Ilmatieteen laitos

Specifications for the Snow Extent product of GlobSnow

Based on the discussions at the Requirements Engineering Review (3 February Geneva), the following specifications are proposed for the Snow Extent Product:

Coverage: Global (excluding glaciers, Greenland, Antarctica and snow on ice: lakes/seas/oceans)
Time period: From 1995 (16 years of historical products at the end of the project in 2011)
Temporal frequency:

o Basic product: Weekly and monthly, with full global coverage, based on medium resolution optical imagery.

o Secondary product: Daily. Daily repeat is not possible for global products; in case of optical imagery because of clouds; in case of SAR because of its insensitivity to dry snow (only wet snow can be detected) and because of limitations in data availability and costs. The relevance of such an intermittent product needs to be discussed with the users.

Spatial resolution: 1 km globally for 1995-1999; from 2000 onwards 1 km globally and 250 m to 500 m for complex terrain
Geometric accuracy: Sub-pixel (aiming at 0.5 pixel)
Thematic accuracy: Binary classification (snow/no-snow) with global total (pooled) error ≤ 5 % for open terrain, sparse forest and non-steep mountainous regions, at solar elevation > 20°. Temporal inference of snow in dark regions or masked as ‘not observed’
Grid/projection: WGS 84 Latitude/longitude grid with 0.01 degree spatial resolution.

The added value of the GlobSnow SE product will be:

The longest historical time series of moderate spatial resolution available
Potentially the highest thematic accuracy available

Snow Extent product

The snow extent product will be created using ESA ERS-2 ATSR-2 and Envisat AATSR data.

ATSR-2 and AATSR operating onboard of ERS-2 and ENVISAT since 1995 and 2002, respectively, provide a 15 years data set for producing global snow extent maps at a resolution of about 1 km. The spectral characteristics and resolution of ATSR-2 and AATSR are summarized in Table 1.

AATSR and ATSR-2 data are available as gridded radiances with 1 km resolution (all bands, nadir and forward looking), stored in the same data format. One full orbit of AATSR-products is about 300 MB (compressed by gzip), one day global AATSR- data set is about 4.2 GB, one year global data set is about 1.5 TB.

Table1:
Characteristics of ERS-2 ATSR-2 and ENVISAT AATSR.

ATSR-2, AATSR Specifiations
Spectral Channels [µm]	0.55, 0.66, 0.87, 1.6, 3.7, 11.0
Resolution	1.03 x 1.03 km at nadir
Swath Width	500 km, 555 pixels
Global Coverage	35 and 168 day cycles
Signal Quantization	12-bit

Both sensors provide VIS and SWIR channels which is especially important for snow / cloud discrimination. The swath width is ca 500 km. As an example Figure 1 shows the coverage of AATSR for the Northern Hemisphere on 24 May 2008. Figure 2 and Figure 3 show detailed views of the 3 and 6 days coverage of Europe. Within 6 days Europe is covered by about 99%, only small diamond shaped data gaps are found south of ca 40°N. AATSR and ATSR-2 data are available as gridded radiances with 1 km resolution (all bands, nadir and forward looking), stored in the same data format. One full orbit of AATSR-products is about 300 MB (compressed by gzip), one day global AATSR- data set is about 4.2 GB, one year global data set is about 1.5 TB.

Figure 1: 1 day coverage of AATSR, Northern Hemisphere, 24 May 2008.

Figure 2: 3 days coverage of AATSR, Europe, 24 -26 May 2008.

Figure 3: 6 days Coverage of AATSR, Europe, 24 – 29 May 2008