Product & Method

Instrumentation

The standard instrument of each SKYNET site 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).

Product

ISDC implements two data analysis flows (SR-CEReS & ESR-MRI) and provides two types of standard products by the above two data analysis flows. The following are typical products provided by the two analysis flows(*1). Related information can be found here.

Aerosol optical thickness (AOT) at wavelengths of 340, 380, 400, 500, 675, 870, 1020 nm
Single scattering albedo (SSA) of aerosols at the same wavelength of AOT
Refractive index (RI) at the same wavelength of AOT
Volume size distribution of aerosols(dV/dlnr)
Angstrom exponent (AE)(*2)

You can download or view the products from here.

(*1) The wavlengths and products differ slightly depending on the instruments used, analysis flow, and measurement defects. Columnar ozone and water vapor estimations, as well as cloud optical thickness using different wavelengths are under development in the Regional Sub-Networks.
(*2) The wavelengths used for AE calculation are the same wavelength of AOT for SR-CEReS; those are wavelengths of 400, 500, 675, 870, and 1020 nm for ESR-MRI.

--- Release of Standard Products---

< SR-CEReS flow >
ISDC releases the near-real time data as Level 2 (L2).

< ESR-MRI flow >
ISDC releases the following levels of data:

L2A: provided only for ESR-MRI; obtained using the precious month calibration constants. They will be released in near-real time.
L2: data products obtained reprocessing L2A data with the updated calibration constnts. They will be released at the beginning of each month, together with the calibration constants(*3).

(*3) The calibration constants are available for the Principal Investigators (PIs) of the instruments and anyone who explicitly requests them to ISDC.

Method & Calibration

In the above two analysis flows, the products are retrieved mainly using SKYRAD.pack, a software package implemented for the POM sky radiometer (e.g., Nakajima et al., 1996) that has been previously validated and compared with other inversion algorithms (Che et al., 2008; Estelles et al., 2012a). The SKYRAD.pack and calibration methods used for each analysis flow are as follows. Related information can be found here.

< SR-CEReS flow >

Core algorithm: SR-CEReS(version 01.00.00) which has been developed based on SKYRAD.pack(version 5.0) by CEReS/Chiba university to produce aerosol to produce aerosol products in automatic and systematic.
Cloud screening: using the method of Khatri and Takamura (2009) but global irradiance data from a pyranometer are not used for consistent analysis over as many observation sites as possible. It corresponds to the combination of 1) spectral variability test (Kaufman et al., 2006) and 2) statistical analysis test (same as the method of Smirnov et al. (2000) but without triplet stability criteria test), including a) check the number of data, b) diurnal stability check, c) smoothness criteria, and d) three standard deviation criteria.
Calibration: solar calibration constants F0 from XIL Method (Nakajima et al., 2020), and solid view angles (original values from Prede company, values calculated by the disk scanning method, or the lamp method, depending on the instrument).
Product & Download: AOT, SSA, RI, dV/dlnr, and AE. To download the products, click here and select "SR-CEReS" in "Method".

< ESR-MRI flow >

Core algorithm: SUNRAD.pack(version 0.9), SKYRAD_MRI.pack (version 2) and SKYRAD.pack(version 4.2). SUNRAD.pack is an open source code provided by ESR which processes only the direct sun measurements and produces AOT and AE (Estelles et al., 2012b). SKYRAD_MRI.pack has been developed in Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA) [Kudo et al. 2021]. In this data analysis flow, the SKYRAD_MRI.pack uses the calibration constants estimated by SKYRAD.pack and retrieves L2A & L2 products.
Cloud screening: performed using a procedure based on the methodology developed by Smirnov et al. (2000) for SUNRAD products; performed using the methodology described in Momoi et al. 2020 for ESR-MRI products. The latter method is still under validation, and cloudy data are not rejected but flagged with number 1.
Calibration: F0 from Improved Langley Method (ILM, Campanelli et al, 2004, 2007) using as core the SKYRAD.pack(version 4.2) and solid view angles (original values from Prede or from disk scanning depending on the instrument).
Product & Download: AOT and AE from SUNRAD.pack; SSA, RI, AF, dV/dlnr, phase function, and asymmetry factor from SKYRAD_MRI.pack. To download the products, click here and select "ESR-sunrad" in "Method" if you need AOT and/or AE. If you need the other parameters such as SSA and dV/dlnr, select "ESR-MRI" in "Method".

Other calibration activities

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.
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 attenids validation field campaigns e.g. SAVEX-D.
SKYNET continue updating calibration methodology as well as retrieval and cloud screening codes as tasks of WGs within the SKYNET International organization.

References