Analysis of the Relation between Spatial Cloud Patterns in Satellite Images and Variability as seen in High-Resolution Irradiance Time Series by means of their Daily Profile Categorization

Kurzfassung

The theoretical clear sky curve of the global horizontal irradiance (GHI), which depends on the geographical location and the date in the year, is known. The GHI, however, is subject to temporal variability due to the occurrence of cloudiness. The objective therefore is to couple spatial cloud information with local, temporally high-resolved GHI measurements. Deriving typical patterns of irradiance time series as a function cloud physical parameters offers an advantage: a more than sufficient availability regarding the amount of data (8 years of recorded 15 minute values) and the dimension of covered area (Europe, Africa and the Middle East) is given. Averaged cloud physical parameters are used to allocate days in category groups. These category groups represent daily GHI profiles that are subdivided according to their properties (daily energy content and irradiance fluctuations). This method is successful for days with no or minor fluctuations where the alloca-tion is based on their daily energy content. For days with major fluctuations the method does not work. Major fluctuations of the irradiance are related to short-duration (minutes to a couple of hours) changes in cloud properties. Thus for detecting fluctuations in GHI time series, an hourly based analysis of the cloud data is necessary.