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GCOS Terrestrial ECV - Snow Cover

Snowfall and other solid precipitation as a fraction of total precipitation is important in hydro-climatic models and as an element in monitoring climate change. About one third of the Earth's land surface may be covered seasonally by snow. Up to 50% of the Northern Hemisphere land surface has snow cover during the Northern Hemisphere winter. It has major effects on surface albedo and energy balance and modifies the overlying atmospheric thickness and surface temperatures. Characteristics of snow such as thickness, seasonal and interannual variability or snow-cover duration affect permafrost thermal state, the depth and timing of seasonal freeze/thaw of the ground as well as ablation on glaciers, ice sheets and sea ice. Snow melt plays a major role in seasonal energy exchanges between the atmosphere and ground; it impacts soil moisture and runoff, thereby affecting water resources. As a consequence, snow cover is a key component of land surface process sub-models for higher latitudes and altitudes.

Contributing Network(s)

Contributing Network(s) Status

  • Synoptic and national networks have significant gaps and are all contracting.
  • Northern and Southern Hemisphere monitored operationally for extent and duration.

Contributing Satellite Data

  • Moderate to high resolution optical for extent/duration
  • Passive microwave for snow water equivalent
  • Geostationary satellites

Contributing Satellite Data Status

Moderate to high resolution optical and microwave sensor system follow-on is programmed.

Current Capability

There are several sources that can provide snow-related data and products, but no central archive (especially for snow depth and snow-water equivalent) currently exists and many national databases are not readily accessible. An in situ dataset (station and transect) for North America for the period 1980 to 2004 for more than 15 000 locations is available from the US National Snow and Ice Data Center (NSIDC). NSIDC has updated the Russian station snow depth data up to 2000 for over 200 stations. In addition, snow-water equivalent is observed in many countries by national, state, provincial, and private networks on a 10-30 day basis. The WWW/GOS surface synoptic reports for the United States are available through the NCEI. The Canadian Meteorological Center has produced global daily 1/3 degree snow-depth analyses, and daily snow-depth data from the WMO data stream. These data are available from NSIDC for the period March 1998 to the present. There is a new effort within the Asia Climate and Cryosphere (CliC) Project to obtain station snow depth data from as many sources in Eurasia as possible.

Snow-cover extent is mapped daily by operational satellites, but sensor channels change and continuing research and surface observations are needed to calibrate instruments, improve retrieval methods, and validate satellite products for snow depth and snow-water equivalent. The National Environmental Satellite Data and Information Service (NESDIS) of NOAA began producing daily Northern Hemisphere snow extent maps in 1966, with weekly maps available from 1966 to 1999 and daily maps available in subsequent years.

Southern Hemisphere snow extent maps have been available since 1999 from the NASA EOS MODIS sensor. NSIDC provides a weekly global snow extent product, which combines optical (MODIS) and passive microwave (SSM/I) data for the period 2000 to the present.

More recently, snow products have also been generated under ESA and EUMETSAT (SAF) auspices.

Global snow water equivalent (SWE) products have been available from AMSR-E since 2002, but they remain to be validated. Refinements to the algorithm continue as validation experiments are undertaken. Plans are underway with space agencies to develop new satellite capabilities for measuring snow parameters.

A global snow product generated from the blending of in situ and satellite data is one of the goals of the ESA GlobSnow Project, which is made up of an historical dataset comprising 15 to 30 years of snow data and which demonstrates an operational near-real time snow information service.

Many problems arise because: (a) snow-cover data are collected, even within one country, by several agencies with differing goals; (b) funding support for snow research is fragmentary and not well coordinated; (c) budget restrictions and attempts to reduce the cost of surface networks often result in reduced coverage or automated measurement using different instrumentation whose compatibility is not yet determined and (d) many existing datasets are not readily accessible.

The submission of in situ snow observations from the WWW/GOS surface synoptic network has continued to show some decline due to financial pressures in many countries that have led to closures of remote northern observation stations. In addition, there are major observational gaps in mountainous areas and in Antarctica. Data receipt from the remaining stations has also been an issue, with few stations including snow data in their submissions to the Global Telecommunication System (GTS) and not all providing the WMO SYNOP reports that normally include snow parameters. Furthermore, there is no systematic global monitoring of the amount and quality of in situ snow-related reports exchanged over the GTS. As a result, the creation of well calibrated satellite products has been made more difficult.

Maintenance of adequate, representative surface networks of snow observations must begin with documentation and analysis of the network densities required in different environments. Resolution of the problem of data inaccessibility requires: promoting political commitment to data sharing, removing practical barriers by enhancing electronic inter-connectivity and metadata, and data rescue and digitization. The provision of necessary resources to improve, and to make available, existing archives of snow data will require national efforts. The emerging WMO Global Cryosphere Watch is expected to provide facilitated access to such data.

Data Access