Methodology

Processing

ESR is a research and development network providing the following products:

A link to the AERONET products will be made available for those ESR sites where Cimel CE318 belonging to AERONET are co-located. This information will be provided for easiness of comparison between network products.

1. The standard inversion from SKYNET is performed only for PREDE POM instruments; it is the basic official analysis developed in cooperation with the SKYNET data center at Chiba (Japan).
Products are: aerosol optical depth at all the available wavelengths, Angström exponent using all the available wavelengths from 400 to 1020 nm, single scattering albedo, phase function,complex refractive index and aerosol size distribution.

Products are provided at 4 different levels:

Level 0: daily analysis are performed using the last available calibration constants. Burden of absorbent gases (O3 and NO2) are chosen from nominal values or previous climatologies. Daily plots of Level O output, when available, are shown in the main page of each site, together with the Level O data previously processed within the same month.They can be daily downloaded.

Level 1: data are re-processed at the end of each month after the calculation of the last solar calibration constants, and, when possible, updating the measured gases contents. They can be monthly downloaded.

Level 2: the following screening criteria are applied to Level 1 data (both 1.1 and 1.2).
- Reject result if retrieval error is greater than 0.07
- Reject result for irregular volume size distribution

Level 3: the following screening criteria are applied to Level 1 data (both 1.1 and 1.2).
- Reject result if AOT(500nm) is less than 0.4
- Reject result if retrieval error is greater than 0.07
- Reject result for irregular volume size distribution

The Cloud screening is performed using the method described by Khatri and Takamura (2009).
Two types of products are possible:

Cloud screening 1: is performed when measurements of global irradiance are not available.
Cloud screening 2: is performed using simulataneous and co-located measurements of global irradiance.


2. The ESR.pack (Estellés et al., 2012) is an open source code, developed in two modules:

Sunrad module. This module processes the direct sun measurements from both PREDE POM and CIMEL CE 318 instruments.
Products are: aerosol optical depth, Angström exponent using all the wavelenghts from 400 to 1020 nm, and columnar water vapor direct.
This module is implemented in two different modes:

More details on the differences between modes 1 and 2 are given by Estellés et al. (2012a; 2012b).

Skyrad module.This module processes the sky radiance measurements of CIMEL CE 318 instruments.
Products are:phase function, single scattering albedo, complex refractive index and aerosol size distribution.

In this web page products are provided from the Sunrad module, mode 2 at 2 different levels:

Level 0: daily analysis are performed using the last available calibration constants. Burden of absorbent gases (O3 and NO2) are chosen from nominal values or previous climatologies.
Daily plots of Level O processing, when available, are shown in the main page of each site, together with the Level O data previously processed within the same month.

Level 1: data are re-processed at the end of each month after the calculation of the last solar calibration constants, and, when possible, updating the measured gases contents. They can be monthly downloaded.

The Cloud screening is performed using a procedure based on the methodology developed by Smirnov et al. (2000). Data from CIMEL CE 318 are cloud screened using the method explained by Holben et al. (1998).

Calibration

In the solar aureole technique, two different calibrations are needed:

The solar calibration constant (F0): it is the signal corresponding to the solar irradiance incident at the top of the atmosphere. In SKYNET and ESR, it is retrieved by an in situ procedure called Improved Langley (Campanelli et al 2004; Campanelli et al. 2007). The Improved Langley is a modified version of the standard Langley plot technique. In the Improved Langley, F0 is retrieved by fitting the natural logarithm of the direct solar irradiance versus the product of the relative optical air mass and the total extinction optical thickness (retrieved by the inversion) instead of the air mass alone, as done with the standard Langley plot. This in situ calibration procedure allows the operators to track and evaluate the calibration status on a continuous basis, keeping the revision of the instrument mostly for maintenance reasons and offering the advantage of reducing the number of instrument expeditions. In this way, the data gaps incurred by the periodical shipments are considerably reduced, and the likelihood of instrumental damages attributable to transport also decreases.

The Solid view angle: it corresponds to the field of view of the instrument. Its value can be approached by the geometric solid viewing angle of the telescope. However, several factors contribute to this value: color aberration of the lens, diffraction at the edges, misalignment of the optical axis, and surface nonuniformity of filters and sensor. As a consequence, an in situ method is made available for determining the actual solid view angle (only for PREDE sun-sky radiometers) from optical data. This in situ method consists on performing a scanning of the irradiance field around the Sun, centered at the origin of a local system of rectangular coordinates (Boi et al. 1999).

References

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Please, contact webmasters:
Victor Estelles
Igor Bertello