**4. Results**

#### *4.1. Validation of the Daily Cloud-Free MODIS Snow Products*

Based on above accuracy assessment metrics, the available station data in Northeast China were screened to quantitatively evaluate the cloud removal results. The accuracies of the three evaluation indicators were obtained (Table 3), and Figure 2 summarizes the monthly accuracy evaluation results for ground observation stations. Overall, the *OA* of the MODIS cloud-free product was more than 0.9, the *UE* and *OE* were less than 0.1, and the *UE* values were greater than the *OE* values. Generally, the accuracies in the snow stabilization season were higher than that in the snow accumulation and melting season. However, in the October and April, the *OA* was the highest, and the *UE* and *OE* were the lowest, which may be attributed to the lack of snow during this period. There seems odd relative to others in the February, which is due to the lack of 9 days MODIS data (19 February 2016– 27 February 2016). In the cloud removal processing, the missing data are thought to be cloud. Therefore, the cloud removal accuracy is very low for these data because there is no spatial and temporal neighbor data available. The analysis reveals that the cloud removal product had good reliability and met the needs of the snow phenology analysis.

**Table 3.** Accuracy evaluation results for ground observation stations.


#### *4.2. Spatiotemporal Variations and Trends in Snow Phenology* 4.2.1. SCD

The distribution of SCD in Northeast China was visually consistent with those of topography and latitude, showing the characteristics of "high SCD in the mountains and low SCD in the plains" and "high SCD at high latitudes and low SCD at low latitudes". The SCD values in the northern Daxingan Mountains were more than 150 d, those of the Xiaoxingan Mountains were mainly within 120–150 d, and the SCD values of the plain area were significantly less than those of the mountainous area. The SCD values of the Sanjiang Plain were within 90–150 d, and the Liaohe Plain had the shortest SCD values of less than 60 d (Figure 3).

**Figure 3.** Spatial distribution of annual average SCD in Northeast China from HY2001 to HY2017.

Figure 4 showed the histogram of annual average SCD computed for a 10-day interval from HY2001 to HY2017. The average SCD in Northeast China ranged from 0 to 230 d, with an obvious bimodal distribution, and the annual average SCD was 93 d. The frequency of SCD in the range from 10–20 d accounted for 9.95%, and that in the range from 120–130 d accounted for 13.92%.

**Figure 4.** Histogram of annual average SCD from HY2001 to HY2017 in Northeast China.

Figure 5a indicates that the SCD in Northeast China mainly increased, accounting for 62.27% of the total, among which only 4.73% increased significantly. The regions where the SCD increased were mainly in the southern Daxingan Mountains, Xiaoxingan Mountains and Changbai Mountains. SCD decreased mainly in the northern Daxingan Mountains and the southwestern Changbai Mountains, accounting for 20.25% of the total area, and only 0.24% of the total area recorded significant decreases. Figure 5b showed the area with an

SCD trend >0 d/a accounted for 60.61%, which was similar to the spatial distribution of SCD with an increasing trend; the area—revealing no trend was concentrated in the Liaohe Plain and Songnen Plain, accounting for 16.80%; the areas with trends of less than 0 d/a accounted for 18.65% and were mainly located in the northern Daxingan Mountains and the southwestern Changbai Mountains. The results of the Theil–Sen trend and Mann–Kendall significance test were consistent, which further verified the accuracy of these trends.

**Figure 5.** Trend of SCD in Northeast China. (**a**) Significance test and (**b**) trend of SCD from HY2001 to HY2017.
