DOI: 10.5285/375ffdb8f0a445e380b4b9548655f5f9
title: ESA Snow Climate Change Initiative (Snow_cci): Daily global Snow Cover Fraction - snow on ground (SCFG) from MODIS (2000-2023), version 4.0

description: This dataset provides daily Snow Cover Fraction on Ground (SCFG) derived from Terra MODIS observations, produced within the ESA Climate Change Initiative Snow project.

SCFG expresses the proportion of land area within each about 1 km x 1 km pixel that is covered by snow. In forested areas, the masking effect of the forest canopy is corrected to estimate the SCFG. The SCFG is given in percentage (%) per pixel.

The SCFG product is available at about 1 km pixel size for global land areas except the Antarctica and Greenland ice sheets and permanent snow and ice areas. The coastal zones of Greenland are included. The SCFG time series spans 24 February 2000 to 31 December 2023.

The SCFG product is based on Moderate resolution Imaging Spectroradiometer (MODIS) data on-board the Terra satellite. For the SCFG product generation from MODIS, multiple reflective and emissive spectral bands are used. In a first step, clouds are masked using an adapted version of the Simple Cloud Detection Algorithm (SCDA) (Metsämäki et al., 2015). For all remaining pixels, the snow_cci SCFG retrieval method is applied, using spectral bands centred at about 0.55 µm and 1.6 µm, and an emissive band centred at about 11 µm. The snow_cci snow cover mapping algorithm is a two-step approach that first identifies pixels which are assessed as snow free, followed by SCFG retrieval for remaining pixels. 
Permanent snow/ice and water bodies are masked using the Land Cover CCI 2000 dataset, supplemented by a manually mapped salt-lake mask. Per-pixel uncertainty is provided in the ancillary variable as an unbiased Root Mean Square Error (RMSE) for all observed land pixels.

Compared with SCFG CRDP v3.0 (https://catalogue.ceda.ac.uk/uuid/80567d38de3f4b038ee6e6e53ed1af8a/), the SCFG CRDP v4.0 includes the following improvements: 
• more permissive pre-classification allowing more pixels to enter the SCFG retrieval; 
• correction function applied to spectral reflectance for improved SCFG retrieval at low solar illumination conditions;
• updated spectral reflectance layers for snow free ground and snow free forest to improve SCFG retrieval;
• updated uncertainty estimation to account for the changes in the SCFG retrieval;
• improved merging method for generating daily global SCFG products;
• updated salt lake mask;
• extended time series, to December 2023.

There are several days with no MODIS acquisitions in the years 2000, 2001, 2002, 2003, 2008, 2016 and 2022. In addition, on multiple days between 2000 and 2006 and in 2023, as well as on single days in 2012, 2015 and 2016, 2018, and 2020, the available MODIS data exhibit either limited spatial coverage, or corruption during data download. SCFG products are provided for all of these days, but they contain data gaps.

The SCFG product is aimed to support cryosphere and climate research applications, including variability and trend analyses, climate modelling and studies in hydrology, meteorology, and ecology.
ENVEO leads the SCFG product development and product generation from MODIS data, with contributions on the product development from Syke.