NCAR developed an enhanced version of the Weather Research and Forecasting model – WRF-Solar® model to improve forecasting of solar irradiance at the surface in hour-ahead to day-ahead range for renewable energy applications. WRF-Solar was the first numerical weather prediction (NWP) model specifically designed to meet the growing demand for specialized numerical forecast products for solar power applications. Specific developments included addition of direct normal irradiance and diffuse irradiance components to the model output, a fast radiative transfer code, new parameterization that improves representation of absorption and scattering by aerosols, accounting for aerosol interactions with cloud microphysics, as well as implementation of an improved shallow cumulus parameterization. While these developments resulted in significant improvements in short-range and day-ahead forecasts for both clear sky and cloudy conditions there are several areas where further improvement could result in significant error reduction in predicted solar irradiance.

The goal of this project is to reduce forecast errors of global horizontal irradiance and direct normal irradiance and to yield better forecasts of irradiance ramps, improvements in estimates of sub-grid scale variability, and more accurate estimates of forecast uncertainty. WRF-Solar V2 will include a number of enhancements including:

• New representation of boundary-layer clouds (both shallow cumuli and the breakup of stratocumulus) including the impact of entrainment on cloud fraction in a grid cell.
• Improved treatment of cloud microphysics, and impacts of aerosol (including black carbon),
• New parameterizations to account for the sub-grid temporal variability of solar irradiance during periods with broken clouds, and
• Detailed analysis to better quantify model uncertainty and improved calibration of WRFSolar V2 using Uncertainty Quantification (UQ) techniques.

With these improvements, WRF-Solar V2 will be a new tool that will lead to improved intra-day and day ahead forecasts. The new version of WRF-Solar will be a community model that will become the new standard in irradiance forecasts.