The Use of a Physics-based DNI Model to Enhance the National Solar Radiation Database (NSRDB)

Direct normal irradiance (DNI) is often interpreted differently in ground measurements and forecasts using numerical weather prediction (NWP) models, leading to substantial bias in DNI computation and forecasting especially under cloudy-sky conditions. To mitigate the bias, we use the Fast All-sky Radiation Model for Solar applications with DNI (FARMS-DNI) to conduct physics-based simulations of solar radiation in the circumsolar region. The DNI is quantified using the sum of the radiation along the sun direction and the scattered radiation within the circumsolar region. FARMS-DNI is implemented in the National Solar Radiation Database (NSRDB) to assess 5-minute DNI in 2-km pixels over the contiguous United States. The DNI computation for 2019-2021 are validated using surface-based observations at the Atmospheric Radiation Measurement (ARM), Surface Radiation Budget Network (SURFRAD), National Renewable Energy Laboratory (NREL), Solar Radiation (SOLRAD), and University of Oregon (UO). The results show that the cloudy-sky DNIs from the NSRDB are improved as evaluated using absolute percentage error and mean absolute error.