Abstract
Seasonal snow is an important component of the global climate system. It is highly variable in space and time and sensitive to short term synoptic scale processes and long-term climate-induced changes to temperature and precipitation. Current snow products derived from different algorithms applied to various satellite data show significant discrepancies in extent and snow mass, a major source of uncertainty for monitoring and climate model verification. The ESA CCI+ Programme addresses seasonal snow as one of nine Essential Climate Variables derived from satellite data.
In the first phase of the snow_cci project (2018 - 2021), reliable and fully validated processing lines for the generation of snow climate data records were developed and implemented. Homogeneous multi-sensor time series of daily snow extent and snow water equivalent with global coverage were generated. Using GCOS guidelines, the product requirements for these parameters are assessed and consolidated using workshops and other engagement with users dealing with different climate applications. The retrieval algorithms for fractional snow extent provide consistent daily products for snow viewable from space (viewable snow) and snow on the surface corrected for forest masking (snow on ground) with global coverage. Input data are medium resolution optical satellite imagery (AVHRR-2/3, A/ATSR-2, MODIS, SLSTR) from 1982 to present. For the snow_cci Climate Research Data Package version 2, an iterative development cycle was implemented to improve and homogenise the snow extent products from different sensors. Independent validation of the snow extent products is performed using high resolution snow maps from Landsat and Sentinel-2 acquired across different seasons and climate zones around the globe from 1985 onwards as well as in-situ snow data following protocols developed within the snow community. Time series of global (non-mountain) daily snow water equivalent (SWE) products are generated from passive microwave data from SMMR, SSM/I, and AMSR from 1979 onwards combined with in-situ snow depth measurements. Long-term stability and quality of the product is assessed using independent snow survey data and by intercomparison with snow mass information from global land surface models.
We will present an overview of the algorithms and systems for generation of snow_cci snow products available at the ESA Open data portal. The 40 years (from 1982 onwards) time series of daily fractional snow extent products from AVHRR with 5 km pixel spacing, and the 20 years timeseries from MODIS (from 2000 onwards, 1 km pixel spacing) as well as the coarse resolution (12.5 km pixel spacing) daily SWE products from 1979 onwards will be presented along with the results of the multi-sensor consistency and validation activities and inter-comparisons with snow products from other sources. The impact of the snow_cci products within the use cases carried out by the Climate Research Group on long term snow extent and mass trends, evaluation of CMIP-6 experiments, and the simulation of long-term changes in Arctic hydrological regimes will be summarized.