Inter-annual variations of snow days over Switzerland from 2000–2010 derived from MODIS satellite data

Abstract. Snow cover plays a vital role in the Swiss Alps and therefore it is of major interest to determine and understand its variability on different spatiotemporal scales. Within the activities of the National Climate Observing System (GCOS Switzerland) inter-annual variations of snow days over Switzerland were derived from 2000 to 2010 based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite. To minimize the impact of cloud cover on the MODIS snow product MOD10C1, we implemented a post-processing technique based on a forward and backward gap-filling approach. Using the proposed methodology it was possible to determine the total number of annual snow days over Switzerland from 2000 to 2010 (SCD_MODIS). The accuracy of the calculated snow days per year were quantitatively evaluated against three in situ snow observation sites representing different climatological regimes (SCD_{in_situ}). Various statistical indices were computed and analysed over the entire period. The overall accuracy between SCD_MODIS and SCD_{in_situ} on a daily basis over 10 yr is 88% to 94%, depending on the regional characteristics of each validation site. Differences between SCD_MODIS and SCD_{in_situ} vary during the snow accumulation period in autumn and smaller differences after spring, in particularly for the Central Alps.