| 15 Sep 2025
A new method for estimating cloud optical depth from photovoltaic power measurements
Abstract. A new method was developed to estimate the cloud optical depth (τc) from photovoltaic (PV) power measurements under overcast sky conditions. It is the first fully physical and universally applicable method utilizing directly PV power measurements. It exploits the recent advances and real-time availability at global scale of aerosol properties, downwelling shortwave irradiance and its direct and diffuse components received at ground level under clear-sky conditions, ground albedo and extraterrestrial irradiance, altogether provided by the Copernicus Atmosphere Monitoring Service (CAMS) radiation service. In addition to CAMS data, wind speed and air temperature from European Centre for Medium-Range Weather Forecasts (ECMWF) twentieth century reanalysis ERA5 products are also used as inputs. An algorithm for selecting overcast sky conditions has been designed too. The estimates have been compared to different data sources of retrievals at four experimental PV sites located in various climates. When compared to retrieved from ground–based pyranometer measurements serving as reference, the correlation coefficient is greater than 0.97. The bias ranges between –3 and 4, i.e., −8 % and 12 % in relative value. The root mean square error (RMSE) lies in the interval [3, 8] ([9, 21] % in relative value). When compared to satellite–based retrievals from Meteosat Second Generation (MSG) and Moderate Resolution Imaging Spectroradiometer (MODIS), both relative errors become comprehensively greater. Nevertheless, our method remarkably reduces the relative bias and RMSE, by up to 10 % and 20 % respectively, compared to the existing state-of-the-art approach. This work demonstrates the accuracy of the method and clearly shows its great potential use whenever PV power measurements are available.
The manuscript presents a rigorous approach to retrieve cloud optical depth (τc) directly from photovoltaic power measurements.
The abstract describes it as “the first fully physical and universally applicable method … to estimate τc from PV measurements”.
It may be noted that the broader concept of exploiting PV-panel electrical output in synergy with a radiative-transfer model to infer an atmospheric optical-depth parameter was introduced earlier in Lolli (2021), Sensors 21, 6342; https://doi.org/10.3390/s21196342, which retrieved aerosol optical depth (AOD) at 550 nm from PV power.
Although the methodology in Lolli (2021) differs, i.e. using the Fu–Liou–Gu model and an iterative inversion, the two studies share the same fundamental synergy of radiative-transfer modelling coupled with PV-power measurements. Including Lolli (2021) as a reference could be useful for readers interested in the historical development of PV-based atmospheric-retrieval techniques.