High latitude ground truthed assessment of NSRDB V3 irradiance using sky condition data

We perform a global horizontal (GHI) and direct normal (DNI) surface solar irradiance comparison between the National Solar Radiation Database (NSRDB V3) and four Canadian Solar Spectral Irradiance Meter Network (CanSIM) stations for 2018-2019. On average, under all-sky conditions, the GHI from the NSRDB had the mean absolute error (MAE) and the root mean square error (RMSE) of 14% and 18%, respectively, while for the DNI those metrics were 15% and 24%, respectively. By using the sky condition classification obtained from a spectral pyranometer, we estimated that 7% to 17% (depending on location studied) of NSRDB cloudy sky data is misattributed as such due to satellite parallax and snow cover errors. The lower bound on parallax error varied from 5% and 12%, while the upper bound on the cloud-snow discrimination error ranged from 2% to 5%.