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Improved techniques for the spatial and temporal measurement of ultraviolet radiation

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Kuchinke, Christopher Paul (2002) Improved techniques for the spatial and temporal measurement of ultraviolet radiation. PhD thesis, University of Tasmania.

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Abstract

One of the more difficult tasks confronting atmospheric researchers today is the
acquisition of long-term radiometric measurements that encapsulate variability in the
sky hemisphere as well as time. High quality spatial measurements would allow for a
greater understanding of atmospheric processes, both specific to the site and between
locations. This information would lead to further development of better spatial
models for predicting future trends. This study develops a timer-controlled variable
sky-view platform (or VSP), designed to encapsulate both the spatial and temporal
variation of radiation in the sky hemisphere for Hobart (42.90 S; 147.33 E). The VSP
essentially allows for a sensor to move up and down inside a fixed cylinder, therefore
causing the sky view factor to vary. It is currently in use with the erythemal
ultraviolet (UV) biometer 501-A manufactured by Solar Light Company Inc. (SLC).
As such, the VSP provides an additional source of UV sky distribution data to the
Hobart UV-climatology database.
Before VSP deployment of the SLC instrument, it was first necessary to fully
characterise its cosine response in an accurate calibration scheme. Here, calibration
of the detector is dependent upon wavelength as well as solar zenith angle, due to the
discrepancy between the actual instrument relative spectral response and the
erythemal response curve. In this study, measurements were compared for the
purpose of characterising a new cosine response function (CRF) for the broadband
detector. Concurrent measurements of spectral and broadband erythemal UV data
have been undertaken at the University of Tasmania in Hobart, Australia.
Measurements spanned solar zenith angles (SZA) of 21°-74°, total ozone values of
220 to 320-DU and relatively small aerosol optical depths. The CRF is both
wavelength and SZA dependent and applicable to all broadband UV-B instruments
based on the original Robertson-Berger design. A methodology is presented for
incorporation of the CRF into the calibration scheme. The concept of spectral
multipliers is introduced, which are unique to each broadband instrument and correct
for the wavelength dependence of the CRF. A method is then described for ongoing
calibration of the detector based on model data alone. The scheme is then tested with
real data to validate its reliability. Overall, by nature of the simultaneous
characterisation of both spectral and cosine response, the CRF allowed for a more
robust calibration for the VSP deployed sensor. A full description of the VSP design, electronics and construction is then given.
Emphasis is on high precision long-term operation under all weather conditions. A
modelling scheme and output results illustrate the advantages of the VSP over current
sky observing instruments. It can accurately parameterise the direct and diffuse
erythemal irradiance as well as the azimuthally-integrated erythemal sky radiance
distribution for every two minutes duration (at the current setting). In addition, all
data can be obtained for clear-sky and cloudy conditions. Analysis of the clear-sky
VSP sky distribution, in conjunction with spectroradiometer radiance measurements,
allows for derivation of the azimuthally-integrated circumsolar component for each
zenith angle.
The thesis concludes with an application of the VSP for the all-sky calibration of
other radiometric instruments. Occulting disk or shadowband arrangements are often
applied to both spectral and broadband sensors in order to retrieve the total diffuse irradiance. Here, extraction of the diffuse component is simply calculated as a
difference between the measured global and direct components. However, difficulty
lies in the application of a suitable diffuse calibration due to the instrument cosine
response. Generally, the sky distribution is assumed isotropic during the correction,
whereas it has been shown in most studies to be nearly always anisotropic. Hence,
without in situ sky distribution measurements it is difficult to obtain the correct total
diffuse measurement. This study utilises VSP-extracted erythemal sky distribution
data. Results are used to correct for the cosine error of instruments that measure
diffuse erythemal radiation data, for both clear-sky and cloudy conditions. A
comparison was then made between corrected diffuse values resulting from
application of an isotropic versus the real sky distribution. Results showed that an
isotropic diffuse correction overestimated the real diffuse signal by a maximum of 4
to 6% for clear skies, with this magnitude decreasing with an increase in SZA. This is
in accordance with the increase in sky isotropy at high SZAs. In contrast, isotropic
corrections for increasing cloud cover displayed greater fluctuation in the
overestimate statistic over relatively short time periods. Here, variability at low to
mid SZAs (for both cloud types) ranged from approximately 2 to 5% for moderate
cloud cover and zero to 10% for high cloud cover. Furthermore, results can again
differ between instruments for the same cloud and SZA conditions due to cosine
response differences. Finally, the overestimate variability under cloudy skies tended
to increase with SZA such that the overall trend displayed less SZA dependence than
that for clear skies. Here, cloud distribution masks the effect of increased sky
isotropy.

Item Type: Thesis (PhD)
Keywords: Ultraviolet radiation
Copyright Holders: The Author
Copyright Information:

Copyright 2002 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

For consultation only. No loan or photocopying permitted until 14 June 2004. Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:43
Last Modified: 11 Sep 2017 22:53
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