On the characteristics and long-term trend of total cloud cover in Iran

Abstract

In this study, the annual, seasonal and monthly trends of total cloud cover (TCC) and associated climatic variables are investigated for a period of 63 (1959–2022) years in Iran based on ERA5 reanalysis data extracted from ECMWF. To analyze temporal trends, the Mann–Kendall test is used. The geographical location within Iran, especially distance from moisture sources and different atmospheric systems, influences cloudiness such that TCC decreases from north to south and from west to east. With respect to seasons, the highest and lowest average TCC is observed in winter and summer, respectively. The annual trend analysis reveals a decreasing trend in TCC (i.e., Mann–Kendall’s tau is negative: −0.40 per decade). On the monthly scale, a statistically significant decrease in TCC occurs during January, February, March, June, August, November, and December. There is a significant decreasing trend in all seasons, among which the maximum decreasing trend is observed in the summer season with a value of −0.31 per decade. Examining the trends of climatic variables shows that on all three temporal scales (annual, seasonal, and monthly) the number of rainy days (NRD) decreases and temperature (T) increases. Spatial analysis of trends (seasonal, annual) suggests the highest decrease in TCC in the west, northwest, east, and southeast, whereas the lowest decrease is in the center of Iran. Spatially, the T trend (annually and spring, summer, and winter seasons) indicates a consistent increase in temperature in the central and eastern parts of Iran. The spatial trend (annual and seasonal) of NRD in the limited parts of northwestern Iran exhibits the highest increasing trend. The results of investigating the anomalies in TCC relative to the long-term average amount of cloud cover on annual and seasonal scales show zero anomalies in most of the years (67% on an annual scale and 73% in summer and 71% in winter).