation of the regions that are significantly out of balance, in an attempt to identify causes for the imbalance.
• Progressive updating of the dH/dt estimates using improved accumulation estimates, flow lines based on better maps of ice-sheet topography, and better estimates of R. This will be continued into the GLAS mission, using accumulation estimates from atmospheric analyses, for comparison with GLAS measurements of surface-elevation change.

PARCA began in 1993 with the primary goal of measuring and understanding the mass balance of the Greenland Ice Sheet. Approximately 25 investigations used in situ measurements and satellite and aircraft remote sensing to study this issue. Measurements include surface elevation and ice thickness change rates from aircraft laser altimetry, shallow to intermediate ice cores (for average accumulation rates and their temporal and spatial variability), velocity around the ice-sheet perimeter at an elevation of about 2000 meters, local ice thickening/thinning rates, and climatological observations from automatic weather stations. Satellite data used include elevation change from radar altimetry; mapping of snow facies and zones of summer melt from passive and active microwave; ice velocities from interferometric synthetic aperture radar (SAR) and from repeat high-resolution visible and SAR imagery; mapping surface and 10-m temperatures, accumulation rates, and surface albedo from microwave and AVHRR data. Data are available either by FTP or through investigator Web sites. Questions about investigator data should be directed to the individual technical contacts responsible for each data set. The National Snow and Ice Data Center (NSIDC) provides summaries and links to these data as a service to our users.