ارزیابی درصدِ ابرپوش در ایران با استفاده از داده‌های سنجندۀ مودیس تررا (مطالعۀ موردی: سال 2007 میلادی)

Introduction
Clouds and water vapor are important modulators of climate and are involved in feedbacks that strongly affect global circulation and energy balance. Typically, 50% of the earth surface is covered by clouds, at any given time. A cloud is defined as a visible mass of condensed water droplets or ice crystals suspended in the atmosphere above the earth surface. The cloud phenomenon plays an important role in the water cycle, radiation, temperature, precipitation, predictors of climate, and etc. As there are not adequate information on the spatial distribution of clouds in Iran, except for synoptic stations data, it is necessary to conduct researches on the physical properties of cloud by remote sensing data. Therefore, these kinds of data can provide us with a more accurate and comprehensive understanding of cloud phenomenon. This study examines the spatial distribution of cloud Fraction (cover) over Iran in 2007 using remotely sensed data. The results of this fundamental research could be applied in practical knowledge such as weather forecasting, climate modeling, seeding clouds, site selection of solar panels, and etc.
 
Materials and methods
The aim of this research is to survey the spatial distribution of Cloud Fraction (CF) over Iran in annual and seasonal timescales. To do this, daily data of cloud product of MODIS Terra (MOD06_L2.A) over Iran was used in 2007. The data was obtained from the ftp link\ftp://ladsftp.nascom.nasa.gov/allData/6/MOD06_L2/.  In this research, we have used spatial resolution of CF in 5 km×5 km scripting in MATLAB. In the first step, overlapping images have been removed and, then, the data within the Iranian border have been extracted from daily data. The data have been transferred into regular network of Iran for doing statistical computations.  
 
Results and discussion
The investigation on the annual mean percent of CF indicates that the value is 25.3% for the morning and 29.7%for afternoon. Thus, the amount of CF in the afternoon is increased compared to the morning times. In winter, the amount of CF for the morning is 48.2% but for the afternoon it is reduced to 44.1% and this reduction condition is not seen in other seasons. In spring, fall and winter seasons the amount of CF is increased in the afternoon compared to the morning. The spatial distribution of annual percent of CF indicates that the maximum is seen over the southern shores of the Caspian Sea and the minimum is observed in south east part of the country. The spatial distribution of seasonal percent of CF shows that the maximum amount of CF is over the highlands of Zagrous and Alborz mountains and the minimum is in south and south-east regions of the country. In this season, the maximum percent of CF is not seen over Caspian shores like other seasons. In spring, the maximum percent of CF can be seen over the southern shores of the Caspian Sea and parts of north-west but the minimum percent of CF can be observed over central areas of south and south-east regions of the country. In fall season, the maximum percent of CF is seen over the southern shores of the Caspian Sea and parts of north-east and the minimum is observed in south and south-east regions. In summer, the maximum percent of CF is in the southern shores of the Caspian Sea and the minimum can be seen over east and west parts of the country. In summer, the extent of minimum percent of CF is changed to other seasons and is far away from south-east regions.  
 
Conclusion
In this investigation, the CF parameter of MODIS Terra was applied in the daily temporal resolutions for the year 2007 to explore the spatial distribution of cloud cover over Iran. As the data did not have a regular geographical coordinated grid, a regular coordinate was initially constructed and CF data were trasnferred to this regular grid. This process was conducted to analyze the climatology of cloud cover. The results from MODIS Terra data for the pass of morning and afternoon times revealed that the maximum annual percent of CF is seen over the southern shores of the Caspian Sea and the minimum is occurred over south-east part of the country which is consistent with the results of Rasooli et al. (2013) and Masoodian and Kaviani (2008). For the seasonal time scales, the maximum percent of CF is occurred over the southern shores of the Caspian Sea for spring, fall and summer seasons but in winter it is seen over the elevations of Alborz and north-west parts of the country. The minimum percent of CF is seen in south-east and east parts of the country for spring, fall and winter seasons. In summer, it is observed over east and west regions where shows that the formation process of cloud is different in winter and summer compared to the other seasons. The validation of CF values in the annual time scale indicates that MODIS overestimes CF by 3% compared to the synoptic stations. This is acceptable when the results are compared with the findings of Bisoolli and Pahl (2001) in Germany with the erros of 6% and Kotarba (2009) in July and January months with the error of 4.38% and 7.28% for the year 2004. In general, the estimation of cloud cover is identical for the two data sets.