DESCRIPTION: SN-OBS-3
Effective snow cover by VIS/IR radiometry

Combined effect, within a product resolution element, of fractional snow cover and other reflective contributors is used to estimate the fractional cover at resolution element level. The product may be processed in different ways and have different quality depending on the surface being flat, forested or mountainous.

  • Coverage: The H-SAF area [25-75°N lat, 25°W-45°E long]
  • Cycle: Daily
  • Resolution: 5 to 10 km (0.05 degrees), depending on the location (best for northern parts, worst for southern parts of the H-SAF area)
  • Accuracy: Around 20 % - Depending on geographical location (flat/forested areas, mountainous regions)
  • Timeliness: Fixed time of the day, product updated to account for data available until 1 h before delivery
  • Dissemination: By dedicated lines to centres connected by GTS - By EUMETCast to most other users, especially scientific
  • Formats: Values in fixed latitude-longitude grid representing a resolution element of the used instrument. Also JPEG or similar for quick-look.

Short description of the basic principles for product generation

The product differs from SN-OBS-1 in so far as, in the snow map, the resolution elements report the fractional snow coverage instead of being binary (snow/snow-free). The possibility of appreciating fractional coverage stems from the lack of observed brightness in respect of what would be if the pixel were fully filled by snow. The forest canopy obscuring the full visibility to the ground is accounted for by applying certain a priori transmissivity information, which must be generated using satellite-borne reflectance data acquired under full dry snow cover conditions. These reflectances must be measured by the same instrument as which is used in the fractional snow coverage estimation. This means that the product can be generated for areas where full dry snow cover stays at least for some period and that during that period, cloud-free reflectance data are gained. Since the transmissivity approach may not work well for bare ground and vegetative lands (different from forest area) on mountainous regions, subpixel reflectance model is used. The topographic normalization is performed preferably, in order to eliminate the terrain effects on the reflectances of the features in the mountainous regions.