Characterising the effect of the transition zone of clouds and aerosol on solar surface irradiance using a dense pyranometer network

Solar energy is an important source of renewable energy, and understanding its variability due to clouds and aerosols is critical for efficient and reliable solar energy systems. The interaction between clouds and aerosols can have a significant impact on surface solar irradiance during broken cloud conditions. Reflections at cloud edges and changing aerosol properties in the vicinity of clouds affect the surface solar irradiance during broken cloud conditions. However, the boundaries between clouds and clear skies with aerosols are not always well defined, and quantifying the frequency and spatial extent of the transition zone between these regions is challenging¹. This study uses a unique dataset of observations from the TROPOS dense pyranometer network to detect and characterize the transition zone between clouds and clear skies with aerosols.

The TROPOS pyranometer network consists of up to 100 individual stations. Datasets from two campaigns are used for this study: 99 stations were distributed over an area less than 10 km² operated during the HOPE² measurement campaign in 2013 in Jülich, Germany; 60 stations were distributed over an area of about 6 km² during the S2VSR³ measurement campaign in 2023 in Oklahoma, USA. The surface solar irradiance is measured at each station with a time resolution of 10 Hz. The transition zone is detected and characterized by applying modified clear sky detection algorithms⁴ to the data. To quantify the spatial characteristics of the transition zones, a method based on optimum averaging/spatio-temporal Kriging is introduced. A main component of this method is the determination of the cloud motion vector. It is determined whether this can be reliably estimated from ground-based measurements, by using cross-correlation techniques, or whether additional information from satellite data is required. First results utilizing the HOPE and S2VSR dataset are presented.

The study aims to quantify the small scale effects of the transition zone on surface solar irradiance and potential photo-voltaic yield. This information is valuable for photo-voltaic site planning and provides scientifically relevant insights into the interaction between clouds and aerosols. Furthermore, the study provides initial considerations for modelling the identified relationships and insights into small-scale surface irradiance variability not captured by current high-resolution satellite observations.

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1 e.g., Calbó et al. 2017, https://doi.org/10.1016/j.atmosres.2017.06.010

2 Macke et al. 2017, https://doi.org/10.5194/acp-17-4887-2017

3 https://www.arm.gov/research/campaigns/sgp2023s2vsr

4 Bright et al. 2020, https://doi.org/10.1016/j.rser.2020.109706