*5.2. Influence of Snow Phenology Parameters*

Both the simulation experiments and satellite data analysis showed that the presence of snow could significantly reduce the VI values, increase the local gradient of the time curve of VI during pregrowth periods, and cause the SOS to be detected. The bias in the SOS (ΔSOS) caused by snow cover depends on snow phenology parameters, especially the SCDc, ESS, and snow-free SOS.

In general, the SCDc represents the maximum duration of snow cover, corresponding to the length of the reduced values in the VI trajectory, which determines whether the reduced VI values could be recovered from temporal filtering. Thus, the larger the SCDc value was, the larger the absolute value of ΔSOS. Compared to the SCDc, the SCDr only represents overall snow-covered days. A high SCDr does not necessarily imply a high SCDc, while a high SCDc does lead to a high SCDr. Thus, the relationship between the SCDr and ΔSOS was rather indirect and was not discussed in detail.

Whether the ESS is earlier or later than the snow-free SOS determines whether ΔSOS is negative or positive, corresponding to the advancement or delay of the SOS detected under snow conditions. The further the temporal distance of ESS to the snow-free SOS was, the larger the absolute value of ΔSOS. These findings were consistent with previous studies [37,38,63,64]. An earlier ESS results in an earlier estimate of SOS, while a later ESS results in a later estimate of SOS. For example, Wang et al. [38] analyzed the correlation between the duration of snow cover and the SOS on the Qinghai–Tibet Plateau and found that snow cover can advance the SOS in the northeastern, central, and southwestern edges of the Qinghai–Tibet Plateau; however, in some areas, longer snow cover duration delayed the SOS. Xie et al. [64] found that a shortened snow duration advances the SOS, whereas a prolonged snow duration delayed the SOS in their study in the European Alps.
