The Variability of the Snow Depth in the Northern Zone of Iran is based on the ECMWF Database of the ERA Interim Edition

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Snow is a vital component of the Earth's climate system because of its interaction with the energy flux and surface moisture on a local to global scale. This parameter significantly increases the relationships with radiation at higher latitudes. Analyzing changes in the amount of snow is essential for the assessment of the impacts of climate variability of a region. Snow cover has major effects on surface albedo and energy balance, and represents a major storage of water. The snow pack strongly influences the overlying air, the underlying ground, and the atmosphere downstream. Snow cover duration influences the growing season of the vegetation at high altitudes. A shortening snow season enhances soil warming due to increased solar absorption. While the importance of information on mountain snowpack is clear, obtaining these measures remains challenging. This is in part because snow depth and snow water equivalent (SWE) are both spatially and temporally variable, and mountain regions are generally difficult to access. Snow depth is one of the key variables for understanding the relationship between hydrological cycles. The flow of many rivers, especially during the warm period of the year, is mainly due to snow accumulation, which varies depending on the amount of snow melting in the time series. As mentioned, snow is an important hydrologic variable and acts as a water source in many parts of the world, especially Iran. In Iran, mountainous regions act as water suppliers for arid and semi-arid areas around them, and the coincidence of these conditions is one of the most important reasons for the creation of aqueducts in the country. This study, using the ECMWF data base of the ERA Interim, evaluates the trend and slope trend of snow depth (SD) in northern Iran. The achievements of this research can be useful for studies on climate change, water resources, flood, and agriculture. As a step toward addressing this challenge, we evaluated Methods to increase the efficiency of snow surveys and to enhance remotely derived estimates.
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In this study eleven districts of North Khorasan, Golestan, Mazandaran, Gilan, Tehran, Alborz, Qazvin, Zanjan, Ardebil, East Azarbaijan and West Azarbaijan have been studied. Interim was produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The database is available as an hourly basis since 1979. In this study, the spatial resolution of 0.125 × 0.125 degrees arc for the period 1980-2016 was used. Non-parametric Mann-Kendall and Sen's Slope methods were used to evaluate the trend and trend slope of snow depth.
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The assessment of the depth of snow in northern Iran in January shows that only 0.063% of the northern zone of the country has a significant increase in the level. These areas are more in the northwest of Iran on the border with Turkey, areas with no significant trend. An increase of 3.82% of the total study region has come from this month. These areas are located in the North Khorasan Provinceand near Bojnurd. Areas with increasing trend at 0.05, 0.01 and 0.001 levels have not been observed in northern zone of the country. The northwest regions of Iran on the border between Iran and Turkey, which show an increasing depth of snow, can be attributed to climate change affecting the systems leading to northwest Iran, with snow depth rising. January showed the lowest amount of snow depth for me-Kendall in winter. In this month, the maximum declined trend was 5.58 and the average trend was -14.3. Also, the average slope of the calculated trend in January was 0.03. This indicates that the depth of the snow with a negative slope of 0.07 cm is decreasing.
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The results show that snow depth in the north of Iran in winter is more than 96% of the studied area with decreasing trend. The significant decrease trend at the level of 0.001 in the winter is the maximum trend, and from January to March, the size of areas under the territory of this level increase the meaning of the trend, so that in January, February and March, respectively, 47.99, 56.08, 71.82 percent of the area of the northern zone of Iran has fallen into a declining trend at a probability level of 99.99 percent. Winter season of the Iran regions in the northwest and east, the increasing snow depth was observed that this trend is not incremental but significant. The maximum decreasing trend is snow depth in the provinces of Tehran, Qazvin, Zanjan and East Azarbaijan. In end of April areas with no significant decreasing trend with more than 51% of the same areas. The pattern of snow depth in the spring follows the same pattern in the winter. The average slope of the trend has also declined in line with the trend slowdown in April. On the contrary, the decreasing trend in autumn is based on the statistical results obtained in the study period. Snowfall increases in autumn in October and November, unlike other months in the northern regions of Tehran and southern Mazandaran province, especially in the central Alborz region.