The GlobSnow SWE product is the first satellite based daily SWE dataset for the non-alpine northern hemisphere that extends from 1979 to 2014. The previous existing daily SWE records have spanned a shorter time period (2002-2014) or described the snow conditions on a monthly basis for a similar period (1978-2014).
The GlobSnow SWE record utilizes a novel data-assimilation based approach for SWE estimation which combines weather station measurements of snow depth with satellite passive microwave measurements. This approach was shown to be superior to alternative algorithms which solely utilize satellite data through comparison with extensive ground reference datasets.
The GlobSnow-1 and -2 projects have developed a long term data record of SWE products covering the non-alpine Northern Hemisphere, based on a time series of remotely sensed observations from the Nimbus-7 SMMR, DMSP F8/F11/F13/F17 SSM/I(S) instruments and ground-based weather station measurements from 1979 until 2014.
There are three SWE products (all on the EASE model grid; see Armstrong and Brodzik, 1995):
- Daily Snow Water Equivalent (Daily L3A SWE), snow water equivalent (mm) for each grid cell for all evaluated land areas of the Northern Hemisphere.
- Weekly Aggregated Snow Water Equivalent (Weekly L3B SWE), calculated for each day based on a 7-day sliding time window aggregation of the daily SWE product.
- Monthly Aggregated Snow Water Equivalent (Monthly L3B SWE), a single product for each calendar month, providing the average and maximum SWE, calculated from the weekly aggregated SWE product.
The GlobSnow-1 project resulted in two versions of the data record, SWE v1.0 and SWE v1.3 (available from FMI). The dataset produced in GlobSnow-2 is identified as the GlobSnow SWE v2.0 data record.
In addition to the SWE retrievals, the SWE products include information on the overall extent of snow cover. The information on snow extent is included in the product by utilizing the following coding for the SWE product, whereby SWE values of:
- 0 mm denotes snow-free areas (Snow Extent 0%)
- 0.001 mm denote areas with melting snow (Snow Extent undefined between 0% and 100%; no SWE retrieval because of the wet state of the snow cover)
- > 0.001 mm denote areas with full snow cover (Snow Extent 100%)
The areas that have been flagged as snow-free or melted are identified using a time-series melt detection approach described in Takala et al. (2009). The areas that are identified as wet snow or have no SWE retrieval, but are identified as snow covered with the time-series melt-detection approach, are denoted with a SWE value of 0.001 mm. The areas that are determined as snow-free or melted by the melt-detection approach, are denoted with a SWE value of 0 mm. All the other areas show a retrieved SWE value (that is in all cases greater than 0.001 mm).
The project was coordinated by the Finnish Meteorological Institute (FMI). Other project partners involved are NR (Norwegian Computing Centre), ENVEO IT GmbH, GAMMA Remote Sensing AG, Finnish Environment Institute (SYKE), Environment Canada (EC), Northern Research Institute (Norut), University of Bern, Meteoswiss and ZAMG.
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