An Analysis of the Effects of Clouds in High-Resolution Forecasting of Surface Short-wave Radiation in South Africa

We assess site-specific surface short-wave radiation forecasts from two high resolution configurations of the South African Weather Service numerical weather prediction model, at 4 km and 1.5 km. The models exhibit good skill overall in forecasting surface short-wave radiation, with zero median error for all radiation components. This information is relevant to support a growing Renewable Energy sector in South Africa, particularly for photovoltaics. Further model performance analysis has shown an imbalance between cloud and solar radiation forecasting errors. In addition, cloud over-prediction does not necessarily equate to under-estimating solar radiation. Overcast cloud regimes are predicted too often with an associated positive mean radiation bias, whereas the relative abundance of partly cloudy regimes is under-predicted by the models with mixed radiation biases. Challenges highlighted by the misrepresentation of partly cloudy regimes in solar radiation error attribution may be used to inform improvements to the numerical core, namely the cloud and radiation schemes.