2011 - Day Zero - Renewables User Group

Short-term Solar Forecasting

A method for intra-hour, sub-kilometer cloud forecasting and irradiance nowcasting using a ground-based sky imager at the University of California, San Diego is presented. Sky images taken every 30 seconds were processed to determine sky cover using a clear sky library and sunshine parameter. From a two-dimensional cloud map generated from coordinate-transformed sky cover, cloud shadows at the surface were estimated. The (binary) cloud conditions were correctly nowcast 76% of the time for a network of six pyranometer ground stations spread out over an area of 2 km2. Cloud motion vectors were generated by cross-correlating two consecutive sky images. Cloud locations up to five minutes ahead were forecasted by advection of the two-dimensional cloud map. Cloud forecast error increased with increasing forecast horizon due to high cloud cover variability over the coastal site.

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Power & Utilities


University of California, San Diego


Jan Kleissl

Department of Mechanical and Aerospace Engineering