Cloud-cover estimation: use of GOES imagery in development of cloud-cover data base for insolation assessment

Abstract

Site selection and system engineering studies of solar energy systems require high quality insolation data of several kinds, primarily direct normal and total horizontal data. The paucity of measured insolation data has been well documented. Insolation models have been used as substitutes for the absence of measured data, but a key problem has been the need for accurate and complete information regarding turbidity, precipitable water vapor, and cloud cover to input into the models. All but cloud cover display reasonably smooth temporal and spatial behavior, but cloud cover varies rapidly both in time and space, and therefore produces large fluctuations in insolation values.It is the purpose of this study to verify the potential of using digital satellite data to establish a cloud cover data base for the United States, one that would provide detailed information on the temporal and spatial variability of cloud development. The study involves the use of four sequential days of GOES data (eighty acquisitions) over -a predetermined test area, and converts those data into a computer archive of cloud cover, one that is accessible using predetermined grid cells of variable size. Key elements include: a) interfacing GOES data from the University of Wisconsin Meteorological Data Facility with the Jet Propulsion Laboratory's VICAR image processing system and IBIS geographic information system; b) creation of a registered multitemporal GOES data base; c) development of a simple normalization model to compensate for sun angle; d) creation of a variable size georeference grid that provides detailed cloud information in selected areas and summarized information in other areas; and e) development of a cloud/shadow model which details the percentage of each grid cell that is cloud and shadow covered, and the percentage of cloud or shadow opacity. In addition, comparison of model calculations of insolation with measured values at selected test sites was accomplished, as well as development of preliminary requirements for a large-scale data base of cloud cover statistics. ACKNOWLEDGMENTThe work reported herein was performed through NASA Task Order RA-152, Amendment 309 0 and was-sponsored' by the United States Department of Energy under IAA DE-AI01-76-ET20356, Mod. A026. Although numerous individuals contributed to this publication, three persons deserve special recognition. We wish to thank Robert Yinger, Southern California Edison Company, for his timely assistance in providing us with 1, 980 WEST Associates insolation data, and Dr. Macgregor Reid for critically reviewing the final draft of this document. To Arlene Calvert goes special kudos for accommodating the many unreasonable demands that were placed upon her in the preparation of the manuscript.