50
H.-I. Bolle
aerosol therefor is an atmospheric warming of a little bit more than half a degree per
day. Model computations with aerosol parameters observed at a hazy day (optical
depth at 550 nm around 0.7) as input data showed a marked effect on the
downwelling shortwave flux. Without aerosols the calculated flux at the surface was
530 Wm- z and with this aerosol load of the order of 400 Wm- z . The infrared flux
with aerosols was 390 Wm· 2 , which agreed with measurements, while the calculated
flux without aerosol was 370 Wm· z . These results indicate that the Sahelian aerosol
has a net cooling effect at the surface.
The spectral distribution of the optical depth during to a sand storm is presented
in Fig. 9. The effect of large particles can clearly be seen in the nearly linear
dependence of the optical depth on wavelength. In the Mediterranean basin the
Saharan aerosol effects may be slightly smaller because of the thinning effect during
the long transport. But even here during scirocca events the optical depth at 525 nm
increases to values around 0.8 as compared to a very clear day when it is around 0.2.
A selection of some characteristic cases of aerosol optical depths measured in the
Mediterranean area is presented in Fig. 9.
5.5 Unknown Budgets
As can be seen from these mostly preliminary data, the span of carbon fluxes is
large. An integration over the Mediterranean area has to account for very different
source and sink situations. One tool to integrate over the Mediterranean ecosystems
would be to apply models for the individual ecosystems and integrate over the area
they occupy. According to a model comparison organized by GAIM (Hibbard, 2000)
most of the applied models overestimate the NPP in the range of 0 to 1000 g C m 2
year' 1 in single cases up to a factor of three while they tend to underestimate the
higher values ofNPP measured in the range of 1000 to 1700 g C m- 2 year I. It would
need more measurements in different parts of the Mediterranean basin to validate
the available models and a careful integration over the Mediterranean ecosystems
e.g. by means of satellite data to decide how much the Mediterranean land surfaces
contribute to the global carbon budget respectively how sensitive the Mediterranean
flora is with respect to climate variability and change. To establish a complete
budget the effect of forest fires has to be rated against the NPP of regrowing forests
which is higher than that of an old forest. To reach a final conclusion the carbon
that remains after fires in the ground, the transport of carbon by rivers into the sea,
and the carbon cycle of the Mediterranean Sea have to be added.
Analog problems exist with respect to aerosols. Emission from industrial plants,
sea spray, and fires, the formation of aerosols by chemical processes, the flux of
Saharan dust particles and the local raise of soil particles by wind from dry surfaces
as well as the raining out of theses constituents are not well known. The high
variability of the concentrations due to wind direction, wind velocity, sedimentation
and precipitation makes it difficult to build reliable aerosol models. A key problem
is how the cloud climatology is affected by the changing aerosol load. For this
purpose also a cloud climatology separately for land and sea would be needed at
H.-I. Bolle
aerosol therefor is an atmospheric warming of a little bit more than half a degree per
day. Model computations with aerosol parameters observed at a hazy day (optical
depth at 550 nm around 0.7) as input data showed a marked effect on the
downwelling shortwave flux. Without aerosols the calculated flux at the surface was
530 Wm- z and with this aerosol load of the order of 400 Wm- z . The infrared flux
with aerosols was 390 Wm· 2 , which agreed with measurements, while the calculated
flux without aerosol was 370 Wm· z . These results indicate that the Sahelian aerosol
has a net cooling effect at the surface.
The spectral distribution of the optical depth during to a sand storm is presented
in Fig. 9. The effect of large particles can clearly be seen in the nearly linear
dependence of the optical depth on wavelength. In the Mediterranean basin the
Saharan aerosol effects may be slightly smaller because of the thinning effect during
the long transport. But even here during scirocca events the optical depth at 525 nm
increases to values around 0.8 as compared to a very clear day when it is around 0.2.
A selection of some characteristic cases of aerosol optical depths measured in the
Mediterranean area is presented in Fig. 9.
5.5 Unknown Budgets
As can be seen from these mostly preliminary data, the span of carbon fluxes is
large. An integration over the Mediterranean area has to account for very different
source and sink situations. One tool to integrate over the Mediterranean ecosystems
would be to apply models for the individual ecosystems and integrate over the area
they occupy. According to a model comparison organized by GAIM (Hibbard, 2000)
most of the applied models overestimate the NPP in the range of 0 to 1000 g C m 2
year' 1 in single cases up to a factor of three while they tend to underestimate the
higher values ofNPP measured in the range of 1000 to 1700 g C m- 2 year I. It would
need more measurements in different parts of the Mediterranean basin to validate
the available models and a careful integration over the Mediterranean ecosystems
e.g. by means of satellite data to decide how much the Mediterranean land surfaces
contribute to the global carbon budget respectively how sensitive the Mediterranean
flora is with respect to climate variability and change. To establish a complete
budget the effect of forest fires has to be rated against the NPP of regrowing forests
which is higher than that of an old forest. To reach a final conclusion the carbon
that remains after fires in the ground, the transport of carbon by rivers into the sea,
and the carbon cycle of the Mediterranean Sea have to be added.
Analog problems exist with respect to aerosols. Emission from industrial plants,
sea spray, and fires, the formation of aerosols by chemical processes, the flux of
Saharan dust particles and the local raise of soil particles by wind from dry surfaces
as well as the raining out of theses constituents are not well known. The high
variability of the concentrations due to wind direction, wind velocity, sedimentation
and precipitation makes it difficult to build reliable aerosol models. A key problem
is how the cloud climatology is affected by the changing aerosol load. For this
purpose also a cloud climatology separately for land and sea would be needed at