بررسی پوشش برف در کشوری همچون ایران، امری بسیار حیاتی است و تمدن ایرانی به پوشش برفی کوهستانهای این کشور وابسته است. هدف از پژوهش حاضر، بررسی تغییرات زمانی و مکانی زمانبندی فصل انباشت پوشش برف در ایران است. بدین منظور در گام نخست، دادههای نسخة ششم سنجندة مودیس تررا و مودیس آکوا برای دورة زمانی 1/1/1379 تا 31/6/1397 بهصورت روزانه و در تفکیک مکانی 500×500 متر برای کل گسترة ایران دریافت شد؛ سپس بهمنظور کاهش اثر ابرناکی در دادههای ماهوارهای، سه روش کاهش اثر ابرناکی بر دادههای خام اولیه اعمال شد. در مرحلة دوم به تفکیک هر سال آبی روی هریک از 7541502 یاختهای که گسترة ایران را میپوشاند، نخستین روز گزارش برف با سنجندة مودیس (برپایة روز اول مهر) استخراج و بهمثابة شروع فصل برف روی یاختة مدنظر ثبت شد. درنهایت آرایهای در ابعاد 7541502×1 ساخته شد که روی هریک از ستونهای آن، شمار روز مربوط به نخستین بارش برف ثبت شده بود. این فرایند روی 18 سال دادة موجود تکرار و درنهایت آرایهای در ابعاد 7541502×18 تولید شد که در آن سطرها، نمایندة زمان و ستونها، نمایندة یاختههای مکانی بودند. بررسیها نشان میدهد در ارتفاعات غربی و شمال شرقی کشور، آغاز فصل انباشت پوشش برف در حال پیشروی به سوی زمستان است؛ این در حالی است که در ارتفاعات شمالی و شمال غرب کشور، شروع فصل انباشت پوشش برف در حال رفتن به عقب یعنی پسروی به سوی پاییز است.
عنوان مقاله [English]
Trend analysis of snow accumulation season start in Iran using remote sensing data
By advances in remote sensing techniques, it has been possible to better monitor environmental phenomena. Satellite data with appropriate temporal and spatial coverage along with high spatial resolutions are valuable resources in environmental studies. During the past two decades, many studies have been conducted to evaluate the temporal variations of accumulation and melting as well as the seasonal characteristics of snow covers using remote sensing data. For instance, Hall et al. (2015) used Landsat satellite data from 1972 to 2013 to study changes in snowmelt time in the Fremont Lake Basin in the United States. The results have shown that snow melting time in this region begins earlier in the 2000s than in the 1972-1999 periods and that increasing nighttime temperature in spring and autumn resulted in earlier melting of snow cover. In another study, Bi et al. (2015) used MODIS data for the period 2003–2013 to investigate the time of beginning and end of snow cover in the northeast Tibetan Plateau. Their findings showed that although during the study period, the timing of snow accumulation and ablation varied, the spatial distribution of the number of snow-covered days has shown a stable behavior.
In the present study, in order to investigate the trend of changes at the beginning of the snow cover accumulation season in Iran, the version six of MODIS Terra (MOD10A1) and MODIS Aqua (MYD10A1) for the time span from 1379/7/1 to 1397/6/31 (1999 to 2007) was used. One of the natural barriers to monitoring snow cover is the cloud issue problem. Cloud contamination causes snow cover to hide beneath the satellite's field of view. Various techniques have been suggested by researchers to reduce cloudiness and better observation of snow cover. In the present study, three techniques including the combination of two satellite and temporal and spatial filtering were applied to the raw data. The application of these three techniques resulted in a significant reduction of the cloud cover effect on the data. After the preparation and construction of the snow cover dataset, the first day of the snow cover reported by the MODIS (based on the first day of October) was computed and that day was defined as the start of the snow cover season, this process was carried out for all the 7541502 pixels in the country. This process was repeated over 18 years of available data and finally, the slope of the regression line was calculated on more than seven million pixels.
Calculations of the trend of spatial variations of the snow cover accumulation season showed that generally there was a significant increasing trend in the western parts of the country on the Zagros Mountains, it means that the first time that snow was accumulated during the snow cover season is advancing towards late fall and winter. In a clearer sense, it means that the first day of snow cover was occurring later in these areas. But in parts of central Zagros, the beginning of the snow cover season had a negative trend, meaning that the early days of the snow cover were occurring sooner than the long-term condition. The negative trend of the beginning of the snow cover accumulation season was also seen in the Alborz Mountains and the northwestern parts of the country. This negative trend appeared to be due to the earlier onset of autumn precipitation in the study period. Investigating temporal changes at the beginning of snow cover accumulation season in Iran, on each of the altitude belts in 50 meters, spatial steps showed that up to the altitude belt of 2750 meters, the beginning of snow accumulation season was advancing toward late fall and winter. However, from the altitudes of 2800 meters upwards, the onset of the snow cover season was turning backward. In a clearer sense, it was moving back toward the early autumn season.
The purpose of the present study was to investigate the temporal and spatial variations of the beginning of snow cover accumulation season in Iran. For this purpose, MODIS Terra and MODIS Aqua data were obtained at the spatial resolution of 500 × 500 meters for the time interval from 1379/7/1 to 1397/6/31 (1999 to 2007). After the construction and preparation of the snow cover database in Iran, the trend of changes at the beginning of the snow cover accumulation season was calculated temporally and spatially. The beginning of the snow cover accumulation season means the first day that a pixel was covered by snow based on the first day of October. For each hydrological year, the beginning of the snow cover season was calculated for each of the MODIS pixels and finally, the regression slope of changes was then computed. The findings showed that snow cover accumulation season was advancing towards winter on the western and northeast highlands of the country while snow cover accumulation season on the north and northwest of the country was forwarding back to fall. The calculations also showed that up to 2750 meters the onset of the snow cover season was moving toward winter, while from the height of 2800 meters, the onset of snow cover season was forwarding back to the fall season.