Trend assessment of sunshine duration, cloudiness, and reference evapotranspiration for exploring global dimming/brightening in Tehran
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Recent studies have shown global warming as a deterministic phenomenon, while assessment of changes in specific variables such as sunshine duration, cloud cover and the amount of solar radiation at ground level increase uncertainties about this phenomenon in response to increasing greenhouse gases. Assessment studies of the mentioned variables have suggested the existence of a global dimming/brightening phenomenon in recent decades. In order to further research to investigate the global dimming/brightening phenomenon, the use of climatic variables such as sunshine duration used as a proxy for solar radiation variable is essential. Thus, this study used the cloudiness and residual sunshine duration time series after removal of the effect of cloud cover to study how the trend of changes in this phenomenon and its impact on reference evapotranspiration. In this regard, daily sunshine duration and cloudiness data measured at Mehrabad synoptic station of Tehran during the period 1951–2008 was used. Since the direct impact of this phenomenon will be on receiving solar radiation and subsequently on evapotranspiration, in this study, the trend of seasonal and annual variations of reference evapotranspiration and its trend accordance with mentioned factors was examined. The results showed that the existence trends in seasonal and annual sunshine duration and cloud cover time series in Mehrabad station are in accordance with global dimming/brightening trend, and it represents the overall occurrence of this phenomenon on a global scale.
KeywordsReference evapotranspiration Sunshine duration Cloudiness Global dimming/brightening Trend Tehran
The authors acknowledge the I.R. of Iran Meteorological Organization for providing the meteorological data. The authors also thank the anonymous reviewers for useful comments, which improved the quality of the paper.
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