Abstract. This paper is an overview of the progress in sky radiometer technology and the
development of the network called SKYNET. It is found that the technology
has produced useful on-site calibration methods, retrieval algorithms, and
data analyses from sky radiometer observations of aerosol, cloud, water
vapor, and ozone. A formula was proposed for estimating the accuracy of the sky radiometer
calibration constant F0 using the improved Langley (IL) method, which
was found to be a good approximation to observed monthly mean uncertainty in
F0, around 0.5 % to 2.4 % at the Tokyo and Rome sites and smaller values of
around 0.3 % to 0.5 % at the mountain sites at Mt. Saraswati and Davos. A new cross IL (XIL)
method was also developed to correct an underestimation by the IL method in
cases with large aerosol retrieval errors. The root-mean-square difference (RMSD) in aerosol optical thickness (AOT) comparisons with other networks took values of less than 0.02
for λ≥500 nm and a larger value of about 0.03 for shorter
wavelengths in city areas and smaller values of less than 0.01 in mountain
comparisons. Accuracies of single-scattering albedo (SSA) and size
distribution retrievals are affected by the propagation of errors in
measurement, calibrations for direct solar and diffuse sky radiation, ground
albedo, cloud screening, and the version of the analysis software called the Skyrad
pack. SSA values from SKYNET were up to 0.07 larger than those from AERONET,
and the major error sources were identified as an underestimation of solid viewing angle (SVA) and cloud
contamination. Correction of these known error factors reduced the SSA
difference to less than 0.03. Retrievals of other atmospheric constituents by the sky radiometer were also
reviewed. Retrieval accuracies were found to be about 0.2 cm for
precipitable water vapor amount and 13 DU (Dobson Unit) for column ozone amount.
Retrieved cloud optical properties still showed large deviations from
validation data, suggesting a need to study the causes of the differences. It is important that these recent studies on improvements presented in the
present paper are introduced into the existing operational systems and future
systems of the International SKYNET Data Center.
We examine the impact of atmospheric aerosols and clouds on the surface solar radiation and solar energy at Nainital, a high-altitude remote location in the central Gangetic Himalayan region (CGHR). For this purpose, we exploited the synergy of remote-sensed data in terms of ground-based AERONET Sun Photometer and satellite observations from the MODerate Resolution Imaging Spectroradiometer (MODIS) and the Meteosat Second Generation (MSG), with radiative transfer model (RTM) simulations and 1 day forecasts from the Copernicus Atmosphere Monitoring Service (CAMS). Clouds and aerosols are one of the most common sources of solar irradiance attenuation and hence causing performance issues in the photovoltaic (PV) and concentrated solar power (CSP) plant installations. The outputs of RTM results presented with high accuracy under clear, cloudy sky and dust conditions for global horizontal (GHI) and beam horizontal irradiance (BHI). On an annual basis the total aerosol attenuation was found to be up to 105 kWh m−2 for the GHI and 266 kWh m−2 for BHI, respectively, while the cloud effect is much stronger with an attenuation of 245 and 271 kWh m−2 on GHI and BHI. The results of this study will support the Indian solar energy producers and electricity handling entities in order to quantify the energy and financial losses due to cloud and aerosol presence.
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