Comparing NWP and satellite-based irradiance forecasts in Fennoscandia

Solar energy production is growing at a very high rate. This growth is present also in Fennoscandia, where solar energy can answer to the renewable energy need during the long summer days. Solar energy is a naturally fluctuating energy source, one that requires the use of solar irradiance forecasts for optimal operation. In northern locations clouds cause the largest fluctuations of irradiance, which is why the forecasting of solar irradiance is often focused on forecasting the movement of clouds. At high latitudes there are also specific challenges due to the location, for instance estimating cloud location at the edge of the geostationary satellite instrument viewing area.

Forecasting irradiance is possible using various methods. Methods using satellite imagery, for instance, are good at forecasting irradiance for a few hours forward, while Numerical Weather Prediction (NWP) methods are recognised as superior for forecasting the next day. Validations have been made for various methods and setups, at several different locations, and for example the satellite-imagery-based method Solis-Heliosat has been shown to work well also at high latitudes. Operational NWP models, however, are not often validated for this specific purpose.

To understand the performance of operational local area NWP models in irradiance forecasting in Fennoscandia, we have compared and validated NWP irradiance forecasts at several measurement stations in Finland and Sweden. In the comparison, we have included the operational weather forecast model in the Nordic countries, the MetCoOp Ensemble Model (MEPS), as well as the MetCoOp Nowcasting Model (MNWC). Additionally, we have included the Solis-Heliosat method and a persistence method in the comparison.

Initial results show MEPS to have a steady, small relative bias error during the forecast, particularly after the first hours. The relative Root Mean Square Error (rRMSE) is also steady with only a slight increase during the whole forecast. MNWC somewhat underpredicts irradiance in the beginning, but the errors improve throughout the forecast. Solis-Heliosat has good initial accuracy, but the quality deteriorates very fast, with MEPS and MNWC outperforming the method after a 1-3 hour lead time. The Persistence method, as expected, has a very good bias, but also a very fast increasing rRMSE with forecast lead time.

The results show the NWP methods to be very suitable for forecasting irradiance. Differences in best performing lead times and varying run times between the methods increase our interest in using both NWP and satellite-based methods in forecasting irradiance, to get the most optimal accuracy for both short and long-term forecasting.