GCOS Terrestrial ECV T5
Snow Cover

Introduction: Snowfall and snow cover play key roles with respect to feedback mechanisms within the climate system (albedo, runoff, soil moisture, and vegetation) and are important variables in monitoring climate change. About one third of the Earth's land surface is seasonally snow-covered and seasonal snow melt is a key factor in runoff regimes in middle and high latitudes as well as in many other high altitude locations. Snow depth and snow-cover duration affect the permafrost thermal state, the depth and timing of seasonal soil freeze/thaw /break-up, and the melting of land ice and sea ice. 
 
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 instrumentationwhose 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 WMO 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 (GCW) is expected to provide facilitated access to such data.
 
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 NSIDC. Updates are expected. 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 NCDC. The Canadian Meteorological Centre 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 CliC Project to obtain station snow depth data from as many sources in Eurasia as possible.

To assist in providing global coverage of snow extent and snow water equivalent, optimal procedures to generate blended products of surface observations of snow cover with visible and microwave satellite data and related airborne measurements need to be agreed upon and implemented by national services and research groups involved in snow mapping. The Climate and Cryosphere Project (CliC) of the WCRP should take the lead in organizing this with GEWEX and other involved working groups.
 
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 MODIS sensor. NSIDC provides a weekly global snow extent product, which combines optical (MODIS) and passive microwave (Special Sensor Microwave/Imager (SSM/I)) data for the period 2000 to the present. Agencies currently generating Northern Hemisphere snow-cover products (particularly NASA groups and NOAA/NESDIS) should also routinely generate and archive Southern Hemisphere products. More recently, snow products have also been generated under ESA and EUMETSAT (Satellite Application Facility (SAF)) auspices. TOPC, in cooperation with the AOPC and the International Association of Cryospheric Sciences (IACS), should approach research and space agencies (through CGMS, CEOS, and the WMO Space Programme) to seek commitment to provide snow-cover products for both hemispheres. 
 
Global snow water equivalent (SWE) products have been available from the Advanced Microwave Scanning Radiometer for EOS (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.
 
Development of snow products that blend multiple data sources and are globally applicable needs urgent focused attention. The research community through WCRP CliC could help lead such an effort. 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. 
 
The TOPC, in consultation with the AOPC, WCRP CliC and WMO Technical Commissions, will consolidate and, where necessary, recommend standards and protocols for measuring snow cover and SWE, design an optimum network, and recommend International Data Centre and analysis centre responsibilities. TOPC’s current cryosphere activities can provide a starting point, but the required activity would need dedicated funding for meetings and workshops in which to agree on standards and protocols (cf. T1), funding for report preparation, and funding for filling gaps in networks. The development of guidelines and standards is one of the tasks of the evolving Global Cryosphere Watch.(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Observations: Regular measurements of terrestrial snow cover are important because snow dramatically influences surface albedo, thereby making a significant impact on the global climate, as well as influencing hydrological properties and the regulation of ecosystem biological activity. The IPCC has found that – on the evidence of satellite data – there is likely to have been a decrease of about 10% in the extent of snow cover since the late 1960’s. Snow forms a vital component of the water cycle. In order to make efficient use of meltwater runoff, resource agencies must be able to make early predictions of the amount of water stored in the form of snow. Coverage area, snow water equivalent and snow pack wetness are the key parameters to be determined in this process. Snow cover information has a range of additional applications such as detecting eas of winterkill in agriculture that result from lack of snow cover to insulate plants from freezing temperatures. Locally, monitoring of snow parameters is important for meteorology and for enabling warnings of when melting is about to occur – which is crucial for hydrological research and for forecasting the risk of flooding. A range of different instrument types can contribute to measurements of snow. Visible/near-infrared satellite imagery provides information of good horizontal and temporal resolution and accuracy on snow cover in the day-time in cloud-free areas. AVHRR provides snow cover information and this will be continued in the future by VIIRS. MODIS data are being used to monitor the dynamics of snow and ice cover over large areas (greater than 10 km2) and, on a weekly basis, to report the maximum area covered by both. The resulting snow maps should be available within 48 hours of MODIS data collection. Passive microwave instruments such as SSM/I, AMSR and CMIS have all-weather and day/night monitoring capability, and are able to estimate the thickness of dry snow up to about 80 cm deep. Data from RADARSAT and ERS-2 have shown the usefulness of SAR remote sensing techniques to determine snow area extent and to monitor the physical conditions of snow. Envisat, ALOS and RADARSAT-2 are providing continuity of such snow information. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observation Plans by Measurement)

NSDIC National Snow and Ice Data Center - Advanced Data Search

References:

• Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523
• The Second Report on the Adequacy of the Global Observing Systems for Climate in Support of the UNFCCC - April 2003 (GCOS-82/WMO/TD Noc 1143)
• ECV T5 Snow Cover - Assessment of the Status of the Development of the Standards for the Terrestial Essential Climate Variables (ECV), Version 8, 21 May 2009 (GTOS-60)
• ECV T5 Snow Cover web page on the FAO/GTOS web site
• NOAA NWS Snow Measurements Guidelines
• All About Snow (NSIDC)
• CEOS EO Handbook - Earth Observation Plans by Measurement
• NOAA/NCDC - Climate Indicators
• BAMS State of the Climate Reports
  

Data, Product, Metadata and Information Access
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