Global Change Abstracts The Swiss Contribution - SCNAT

Global Change Abstracts – The Swiss Contribution | Atmosphere 41
cloud was in contact with the ground and the
measuring station. Both water and ice clouds were
examined at different times of the year. The second
series of experiments is the CLOud Processing
of regional Air Pollution advecting over land and
sea (CLOPAP) series, where ageing pollution aerosol
from UK cities was observed, from an airborne
platform, to interact with warm stratocumulus
cloud in a cloud- capped atmospheric boundary
layer. Combining the results it is shown that aged
pollution aerosol consists of an internal mixture
of organics, sulfate, nitrate and ammonium, the
organic component is dominated by highly oxidized
secondary material. The relative contributions
and absolute loadings of the components
vary with location and season. However, these
aerosols act as Cloud Condensation Nuclei (CCN)
and much of the organic material, along with the
other species, is incorporated into cloud droplets.
In ice and mixed phase cloud, it is observed that
very sharp transitions (extending over just a few
metres) are present between highly glaciated regions
and regions consisting of supercooled water.
This is a unique. finding; however, aircraft
observations in cumulus suggest that this kind of
structure may be found in these cloud types too. It
is suggested that this sharp transition is caused by
ice nucleation initiated by oxidised organic aerosol
coated with sulfate in more polluted regions
of cloud, sometimes enhanced by secondary ice
particle production in these regions.
Faraday Discussions, 2008, V137, pp 205-222.
08.1-15
Limits on climate sensitivity derived from recent
satellite and surface observations
Chylek P, Lohmann U, Dubey M, Mishchenko M,
Kahn R, Ohmura A
USA, Switzerland
Modelling , Meteorology & Atmospheric Sciences
An analysis of satellite and surface measurements
of aerosol optical depth suggests that global average
of aerosol optical depth has been recently
decreasing at the rate of around 0.0014/a. This
decrease is nonuniform with the fastest decrease
observed over the United States and Europe. The
observed rate of decreasing aerosol optical depth
produces the top of the atmosphere radiative forcing
that is comparable to forcing due to the current
rate of increasing atmospheric concentration
of carbon dioxide and other greenhouse gases.
Consequently, both increasing atmospheric concentration
of greenhouse gases and decreasing
loading of atmospheric aerosols are major contributors
to the top-of-atmosphere radiative forc-
ing. We find that the climate sensitivity is reduced
by at least a factor of 2 when direct and indirect
effects of decreasing aerosols are included, compared
to the case where the radiative forcing is
ascribed only to increases in atmospheric concentrations
of carbon dioxide. We find the empirical
climate sensitivity to be between 0.29 and 0.48 K/
Wm(-2) when aerosol direct and indirect radiative
forcing is included.
Journal of Geophysical Research Atmospheres,
2007, V112, ND24, DEC 13 ARTN: D24S04.
08.1-16
Long-term trend analysis of aerosol variables
at the high-alpine site Jungfraujoch
Collaud Coen M, Weingartner E, Nyeki S, Cozic J,
Henning S, Verheggen B, Gehrig R, Baltensperger U
Switzerland
Meteorology & Atmospheric Sciences
This study reports the first long-term trend analysis
of aerosol optical measurements at the highalpine
site Jungfraujoch, which started 10.5 years
ago. Since the aerosol variables are approximately
lognormally distributed, the seasonal Kendall test
and Sen’s slope estimator were applied as nonparametric
methods to detect the long- term trends for
each month. The yearly trend was estimated by a
least-mean-square fit, and the number of years
necessary to detect this trend was calculated. The
most significant trend is the increase (4-7% yr(-1))
in light-scattering coefficients during the September
to December period. The light absorption and
backscattering coefficients and the aerosol number
concentration also show a positive trend during
this time of the year. The hemispheric backscattering
fraction and the scattering exponent
calculated with the smaller wavelengths (450 and
550 nm), which relate to the small aerosol size
fraction, decrease except during the summer,
whereas the scattering exponent calculated with
the larger wavelengths (550 and 700 nm) remains
constant. Generally, the summer months at the
Jungfraujoch, which are strongly influenced by
planetary boundary layer air masses, do not show
any long-term trend. The trends determined by
least-mean-square fits of the scattering and backscattering
coefficients, the hemispheric backscattering
fractions, and the scattering exponent are
significant, and the number of years necessary
to detect them is shorter than 10 years. For these
variables, the trends and the slopes estimated by
the seasonal Kendall test are therefore confirmed
by the least-mean- square fit results.
Journal of Geophysical Research Atmospheres,
2007, V112, ND13, JUL 13 ARTN: D13213.