Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
7
11
2007
10.5194/acp-7-2817-2007
http://www.atmos-chem-phys.net/7/2817/2007/
http://www.atmos-chem-phys.net/7/2817/2007/acp-7-2817-2007.html
http://www.atmos-chem-phys.net/7/2817/2007/acp-7-2817-2007.pdf
2817
2837
2007-06-07
Towards closure between measured and modelled UV under clear skies at four diverse sites
J. Badosa
jordi.badosa@gmail.com
R. L. McKenzie
M. Kotkamp
J. Calbó
J. A. González
P. V. Johnston
M. O'Neill
D. J. Anderson
Departament de Física, Grup de Fisica Ambiental, Universitat de Girona, Girona, Spain
National Institute of Water and Atmospheric Research (NIWA), Lauder, New Zealand
Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, USA
Bureau of Meteorology (BoM), Melbourne, Australia
The purpose of this work is determine the extent of closure between
measurements and models of UV irradiances at diverse sites using state of
the art instruments, models, and the best available data as inputs to the
models. These include information about aerosol optical depth (unfortunately
not extending down as far into the UVB region as desirable because such
information is not generally available), ozone column amounts, as well as
vertical profiles of temperature. We concentrate on clear-sky irradiances,
and report the results in terms of UV Index (UVI).
<br><br>
Clear-sky data from one year of measurements at each of four diverse sites
(Lauder – New Zealand, Mauna Loa Observatory – Hawaii, Boulder – Colorado,
and Melbourne – Australia) have been analysed in detail, also taking account
of different measurements of ozone, including satellite-derived values, as
well as ground measured values, both from Dobson instruments and as
retrieved from the UV spectra under study. Previous studies have generally
focussed on data from a single site, and for shorter periods. As such, it is
the most comprehensive study of its kind to date.
<br><br>
At Lauder, which is the cleanest low altitude site, we obtained agreement
between measurement and model at 5% level, which is consistent with the
best agreement found previously. At Mauna Loa Observatory, similar agreement
was achieved, but model calculations need to allow for reflections from
cloud that are present below the observatory. At this site, there are
occasional problems with using satellite-derived ozone. At Boulder, mean
agreements were similar but the dispersion around the mean was slightly
larger, corresponding to larger uncertainties in the aerosol inputs to the
model. However, at Melbourne, which is the only non-NDACC (Network for the
Detection of Atmospheric Composition Change) site, there remain unexplained
discrepancies. The measured values are significantly lower than the
calculated values. We investigate the extent to which this discrepancy can
be explained by incomplete knowledge of aerosol extinctions in the UV at
this site. We conclude that further information about aerosol optical depth
and single scattering albedo in the UVB region is needed to resolve the
issues. For more polluted sites (the four considered locations show in
general small aerosol load), the uncertainties in the aerosol input
parameters would lead to less confidence on the modelling approach. At the
three NDACC sites, the closure provided by the study gives confidence in
both the measurements and our ability to model them. This study revealed a
limitation in the use of PTFE diffusers when temperatures are lower than
approximately 20°C. It also documents the range of clear sky UVI values
expected at these diverse sites.
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