Document Type : Research Paper
Authors
1 MSc Student, Department of Water Engineering and Oceanic and Atmospheric Research Center, Faculty of Agriculture, Shiraz University, Shiraz, Iran
2 Assistant Professor, Department of Water Engineering and Oceanic and Atmospheric Research Center, Faculty of Agriculture, Shiraz University, Shiraz, Iran
Abstract
Introduction
Clouds play a vital role in Earth’s energy balance, and changes in their properties, frequency, and distribution can have either positive or negative feedback effects on global warming. Cloud formation and its impact on the Earth’s climate remain key topics in environmental sciences. By influencing the global energy budget, clouds alter convection patterns and cloud distribution, significantly affecting the global water cycle. Total cloudiness refers to the extent of coverage by any type of cloud in the sky, and this concept is used to describe the proportion of the sky covered by clouds. Cloud cover refers to the fraction of the sky covered by any type of cloud, typically expressed in oktas, ranging from 0 to 9. The aim of this study is to conduct a spatial and temporal analysis of significant changes in TCC on a seasonal scale over six decades across Iran. This analysis is based on statistical and observational evaluations during the selected study period. Due to Iran’s climatic characteristics, summer season—which is generally associated with atmospheric stability, low precipitation, and minimal cloud cover—was excluded from the scope of the study, this allows the research to focus on the seasons that experience the most atmospheric activity and variations in cloud cover.
Materials and methods
The monthly total cloud cover data used in this study were extracted from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis dataset, with a spatial resolution of 0.25°. The study period spans 60 years (1961–2021). By averaging the monthly data (three months per season), a seasonal time series of total cloud cover was generated over a 60-year period. For each season the distribution of total cloud cover over the six decades of the study was compared across different grid points. The significance of differences in cloud cover for each decade compared to other decades at each grid point was determined using the Mann–Whitney U test, with randomness mitigated via Monte Carlo randomization technique with 10,000 iterations, at a 95% significance level. Statistically significant differences were expressed as percentages.
Results and discussion
Analysis of the 60-year climatological mean of TCC over Iran reveals a decreasing gradient in total cloud cover variability from the northwest to the southeast during spring, autumn, and winter. Comparison of the seasonal distribution of total cloud cover in Iran revealed that spring cloud cover variability has significantly increased in the southern and southwestern regions of the country, while a notable decrease has been observed in the northeast, east, and southeast. In autumn, significant changes are limited to small areas in the northern part of the country, while most regions show no notable variations. However, in winter a widespread significant declines in TCC, particularly across the Zagros Mountains and southwestern Iran. This decline was most pronounced during the sixth decade (2011–2021), affecting regions that historically experienced moderate to high cloud cover.
Conclusion
Analysis of significant changes in total cloud cover revealed that during spring, significant decreasing and increasing cycles are observable across different decades, with these variations being most pronounced in the arid and semi-arid regions of the east and southeast of the country. Therefore, in future research, investigating potential changes in the formation and arrival of precipitation systems affecting the region could be considered. In autumn, significant change in cloud cover were rarely observed, highlighting the need to investigate monthly cloud cover variations to detect potential fluctuations in autumn cloudiness. For winter, the widespread and significant reduction in cloud cover, particularly in the sixth decade compared to the previous five decades, is of great importance. Notably, when comparing the fifth and sixth decades, the entire country experienced a significant decrease in cloud cover. These findings is significant for understanding energy balance and potential changes in winter precipitation, and could be a focus of future research. The periodic cloud cover variations observed in this study could have implications for water resource management, rainfed agriculture, and evapotranspiration, necessitating further in-depth research.
Keywords
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