Detection of cloud-free skies based on sunshine duration and on the variation of global solar irradiance

Automatic detection of cloud-free skies became more and more important in recent years to evaluate and interpret surface radiation measurements in the context of the greatly debated solar dimming and brightening and to investigate possible changes of the greenhouse effect using longwave downward radiation measurements. To automatically separate cloud-free from cloudy sky situations within the MeteoSwiss automatic meteorological network (ANETZ) a new method based on sunshine duration and hourly global solar irradiance measurements, referred to as sunshine duration method (SSD-M), is presented. Results obtained with this new method are evaluated with the approved automatic partial cloud amount detection algorithm (APCADA). SSD-M identifies at the six investigated radiation sites between 88 and 98 % of cloud-free cases detected by APCADA. The probability of false detection of SSD-M does not exceed 7 % at any of the sites. The new method presents the advantage to use standard solar radiation parameters that are usually measured at advanced meteorological stations. The use of solar measurements restricts the method to detect daytime cloud-free situations only.