Enhancing the Understanding and Application of the Clearness Index

Defined as the proportion of solar radiation transmitted through the atmosphere to the Earth's surface, the clearness index (CI) is a vital parameter widely applied in characterizing atmospheric transmittance and sky conditions. However, its application accuracy remains inadequately investigated. This study enhances the understanding and application of CI through three key advancements:

• Establishing CI Thresholds for Sky Conditions: Standardized CI thresholds for clear-sky (>0.7) and overcast-sky (<0.2) conditions are proposed using synoptic total cloud cover, refining the previously broad ranges. Their logarithmic relationship with solar elevation angles enables accurate identification of sky conditions throughout the day.
• Advancing Physical Threshold Testing: A CI-DF polynomial envelope, combining CI and diffuse fraction (DF), is introduced to enhance the physical threshold testing procedure. This innovation automatically and effectively filters out outliers, particularly the often-overlooked abnormally low values, thus improving the quality control of surface observations.
• Developing a Radiation Decomposition Model: A model is established to accurately estimate direct radiation at daily and hourly scales, leveraging the logistic growth relationship between CI and the direct clearness index. This supports the growing global transition to renewable energy applications.

These findings highlight the importance of more accurate applications of CI in atmospheric radiation and energy meteorology studies.