# Synergistic Potential of Optical and Radar Remote Sensing for Snow Cover Monitoring
## 3 Data and Methods
### Methods
#### 3.1.1 Snow Characteristics  
The focus montane regions are distributed throughout the Spanish territory, necessitating the merging of data from multiple MODIS granules (h17v04, h17v05, and h18v04) using the HDF-EOS to GeoTIFF Conversion Tool. For the comparison among the different mountain ranges, we used the Probability of Snow Cover Presence Index (PSCPI) [50], which is computed pixel-by-pixel as follows:  
\[PSCPI=\frac{N_{S}}{N_{A}}\times 100 \tag{1}\]  
where N\({}_{\text{S}}\) is the number of days with snow cover presence (days where the NDSI equals or exceeds the standard threshold of 0.4) and N\({}_{\text{A}}\) is the number of days with available NDSI data, so that it refers to the average number of days covered by snow over the 23-year period of records. In this study, snow cover on a pixel was defined when the NDSI is  
Figure 2: Flowchart of the methodology.  
equal or exceeds the standard NDSI threshold of 0.4 [51]. This threshold has traditionally been used and corresponds to a fractional snow cover equal to or exceeding 50% [52; 53]. Recent research suggests that this threshold value may not be optimal for local-scale studies [54]; however, it has been demonstrated to be suitable for products with moderate spatial resolution (around 500 m or greater) for SCA classification [54; 55]. Subsequently, we determined the average PSCPI at 10 m elevation intervals and then defined the area of interest of each mountain range as the regions where the average PSCPI exceeds 1.5%. To establish the elevation interval, it was essential to ensure an adequate number of MODIS cells for calculating the average PSCPI in the smallest mountain range within the Iberian Peninsula. It is important to note that the Iberian mountains vary considerably in size, with larger mountains providing a greater number of MODIS cells for calculating the average PSCPI. These defined domains were utilized to analyze various snow-related questions proposed in this study, providing insights into SCA dynamics across different elevations within each mountain range.  
Firstly, we realized an intercomparison of PSCPI variations with elevation among the mountain ranges of the Iberian Peninsula. Secondly, we compared the duration of the snow-covered season delineated across diverse elevation intervals defined at 500 m increments with the aim of discerning potential differences among mountain ranges and within specific elevation bands. Snow season duration is defined as the time interval with consistency on snow cover, which is determined by onset and offset dates. In this research, the onset of the snow season was identified as the first day when the PSCPI surpasses 10% and remains at or above this value for at least two-thirds of the time over the following four weeks. The offset of the snow season was qualitatively defined as the first day when the PSPI reaches minimum values and remains at those levels for at least two consecutive weeks. Furthermore, we examined the relationship between latitude and the elevation of the snowline. This analysis involved determining snowline elevations under various PSCPI thresholds. The strength of the relationship between the latitude and elevation of the snowline was assessed using the coefficient of determination (R2) from a linear regression.