摘要：Surface irradiance measurements with high temporal resolution can be used to detect clear skies, which is a critical step for further study, such as aerosol and cloud radiative effects. Twenty-one clear-sky detection (CSD) methods are assessed based on five years of 1-min surface irradiance data at Xianghe—a heavily polluted station on the North China Plain. Total-sky imager (TSI) discrimination results corrected by manual checks are used as the benchmark for the evaluation. The performance heavily relies on the criteria adopted by the CSD methods. Those with higher cloudy-sky detection accuracy rates produce lower clear-sky accuracy rates, and vice versa. A general tendency in common among all CSD methods is the detection accuracy deteriorates when aerosol loading increases. Nearly all criteria adopted in CSD methods are too strict to detect clear skies under polluted conditions, which is more severe if clear-sky irradiance is not properly estimated. The mean true positive rate (CSD method correctly detects clear sky) decreases from 45% for aerosol optical depth (AOD) ≤ 0.2% to 6% for AOD > 0.5. The results clearly indicate that CSD methods in a highly polluted region still need further improvements.
