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In the ECMWF model (IFS), snow is represented by an additional layer on top of the uppermost soil level. The whole grid box may not be covered in snow. The snow cover gives the fraction of the grid box that is covered in snow. The method for calculating snow cover depends on the particular version of the IFS and for ERA-Interim is computed directly using snow water equivalent (ie parameter SD (141.128)) as:
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snow_cover (SC) = min(1, RW*SD/15 ) where |
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RW is |
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density of water equal to 1000 and RSN is density of snow (parameter 33.128). |
= 1000. The Physical depth of snow (SD) where there is snow cover is :SD = equal to RW*SD/(RSN*SC) where RSN = density of snow (parameter 33.128).
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