Towards a Mediterranean GEWEX
367
6 The Concept of Mediterranean Anchor Stations
The field experiment EFEDA carried out in Spain, 1991, covering an area of about
10 4 km" in Castilla - La Mancha lasted not much longer than one month (June).
This was just the time when the land-surface processes switched from the wet to the
dry regime but it was too short to include processes in the study which have much
longer response times such as some hydrological processes and their impacts on
vegetation. It was therefor desirable to carryon with at least a reduced number of
observations for a much longer time period. At the ground only a few stations could
further be operated but additional information could be deduced from satellite data.
EFEDA included an intensive comparison program between quantities measured
at the ground and information inferred from satellites. Because of the strong
seasonal variability of surface parameters and the large variety of ecosystems found
in the Mediterranean area it was at that time concluded that a number of stations at
which if not permanently then from time to time surface and atmospheric state
variables are measured which are also assessed from space would tremendously help
to improve the accuracy of the of the data derived from the sateIlite measurements
and would protect from misinterpretations which could occur if the area is not
enough known to the analyser of remote sensing data.
It was therefore proposed to arrange for a limited number of sites which should
be established in areas representative for the various Mediterranean environments
and climatic zones. Very important would be to measure such quantities which are
not routinely measured by operational services such as all six important components
of the radiation (solar, reflected, longwave up- and downweIling, PAR and UV),
sensible and latent heat fluxes, optical depth of the atmosphere, water vapour
amount of the atmosphere, soil moisture, surface emissivity, vegetation cover and
state and if there is a requirement additional special quantities of importance for the
region. The area should be large enough to fiIl a number of high resolution satellite
image pixel to make a thorough comparison. Then these high resolution pixel could
be averaged over the pixel size of medium resolution instruments such as on board
of meteorological satellites. These medium resolution images have to be used to
produce day-by-day data series which are needed to remove the influence of clouds
and to obtain continuous data series which high resolution imagery cannot provide.
These "Anchor Stations" should also offer the opportunity to perform limited
field experiments in case new sensors respectively the algorithms used to evaluate
the measurements with new sensors have to be calibrated and their performane
tested. They may also serve for training courses. To date there are so far only few
and not completely equipped stations which could serve as Anchor Stations. Their
number should greatly be enhanced and each country around the Mediterranean sea
should establish at least one of these stations which should build a network.
A Tunesian - Italian initiative recently took up this idea of an anchor station
network for trasect from south Tunesia to the Ligurian coast. Such a transect indeed
would cut through a number of representative Mediterranean ecosystems all of them
with different problems and probably different responses to climate change. It would
367
6 The Concept of Mediterranean Anchor Stations
The field experiment EFEDA carried out in Spain, 1991, covering an area of about
10 4 km" in Castilla - La Mancha lasted not much longer than one month (June).
This was just the time when the land-surface processes switched from the wet to the
dry regime but it was too short to include processes in the study which have much
longer response times such as some hydrological processes and their impacts on
vegetation. It was therefor desirable to carryon with at least a reduced number of
observations for a much longer time period. At the ground only a few stations could
further be operated but additional information could be deduced from satellite data.
EFEDA included an intensive comparison program between quantities measured
at the ground and information inferred from satellites. Because of the strong
seasonal variability of surface parameters and the large variety of ecosystems found
in the Mediterranean area it was at that time concluded that a number of stations at
which if not permanently then from time to time surface and atmospheric state
variables are measured which are also assessed from space would tremendously help
to improve the accuracy of the of the data derived from the sateIlite measurements
and would protect from misinterpretations which could occur if the area is not
enough known to the analyser of remote sensing data.
It was therefore proposed to arrange for a limited number of sites which should
be established in areas representative for the various Mediterranean environments
and climatic zones. Very important would be to measure such quantities which are
not routinely measured by operational services such as all six important components
of the radiation (solar, reflected, longwave up- and downweIling, PAR and UV),
sensible and latent heat fluxes, optical depth of the atmosphere, water vapour
amount of the atmosphere, soil moisture, surface emissivity, vegetation cover and
state and if there is a requirement additional special quantities of importance for the
region. The area should be large enough to fiIl a number of high resolution satellite
image pixel to make a thorough comparison. Then these high resolution pixel could
be averaged over the pixel size of medium resolution instruments such as on board
of meteorological satellites. These medium resolution images have to be used to
produce day-by-day data series which are needed to remove the influence of clouds
and to obtain continuous data series which high resolution imagery cannot provide.
These "Anchor Stations" should also offer the opportunity to perform limited
field experiments in case new sensors respectively the algorithms used to evaluate
the measurements with new sensors have to be calibrated and their performane
tested. They may also serve for training courses. To date there are so far only few
and not completely equipped stations which could serve as Anchor Stations. Their
number should greatly be enhanced and each country around the Mediterranean sea
should establish at least one of these stations which should build a network.
A Tunesian - Italian initiative recently took up this idea of an anchor station
network for trasect from south Tunesia to the Ligurian coast. Such a transect indeed
would cut through a number of representative Mediterranean ecosystems all of them
with different problems and probably different responses to climate change. It would