**3. Methodology**

*3.1. Data Sources*

3.1.1. MODIS Data

Daily MOD10A1 (V006) data with a 500 m resolution and containing 6574 scenes during 2001–2018 were downloaded with the help of the Google Earth Engine (https://code.earthengine.google.com (30 October 2021)). The product has four data types: NDSI data, quality assessment, albedo, and orbital information. In this research, only NDSI\_Snow\_Cover data were used as the main data source to extract snow products, and the NDSI of snow cover always greater than 0, but not all surface features with NDSI > 0 is snow cover. Detailed attribute information is shown in Table 1 [32].



#### 3.1.2. Snow Depth Records

The snow depth records in Northeast China from 1 January 2013 to 31 December 2018 were collected from the National Cryosphere Desert Data Center (http://www.ncdc.ac.cn (30 October 2021)), and these records in snow season (October to April of the following year) were used to validate the cloud removal accuracy. The station distribution is shown in Figure 1. Generally, SD ≥ 1 cm indicates snow cover; otherwise, the station is regarded as snow-free [6,12,33].
