Measurement and modeling of spectral UV irradiance at
I did my PhD studies at the
Fraunhofer Institute for Atmospheric
Technical University of Ilmenau. The thesis, which
was finished in 1996, is written in German -
if you are interested, I can send you a copy. The main results,
however, are published in refereed Journals anyway, see below.
- The main aim of this work was to combine experimental and theoretical
methods to investigate the transfer of ultraviolet radiation
through the Earth's atmosphere.
- Global spectral irradiance
has been measured continuously over
two years, using a Bentham DTM 300 double monochromator.
- The global irradiance data were supplemented by
direct spectral irradiance measurements using a narrow-angle
entrance optics on a sun-tracking device. These data were used to derive
a time series of total ozone
and aerosol optical depth
which are the most important atmospheric
parameters determining the UV irradiance under
cloudless sky conditions.
- Using and adapting a
radiative transfer model,
it has been shown that, at unpolluted locations like
Garmisch-Partenkirchen, global spectral irradiance
can be simulated within about ±5%, using only total ozone
and spectral aerosol optical depth as input to the model.
- The evaluated model is an ideal tool to study the effects of
various parameters on UV irradiance because it provides the normalization
which is required to separate certain effects. For example, to quantify the
attenuation by aerosols, a measured spectrum can be compared with a spectrum simulated
for the conditions at the measurement time (that is, the actual solar zenith angle,
surface albedo, and total ozone), but for aerosol-free atmosphere.
Such investigations are nearly impossible using measurements only because
conditions which differ by only one of the parameters are hardly found,
even in a data set as large as the one available here.
- Aerosols have been shown to reduce global
irradiance by typically 5-15%, even at an unpolluted
site. In particular, it was found that the decrease
of global irradiance scaled approximately linear
with the aerosol optical depth, being about 20%
for an optical depth of 1.
- Enhanced albedo due to snow cover
in winter was found to increase the global UV irradiance
by about 10-15%, corresponding to an average effective albedo
of 0.38. This relatively low value can be explained by large
fractions of the surrounding area which are woodland or city.
- The monthly averaged reduction
of UVA irradiance due to clouds
was calculated by normalizing each of the 60,000 measured
spectra to the corresponding, model-simulated, cloudless
- Thick clouds were found to
enhance absorption of UV
irradiance by increasing the photon path within the cloud.
A longer path corresponds to more absorption, and in an
extreme case, the tropospheric pathlength was found to be
enhanced by a factor of 10 which, concerning its effect
on spectral UV irradiance, acted effectively like a doubling
of total ozone. Although such effects occur only when
the irradiance is low anyway, they have serious consequences
for remote sensing methods which make use of scattered
radiation to infer atmospheric parameters like total ozone.