Physical process and statistical properties of solar irradiance enhancement observed under clouds

Abstract

Solar irradiance intensity is usually reduced by cloud shading. However, an enhancement of solar irradiance has rarely been detected. For the detection of irradiance enhancement events, high-speed pyranometers are employed here. The enhancement is detected in almost one-third of days in a month on average in central Japan. From sky-camera observation and meteorological analysis, the enhancement occurs owing to the superimposition of the diffuse irradiance scattered at the edges of cumulus clouds near the sun on the direct irradiance passing through a gap in the clouds, which exist along meteorological fronts. Events of solar irradiance enhancement were detected in a five-month observation period and were analyzed statistically. Some of the events with the largest enhancement are amplified by more than 1.6, and their durations are from 30 s to 4 min. Statistical analyses showed events with either large amplifications or long durations but not events with both large amplifications and long durations.