Methodology

The standard instrument of each SKYNET sites is the sun-sky radiometer Prede Co. Ltd. model POM. The sun-sky radiometer is a scanning spectral radiometer able to perform routine and long-term automated measurements of direct and scattered solar radiations at seven wavelengths from 315 to 1020 nm (model POM-01) or eleven wavelengths from 315 to 2200 nm (model POM-02).

SKYNET provides the following optical and microphysical properties of aerosols;

• Aerosol optical thickness (AOT) at the following standard wavelengths: 340, 380, 400, 500, 675, 870, 1020 nm
• Single scattering albedo (SSA) of aerosols at the same standard wavelength of AOT
• Volume size distribution of aerosols
• Refractive index
• Phase function and asymmetry factor

These products are retrieved using SKYRAD.pack, a software package implemented for the POM sky radiometer (e.g., Nakajima et al., 1996, Hashimoto et al., 2012) that has been previously validated and compared with other inversion algorithms (Che et al., 2008; Estelles et al., 2012).

For better quality products, cloud screening is conducted using a code, the Cloud Screening Sky Radiometer (CSSR) (Khatri and Takamura, 2009).

Products are made available in near-real time for on-line sites and regularly for off-line sites.

Columnar ozone and water vapor estimations, as well as cloud optical thickness using different wavelengths are under development in the Regional Sub-Networks.

The sun-sky radiometers are monthly calibrated on site using the improved Langley method (Nakajima et al., 1996, Campanelli et, 2004, 2007).

Inter-calibration of the sun-sky radiometers with a master instrument calibrated using Langley method at a high mountain (e.g., Mt. Mauna Loa) is occasionally performed.

Furthermore, SKYNET has conducted inter-comparison with AERONET Cimel sun-photometers and precision filter radiometers (PFRs) at two SKYNET observation sites (Chiba and Valencia) in recent years (Estelles et al., 2016) and will continue this inter-comparison observation according to the possibility of the instruments to remain co-located in these sites. A traceability program to standard GAW-PFR has be planned in Europe, leaded by the ESR (Campanelli et al, 2016).

SKYNET continue updating retrieval and cloud screening codes as well as calibration methodology as tasks of WGs within the SKYNET International organization.

SKYNET is also attending validation field campaigns e.g. SAVEX-D.

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References

Campanelli, M., T. Nakajima, B.Olivieri: Determination of the solar calibration constant for a sun-sky radiometer, Applied Optics, Vol. 43 No. 3, 20 January 2004

Campanelli, M., V. Estelles, C. Tomasi, T. Nakajima, V. Malvestuto and J. A. Martinez-Lozan: Application of the SKYRAD improved Langley plot method for the in situ calibration of CIMEL sun-sky photometers, Applied Optics. Vol. 46, No. 14 May, 2007

Campanelli, M., V. Estelles, H. Diemoz, N. Kouremeti, S. Kazadzis, R. Becker, S. Vergari, S. Dietrich: The SKYNET radiometer Network: Aerosol Optical Depth retrieval performance at the ERC-IV campaign and long-term comparison against GAW-PFR and AERONET standard instruments, WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (CIMO TECO 2016), 2016

Che, H., G. Shi, A. Uchiyama, A. Yamazaki, H. Chen, P. Goloub, and X. Zhang: Intercomparison between aerosol optical properties by a PREDE skyradiometer and CIMEL sunphotometer over Beijing, China, Atmos. Chem. Phys., 8, 3199-3214, doi:10.5194/acp-8-3199-2008, 2008

Estelles, V., M. Campanelli, M. P. Utrillas, F. Exposito, and J. A. Martinez-Lozano: Comparison of AERONET and SKYRAD4.2 inversion products retrieved from a Cimel CE318 sunphotometer. Atmos. Meas. Tech., 5, 569-579, doi:10.5194/amt-5-569-2012, 2012

Estelles, V., N. Kouremeti, M. Campanelli, J. Grobner, J.A. Mari nez-Lozano, S. Kazadzis: Preliminary aerosol optical depth comparison between ESR/SKYNET, AERONET and GAW international networks. International SKYNET workshop, Rome (Italy) 2016.

Hashimoto, M., T. Nakajima, O. Dubovik, M. Campanelli, H. Che, P. Khatri, T. Takamura, and G. Pandithurai: Development of a new data-processing method for SKYNET sky radiometer observations, Atmos. Meas. Tech., 5, 2723-2737, 2012.

Khatri, P., and T. Takamura: An algorithm to screen cloud affected data for sky radiometer data analysis, J. Meteor. Soc. Japan, 87, 189-204, 2009.

Nakajima, T., G. Tonna, R. Rao, P. Boi, Y. Kaufman, and B. Holben: Use of sky brightness measurements from ground for remote sensing of particulate polydispersions, Appl. Opt., 35, 15, 2672-2686, 1996.