A snow reanalysis for Italy: IT-SNOW

Quantifying the amount of snow deposited across the landscape at any given time is the main goal of snow hydrology. Yet, answering this apparently simple question is still elusive -- particularly in complex and high-elevation terrains where data are sparse. To contribute to the advancement of snow hydrology in Mediterranean regions, we present the first serially complete and multi-year snow reanalysis for Italy (IT-SNOW). IT-SNOW covers the period from September 2010 to August 2021, with future updates envisaged on a regular basis. This reanalysis is the output of a real-time snow and glacier monitoring chain – S3M Italy -- developed for the Italian Civil Protection Department by CIMA Research Foundation. Spatial resolution is 500 m, with input data coming from thousands of weather stations across the Italian territory. By assimilating blended snow-covered area maps from Sentinel-2, MODIS, and the Eumetsat H-SAF products, as well as interpolated snow-depth maps from in-situ data, IT-SNOW optimally combines dynamic modeling and data towards reconciled estimates of snow amount and water equivalent at various scales. IT-SNOW was validated using Sentinel-1-based maps of snow depth and in-situ snow data in the Alps and the Apennines, with little bias compared to the former and typical Root Mean Square Errors of 30 to 60 cm and 90 to 300 mm for snow depth and Snow Water Equivalent, respectively. A comparison at 102 gauge stations showed a strong (0.87) correlation between peak SWE in IT-SNOW and measured annual streamflow, with snow being 22% of annual streamflow on average. IT-SNOW is freely available at the following DOI: https://doi.org/10.5281/zenodo.7034956 and we encourage users to validate and provide critical feedback for future releases.