In research on solar energy and climatology, "clear sky" or "cloudless" conditions are very important. Clear-sky detection (CSD) methods are developed based on the instantaneous surface irradiance, which is highly affected by cloud variations.
However, a challenge that all CSD methods face is that the detection accuracy deteriorates when aerosol loading increases.
"The lack of accurate clear-sky detection data makes it difficult to assess existing clear-sky detection methods in polluted areas," said LIU Mengqi, a PhD student from the group of Prof. XIA Xiang'ao at the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences. LIU is also the first author of a study recently published in Atmospheric and Oceanic Science Letters.
In the study, Prof. XIA and his team evaluated 21 CSD methods using five years of 1-min surface irradiance and visually inspected total-sky imager data at Xianghe - a heavily polluted station on the North China Plain.
The pyranometer at Xianghe observation station (Image by LIU Mengqi)
The researchers found that CSD methods with higher cloudy-sky detection accuracy rates produced lower clear-sky accuracy rates, and vice versa. Moreover, when aerosol loading increased, the CSD accuracy rate decreased significantly.
"Our results provide scientific guidance to existing clear-sky detection methods that are currently not applicable in polluted conditions," said Prof. XIA. "In the future, this will give us the opportunity not only to improve existing methods, but also hopefully to propose a new clear-sky detection method."
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