Cloud enhancement of global horizontal irradiance in California and Hawaii

Clouds significantly attenuate ground-level solar irradiance causing substantial reduction in photovoltaic power output capacity. However, partly cloudy skies may lead to temporary enhancement of local Global Horizontal Irradiance (GHI) above the clear-sky ceiling and, at times, the extraterrestrial irradiance. Such enhancements are referred to here as Cloud Enhancement Events (CEEs). In this work we study these CEEs and assess quantitatively the occurrence of resulting coherent Ramp Rates (RRs). We analyze a full year of ground irradiance data recorded at the University of California, Merced, as well as nearly five months of irradiance data recorded at the University of California, San Diego, and Ewa Beach, Hawaii. Our analysis shows that approximately 4% of all the data points qualify as potential CEEs, which corresponds to nearly 3.5 full-days of such events per year if considered sequentially. The surplus irradiance enhancements range from 18 W m−2 day−1 to 73 W m−2 day−1. The maximum recorded GHI of ∼1400 W m−2 occurred in San Diego on May 25, 2012, which was nearly 43% higher than the modeled clear-sky ceiling. Wavelet decomposition coupled with fluctuation power index analysis shed light on the time-scales on which cloud induced variability and CEEs operate. Results suggest that while cloud-fields tend to induce variability most strongly at the 30 min time-scale, they have the potential to cause CEEs that induce variability on time-scales of several minutes. This analysis clearly demonstrates that CEEs are an indicator for periods of high variability and therefore provide useful information for solar forecasting and integration.