Identifying changes in the beginning end and length of the snow season in Iran using MODIS data

The purpose of this research is to analyze the beginning, end and length of the snow season in Iran. For this purpose, daily data from the MODIS, spanning from 20/03/2000 to 19/03/2020, was obtained from the NASA website at a spatial resolution of 500 meters. First, temporal and spatial filters were applied to eliminate the impact of cloudiness on the data. Furthermore, the first day when a cell was reported to have snow cover (based on the elapsed days since the start of Mehr) was determined for every water year. This process was conducted separately for each water year, ultimately yielding a matrix of dimensions 20 × 7541502, where each row represented the first day that the MODIS detected snow on a given cell, and each column represented a spatial cell. Similarly, the same procedure was employed to calculate the end and length of snow season. The findings indicate that, in general, the beginning of snow season is shifting towards the winter season in the western parts of the country. Conversely, in the northwestern regions, including the northern slopes of Alborz and certain areas of the southern Zagros, the beginning of snow season is transitioning towards the autumn season. Moreover, in many highland areas in the west and northwest, as well as on the Alborz mountain range, the end of the snow season is progressively occurring towards late winter and spring.