*3.1. Data*

The SD data were obtained from the long-term series of the daily snow depth dataset in China (1979–2020), released by the National Tibetan Plateau data center (http://data. tpdc.ac.cn, accessed on 11 October 2021). The data were derived from the inversion of daily passive microwave brightness temperature (SMMR, SSM/I, and SSMI/S), with a spatial resolution of 0.25◦ [51]. Based on the daily SD data, the monthly and annual maximum SD data were obtained using the maximum value composite (MVC) method, which represented the maximum value in the process of snowmelt and accumulation [52]. The snow cover data were adopted from the MODIS Daily Cloudless 500 m Snow Area Product Dataset over China during the period from 2000 to 2019, released by the National Cryosphere Desert Data Center (http://www.ncdc.ac.cn, accessed on 28 October 2021), used for LSSP calculation [53]. A DEM with a 30 m spatial resolution released by the Chinese Academy of Sciences Geospatial Data Cloud (http://www.gscloud.cn, accessed on 5 July 2018) was used to calculate the slope and aspect across the SGP. The surface net solar radiation data were released by the European Centre for Medium-Range Weather Forecasts (ECMWF) (https://www.ecmwf.int/, accessed on 17 April 2019), with a spatial resolution of 0.25◦ and a temporal resolution of 3 h. The meteorological data, including precipitation, maximum temperature, and minimum temperature, were from the National Meteorological Information Center of China Meteorological Administration (http://data.cma.cn/, accessed on 21 August 2021). Snow depth observation data came from the National Meteorological Information Center of the China Meteorological Administration and are used to test the accuracy of the downscaling. The time spans of the above data (except for the DEM) were unified to the same period from 2002 to 2018 for simultaneity.
