عنوان مقاله [English]
Climatology of snow cover accumulation and melting in Iran using MODIS data
Snow cover plays an important role in the water and energy cycle of the world due to its high albedo and thermal properties and it can also reflect global climate change (Dozier et al., 2008; Shi, 2012).
Precise monitoring of the extent of snow cover is an issue that has received much attention (Lampkin and Yool, 2004). Monitoring snow parameters, such as the extent of snow cover and snow equivalent water, are a challenging topic for meteorologists and climatologist. Snow cover plays an important role in balancing the earth's energy due to its high albedo and affects the climate (Akyurek et al., 2010). Many studies have been conducted to investigate snow cover and its changes. for example, Khadka et al. (2014), used the MODIS data for the period 2000–2009 to analyze snow trends in the Tamakoshi Basin of the Himalayan Mountains, The findings of the researchers showed that during the ten years of the study, there was a decreasing trend in the area of snow zones during spring and winter, while the area of snow zones had an increasing trend in the autumn season (Khadka et al. 2014). Sharma et al. (2012) applied MODIS data for the years 2000–2011 to analyze the snow trends in the Jalom River Basin and its sub-basin located in the northwest of the Himalayas. The results showed decreasing trend in all sub-basins with the highest negative rate for Banihal sub-basin (Sharma et al. 2012). Maskey et al. (2011) studied the trend of snow cover in and around Nepal for the years 2000–2008. For this purpose, they applied MODIS data. The findings show that in January for three elevation zones below 6000 m and in March for two elevation zones above 5000 m snow cover trend are seen. In the fall, snow cover showed increasing trend for four altitudes above 4000 m (Maskey et al).
Materials and methods:
In this study, the daily data of MODIS Terra (MOD10A1) and MODIS Aqua (MYD10A1) version 6 were applied for the period from 1380/7/1 to 1397/6/31. One of the problems that is always a big obstacle to monitoring snow cover is the cloud issue. Clouds cause the underlying snow cover to be hidden from the satellite view. To reduce cloudiness and better observation of snow cover, various methods and strategies have been suggested by various researchers to minimize cloud cover effects. The applied filtering methods included combination of MODIS Terra and MODIS Aqua, spatial combination with four pixel neighbor and 1 to 5-day temporal windows. All calculations were performed using programming operations in MATLAB software and at the climatological laboratory of the University of Isfahan.
Results and discussion:
In order to better present the research findings and to classify them, calculations related to the accumulation and melting timing of snow cover in Iran have been provided spatially and temporally for each of the accumulation and melting parameters. Climatological survey of snow cover accumulation in Iran shows among the first days that the earth is covered with snow and the elevation there exist a strong relation, in other words, the first days of snow cover occur at the highest altitudes and slowly move to the lower elevations as we move toward autumn. Analyses shows the first days of snow cover in the first decade of Mehr are seen on the Alborz Heights and the Sabalan Heights in the northwest. Snowfall days in other northwest elevations begin around the second half of late Mehr. The onset of the snow cover season on the central Zagros highs begins around the first decade of Aban and covers many of the lower elevations until late Azar. It seems that the delay of the onset of the snow cover season on the Zagros Mountains compared to the northern and northwestern highlands of the country is due to the delay in autumn rainfall rather than the necessarily warmer temperature.
A long-term study of the average snow cover melting in Iran shows that there is a very strong relationship between altitude and the last day when the ground is covered with snow. The analyses show that, on average, the last day when the ground is covered with snow starts from lower altitudes and slowly migrates to higher altitudes. For example, in the lower elevations of the Zagros Mountains the snow cover on the ground is melted in the middle of February, in the higher elevation areas due to the colder weather the last day when the ground is covered in snow are seen in the late of winter.
However, there is snow cover in the Zardkouh highlands until the end of June and even early July, and afterwards, these areas lose their snow cover. On the Alborz Mountains due to their higher altitude and higher latitude, snow cover will persist even in some areas until late Amordad and early Sharivar. During the months of snow accumulation in Iran, which covers the months of Mehr to Azar, snowline moves downwards at an average rate of 54 m / day, while during the months of snow melting (Dey to Shahrivar) snowline migrate to higher altitudes at the rate of 15 meters per day.
The purpose of the present study is to study the climatological accumulation and melting of snow-covered days in Iran. In this regard, the sixth version of the MODIS Terra and MODIS Aqua daily data at the spatial resolution of 500 × 500 m for the period from 1380/7/1 to 1397/6/31 were downloaded from the NASA Web site. In order to reduce the cloud effect on the data, three data refinement techniques were applied on the raw data including combination of MODIS Terra and MODIS Aqua, spatial combination with four pixel neighbor and 1 to 5-day temporal windows were applied to the raw data. The findings of this study show that the first snow-covered days are seen in the first decade of Mehr over the Alborz and Sabalan highlands and gradually snowline stretches to lower altitudes at 54 m / day. But during the melting months of snow cover, which includes Dey to Sharivar, snowline migrates to higher altitudes at a rate of 15 m / day. The reason for the slower rate of snowmelt migration to higher altitudes in the melting season can be attributed to the highly effective role of temperature in the melting season, but in the snow cover accumulation season, the role of temperature and precipitation mechanisms lead to the snow cover moving to lower altitudes.
Keywords: Iran, Snow accumulation, Snow melting, Snowline, MODIS.