气候变化领域集成服务门户

Global irradiance is an important natural resource which is fundamental in various processes. Direct observations of components of global irradiance are scarce globally and models are required to improve their spatial coverage. Using a 30-year dataset, this study investigates relationships between measured global irradiance and diffuse irradiance at different periods of the day at both coastal and inland locations in South Africa. Historical hourly global and diffuse irradiance data from seven weather stations representing different climate regions were used to determine correlations of clearness index (K-T) and diffuse fraction (K) in the morning and afternoon. Results show that K is inversely related to water vapour pressure deficit. Global and diffuse irradiances are largest and smallest during dry and wet seasons, respectively. Areas that are dry in summer have higher global irradiances and regions that are wet in winter have higher diffuse irradiances during these respective periods. For the same clearness index, the diffuse fraction in the afternoon is larger than in the morning. This suggests that diffuse irradiance is generally greater in the afternoon than in the morning. Models that are commonly used to estimate diffuse irradiance from global irradiance underestimate afternoon irradiances by approximately 10% and overestimate morning irradiances by 4%. The relationship between K-T and K established in this study does not generally change in magnitude in the long term. However, future studies can investigate variability of these indices at micro-scales considering also the impact of other attenuating factors such as atmospheric precipitable water depth. Accessibility of atmospheric datasets can assist in improving modelling of global irradiance in South Africa using multivariate techniques.