Observation-Based Analog Ensemble Solar Forecast in Coastal California
Elynn Wu1, Monica Zamora Zapata1, Luca Delle Monache2, Jan Kleissl1
1Center for Renewable Resources and Integration, Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA, United States
/2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, United States

Historical observations from radiosondes, buoys, and satellite images are used to generate a probabilistic analog ensemble (AnEn) solar forecast. In coastal California, Stratocumulus (Sc) clouds appear most frequently during late spring and summer months. Sc clouds form at night and begin to dissipate after sunrise, obstructing solar production in the morning hours. The AnEn method categorizes cloudy (as either well-mixed or decoupled) and clear events at the forecast initial time and uses several meteorological variables to find the closest analog. The AnEn forecast is validated at the NKX weather station in San Diego during May to September 2014-2017. Both AnEn mean and median forecasts have a lower root mean square error than 24-hour persistence forecasts. The error is lowest for the clear cases and largest for the cloudy decoupled cases. The AnEn forecast is able to capture Sc dissipation for the well-mixed cases in the early morning, but decoupled cases display higher variability throughout the day and are much harder to predict as a result.