Comparison of irradiance forecasts from operational NWP model and satellite-based estimates over Fennoscandia

With the increase of solar power use, the need for solar irradiance forecasts is also increasing. Solar power is a naturally fluctuating energy source, which makes solar irradiance forecasts necessary for e.g. grid integration and management. Both the use of solar energy and the need for forecasts are present also in the Nordic countries, where the sub-Arctic latitude brings its own challenges.

Various methods exist for forecasting solar irradiance. Numerical Weather Prediction (NWP) models have been found superior for forecasting for the following days, while satellite-based models are found suitable for the first few hours of the forecast. The satellite-based model Solis-Heliosat has been found to perform well also in the high latitudes. The current operational NWP models in Finland, however, have not been yet extensively validated for this purpose.

To determine the suitability of the operational NWP models for forecasting solar irradiance in the Nordic countries, we have comparatively validated the MetCoOp Ensemble Model (MEPS), and the MetCoOp Nowcasting Model (MNWC) against in situ irradiance measurements at several stations in Finland and Sweden. We have also included the Solis-Heliosat model in the study, to improve our understanding on the differences and the relative accuracy between the NWP and satellite-based models. As a benchmark, two persistence models are included. The comparison is made for one summer, including all model runs and all MEPS ensemble members, with both hourly and 15 minute output depending on the model.

The results show all models to somewhat under predict irradiance. MEPS shows very good performance in the full length of the forecast, while Solis-Heliosat is better in the first 2-3 hours of the forecast. Solis-Heliosat has some difficulty with the forecasts starting in the morning, whereas MNWC slightly struggles in the afternoon.

Overall we find the NWP models very suitable for forecasting solar irradiance in Finland and Sweden, particularly with the full forecast horizon of MEPS, and the 15-minute time step of MNWC. Nevertheless, Solis-Heliosat brings further value to the beginning of the forecast.

Kallio‐Myers, V., Riihelä, A., Schoenach, D., Gregow, E., Carlund, T., & Lindfors, A. V. (2022). Comparison of irradiance forecasts from operational NWP model and satellite‐based estimates over Fennoscandia. Meteorological Applications, 29(2), e2051.