A cloud is a hydrometeor consisting of minute particles of liquid water or ice, or of both, suspended in the free air and usually not touching the ground. It may also include larger particles of liquid water or ice as well as on-aqueous liquid or solid particles such as those present in fumes, smoke or dust (WMO, 1975). Clouds, which display great variability in space and time, as well as in type, can influence climate through many complex interactions involving the hydrological cycle; however, their dominant role is in controlling the 3-dimensional field of radiative fluxes in the atmosphere. In turn, these radiative fluxes drive the thermally forced general circulations of the earth's atmosphere and oceans. These circulations then form clouds and a major climate feedback loop is joined (Rossow and Schiffer, 1983). Many researches have been done about sky conditions classifications abroad of Iran such as, Nakumura (1985), Qian et al (2012), Stove et al (1989) and Filipiak et al (2009). Bannayan et al, 2012 classified sky condition in northeast of Iran, they classified cloud amounts of synoptic meteorology stations in cloudy day (7-8 octas), partly cloudy (3-6 octas) and clear sky (0-2 octas).
Methods and Material: We have applied Cloud Fraction Parameter from Cloud product of MODIS sensor on board Terra satellite (MOD06). Data were downloaded from site of ftp://ladsweb.nascom.nasa.gov/allData/ 6/MOD06_L2. Statistical period is from 2000 to 2013 solar year (2000 to 2014), so, the research has been done based on solar year. Cloud products of MODIS (MOD06) do not have neatly geographic data network on daily, so, we have used new method for gridding of data, At first, a network of geographical coordinates with the spatial resolution arrays of 5 × 5 km, based on the framework of Iran (25-40 degrees north latitude and 44-64 degrees east longitude) was prepared as a regular network or reference with dimensions of 618 × 353 (618 columns and 353 rows). Using geographic coordinates latitude and longitude of each cell in regular network, a framework was prepared 1.5 times the distance between each pixel dimensions (a radius of less than 7.5 km) and was spread on the granules geographical coordinates and the CF data within the framework of the granules were transferred to target cells in terms of latitude closest to the regular network. To transfer CF data parameters in each path to a regular network on a daily basis, this process was carried out for each individual cell in a regular network. So, CF data were ready for statistical computing and As a new database, next steps of research was carried out based on this database. Then, Sky conditions classified based on clear sky days (0 - 25%), partly cloudy days (25% - 62.5%) and cloudy days (>62.5% to 100%). Daily data composite to sessional and monthly data. Then, spatial distribution and long-term average of sky condition classifications were identified in monthly and seasonal scales on Iran.
In Current research, the sky condition has been investigated based on Cloud Fraction parameter in cloud product of MODIS Terra satellite, at first, sky condition was classified to clear sky, partly cloudy and cloudy days based on thresholds considered. After calculating the long-term average and spatial distribution of sky condition classifications were analyzed. Due to the irregular geographical coordinate of daily granules of MODIS on country, first, the Cloud Fraction (CF) data were transferred to a 5× 5 km regular network to be able to analyze long term climatology of cloud cover. The findings of this study reveal that the highest frequency of cloudy days is in winter with 36 days and the lowest is seen in summer with 7.8 days. The highest (lowest) frequency of clear sky days is seen in summer season with 79.6 (45.2) days. On monthly time scale, the highest (lowest) frequency of cloudy days is in the February (September) with 12.8 (1.8) days and the highest (lowest) frequency of clear days is in the September (February) with 27.9 (14.1) days. The spatial distribution of cloudiness indicated that the maximum of cloudy days in spring, summer and fall is seen over the south and west parts of the Caspian shores, however, in winter the maximum of cloudy days is over mountainous regions of the north. The minimum of cloudy days in spring, fall and winter is seen over south-east of the country but in spring it is far away from the south and south-east areas of the country and it is seen over central parts of Iran. The maximum of partly cloudy days is seen over mountainous areas and the minimum is over southern, central areas and also the hills and the plains. In general, the frequency of cloudy (clear) days decreases (increases) from north to the south-east but in summer it increases over south-east and in the north of Hormuz Strait and in winter over the mountainous regions of the north it has an increasing tendency and the decreasing (increasing) of cloudy (clear) days is no longer seen.
The aim of this research is identification of spatial distribution and frequency monthly and seasonal sky condition classifications on Iran, according to this, it was found that the spatial distribution obtained from the study method showed that the maximum frequent cloudy days is seen in the southern and western shores of the Caspian Sea in the Summer, Spring, and Autumn, and its minimum is seen in the southeast of country in spring, winter, autumn. these results are correspond with Rasooli et al (2014), but the maximum (minimum) territory of cloudy days and clear sky days vary over Iran in summer and Winter Compared with other seasons and this situation have also seen in some months that it differ with Rasooli et al (2014) results. Generally, the frequency cloudy days (clear sky days) decrease (increase) from north to south of country, but, this path is impaired in north of Strait of Hormuz and southeast of country in summer (July, August, September) and over the mountain heights of north of country in winter (December, January, February). The most frequent cloudy days occur in the winter and the lowest it occur in the summer. The most frequent clear sky days occur in the summer and the lowest it occur in the winter. In the monthly scale, the most frequent cloudy days occur in the February and the lowest it occur in the September.
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