5th Swiss Geoscience Meeting, Geneva 2007
Short-term variability of erythemal UV radiation due to
clouds
Walker Daniel* **, Vuilleumier Laurent*
* Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland
(daniel.walker@meteoswiss.ch)
** Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland
This project is part of the ongoing COST action 726 whose goal is the
reconstruction of erythemal ultraviolet radiation for time periods and locations
where no measurements are available. Ultraviolet radiation (UV) is part of the
solar spectra reaching the earth's surface, influencing human beings and
therewith having an impact on public health. Erythemal UV radiation is a
measure to quantify these health effects by weighting different wavelengths
accordingly to their harmfulness for the human skin. Furthermore, UV radiation
also shows impacts in biology, various ecosystems and materials sciences.
Medics and policy makers request more information about the spatial
distribution and temporal evolution of this part of solar radiation.
In Switzerland, erythemal UV radiation is measured operationally at four
locations in different climate regimes since mid to end of the 90ies. The
available time series are spatially too sparse and temporally too short to answer
current needs including trend analyses. Therefore, radiative transfer models are
used to approach this data problem. However, simulating the effect of clouds on
UV radiation is difficult and treating scattered cloud coverage is extremely
difficult even with 3D-models. On the other hand, being able to handle not only
clear-sky situations but various cloud conditions is actually the crucial part in UV
reconstruction. An alternative way to describe the influence of clouds on UV
radiation is the use of global shortwave radiation as a proxy. This parameter
holds information about the transmittance of the atmosphere and describes the
cloud effect over the whole solar spectra. Furthermore, the advantage of global
shortwave radiation is the vast availability of measurements and the long time
series available in Switzerland.
Semi-empirical relationships between UV and global shortwave radiation are
presented for the different locations in Switzerland where UV is measured.
These relationships lead to cloud modification factors (CMF) which are used for
estimating UV doses for all-sky conditions. The connection between the CMFs
in the UV and shortwave radiation is linear and differs for almost overcast and
partly cloudy skies. Furthermore, the dependencies of the derived relationships
on the solar zenith angle and total ozone content are investigated. Correlations
between CMFs in UV and global shortwave radiation up to 0.89 are found with
higher values for heavy cloud coverage.