Spatio-temporal Variations of Snow-covered Days in the Northwest of Iran using Remote Sensing Data

In this research, the spatiotemporal variations of snow-covered days (SCDs) in this region were analyzed using the data of the sixth version of MODIS Terra and MODIS Aqua sensors on a daily basis in the period of 2003-2020. In order to reduce the cloud cover effect, three algorithms were applied to the data. For the digital elevation model, the Digital Surface Model (DSM) of the Japan Space Exploration Agency was used. The relation between two snow-cover phenological components (SCAs and SCDs) and the relation between the SCDs and the altitude were investigated. The findings show an increase in SCDs in the months of November, December, and January. Maximum SCDs are observed in January in Sabalan Mountain and then Sahand. The reduction of SCDs in the spring and summer months is also affected by the two factors of latitude and altitude. The absolute maximum of SCDs in this region is observed at 160 days/ year in the mountain of Sabalan. Examining the changes in SCDs in March and April shows a decrease in SCDs in high-altitude classes. At the same time, it shows the increasing pattern of SCDs in November and December at many altitude levels. Analysis of the relation between SCA and SCDs in different months illustrated that SCAs has decreased in regions with more SCDs (heights) due to the reduction of topographic areas. The relation of SCDs and altitude also showed that the minimum of SCDs occurred in all altitude levels (even altitudes above 3500 m with 4 days) in August and the maximum occurred in December with 22 days at the altitude of 3500 m. SCDs decrease with increasing altitude in mountainous areas of 3500 to 4000 m, due to the increase of land slope and instability of SC in steep areas.