Methodology

Instrumentation

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).

Products and algorithms

Objective and
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SKYNET provides the following optical and microphysical properties of aerosols;

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 semi-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.

Calibration

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. SKYNET has also the intention to participate to inter-comparison programs and campaigns.

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

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
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.