Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 72)

7.4.2.3 Objective Algorithm

The objective algorithm described by Harrison and Michalsky provides a means to remove observations

that may contaminate the Langley calibration method using a quantitative approach. The methodology

is used on airmass between 2 and 6 where airm ass changes are rapid, but the problem of atm ospheric

refraction increasing the uncertainty of the analyses is avoided.

The method for direct pointing instruments consists of four steps to remove observations that have

been contaminated:

(1) A forward finite-difference derivative filter is used to remove regions where the slope of the

dI/dm curve is positive indicating atmospheric variability not consistent with uniform airmass-

turbidity processes such as cloud contamination. By determining the minimum value of the

derivative, the process removes observations for a time period equal to the time between

the positive derivative and the minimum derivative on both sides of the minimum.

(2) A second forward finite-difference derivative filter is then used to determine regions of strong

second derivatives. In these regions, if the first derivative is negative and the value greater

than twice the mean value of the first derivative for the observations, the regions are eliminated.

This method eliminates observations that may have been contaminated by cloud, but missed

using Step 1 and those regions where the data has been truncated.

(3) Perform a least-squares fit to the remaining data. The standard deviation of the residuals

about the fit is calculated and all points that have a residual greater than 1.5 times the standard

deviation are eliminated. A second least-squares fit is computed on the remaining observations.

(4) More than a of the original observations must be found valid in this manner and the standard

deviation of the residuals about the regression line must be less than 0.006 before the Langley

calibration is accepted.

The methods described in Sections 7.4.2.2 and 7.4.2.3 can be combined.

7.4.3 Lamp Calibrations

Standard Lamps have not been used generally in calibrating spectral radiometers used in the measurement

of AOD. This has arisen because of the lack of high quality top-of-the-atmosphere solar spectral data

that existed until recently. As AOD is a relative measure, even a perfectly calibrated system, in absolute,

terms could not be used to obtain optical depths without a high solar spectrum with uncertainties lower

than those needed for the calculation of AOD. Top-of-the-atmosphere spectra are now available that

allows for absolute calibrations of spectral radiometers.

Standard lamps have been used successfully for the calibration of absolute spectral intensity. Most

national standards’ laboratories can provide calibrations of this type at the wavelengths used in standard

AOD measurements. Lamp output uncertainty varies by wavelength, with the greater uncertainties

in the UV portion of the spectrum. This uncertainty is due to the decreased lamp output in this portion

of the spectrum and because small changes in the output current of the voltage source cause large

changes in lamp irradiance. A 0.1% variation in current changes the lamp output by 0.9% at 300 nm

and by 0.4% at 500 nm.

Standard uncertainties in calibrated-lamp output and transfer uncertainties remain large in comparison

with an AOD uncertainty of better than 1%. At 500 nm, the uncertainty associated with a lamp calibrated

by the National Institute of Standards and Technology (NIST) of the United States of Am erica is

approximately ± 1%. Transfer of this standard to a secondary standard (one used by certified calibration

laboratories) in the visible wavelength region is about ± 0.5%. Further uncertainties will be added with

the transfer from the secondary standards’ laboratory to the calibrated lamp used in the calibration

of the radiometer. The increasing uncertainty with multiple transfers suggests that standard lamp

calibrations be used only when other means are unavailable.

The use of the same methodology described below, when used with a detector-based standard, provides

an instrument calibration based only on the uncertainty associated with the standard detector.

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