عنوان مقاله [English]
Clouds cover major portion of the earth’s surface and play an important role in climatic system. Clouds affect the radiation energy balance of the earth’s climate system by absorbing or scattering solar radiation and long wave radiation and emitting thermal radiation. Cloud properties are closely related to cloud cover patterns, a shift in cloud regime would result in changes in cloud fraction and the cloud microphysical properties and both of these (cloud fraction and cloud microphysics) influence the radiation forcing (Rapp, 2015). Clouds have a strong effect on precipitation distribution, tropospheric temperature profile, climate change, radiation budget, global hydrology budget. Thus, they have an important influence on global climate. The purpose of this research is the study of this variety of cloud fraction in Iran during all months over 2001-2015 with respect to the latitude, topographic forcing, and vegetation cover. Latitude, altitude, slope, aspect and vegetation are geographic characteristics in an area which determine and control many climatic parameters such as temperature, precipitation and etc. Analysis of spatiotemporal variations of cloud fraction based on the characteristics in a vast country like Iran has not been considered by researcher. Satellite imagery is one of the most efficient data source to monitor cloudiness. The spatial and temporal variation of cloud type Ping as deep cloud (Ping et al., 2014), stratospheric clouds (Pitt et al., 2007) and different cloud type (Halladay et al., 2012) have been studied by researchers over the world. Some researchers consider relation between cloud fraction and climate and geographic parameters e.g. Sato et al., 2007. Some other researchers reviewed cloudiness studies e.g. Bromwich et al., 2012. Iran is located between 24.5 to 39.5 north latitude and has topographic range between -28 to 5595m. NDVI value reaches a maximum value in June (0.897) during 2001-2015 time period. This research uses DEM 30 meter and Normalized Difference Vegetation Index in order to analyzie the effets of geographic parameters on cloud fraction. Monthly mean values of cloud fraction are extracted from MOD08/MYD08 MODIS products. We have then validated accuracy of MODIS mean monthly of cloud fraction aboard the Terra and Aqua using ERA-Interim and station data. Results show that there is a good agreement between them but the data is more accurate in cold month, and in the mornings, so that, polynomial coefficients of determination are higher against the stations data and in the morning times due to hourly stations weather data which corresponded to the satellites overpass. The geographic characteristics results showed that cloud fraction increases with increase in latitude except summer seasons due to monsoon system. In order to showing topographic forcing on cloud fraction, this parameter is divided into intervals of 15% for each months and then altitude, slope and aspect that were extracted for each interval. Topographic forcing presents the interesting role of slopped convection in mountain area in average elevation (500-1500 meter) over spring and autumn. Vegetation also has nearly direct relation with cloud fraction. Investigation of temporal variations of cloud fraction showed that the maximum value of STD is obtained in autumn for both satellites. Furthermore, significant trend was not observed in many months, but month of December showed decreasing trend by 2 to 3 annually. This research is the first attempt in the field of cloud climatology in recent decades and further analysis are needed to show the ongoing climate change effects on cloud climatology in this region. Study of cloud vertical profiles can be the next research in this field.