Analysis of spatiotemporal variations of cloud fraction based on geographic characteristics over Iran

The goal of the present study is to provide a climatic description of spatiotemporal variations of cloud fractions based on geographic characteristics over Iran during daytimes. Analyses are performed on the basis of 15 years of data collected from the MODIS instrument aboard Terra and Aqua satellites, data from 120 weather stations, ERA-Interim reanalysis, and geographic data (latitude, altitude, and vegetation cover). We have compared the consistency of MODIS monthly mean of cloud fractions aboard the Terra and Aqua with ERA-Interim and weather stations data. The results showed good agreement (R² = up to 74 %) between them but the data is more consistent during the cold months. Weather station data has a better overall agreement with MODIS compared to ERA-Interim. Temporal variations of cloud fraction showed maximum standard deviation in autumn for both satellite observations. Trend analyses applied to both data indicate non-significant trends for most months, although a significant decreasing trend of 2.2% per year during December and an increasing trend of 2.1% per year during May were observed. We found that the maximum and minimum monthly means of cloud fraction correspond to December to February (56 to 63%) and June–July–September (8 to 26%), respectively. Analysis of latitudinal forcing on cloud fraction showed that an increase in cloud fraction is coupled with an increase in latitude; this pattern is true only up to 31.5° N in summer. The reverse pattern is related to the Asian Monsoon system over Iran in low latitudes. Topographic forcing in this mountainous area often leads to orographic convection in the elevations of 500–1500 m during spring and autumn and coastal convection during summer. Vegetation also has a strong positive correlation with cloud fraction.