Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
31
remained there.
The situation is different in areas where enough water is available like in the
large irrigation systems which partly use fossil water resources from underneath of
the Sahara. Here the surface temperature is kept low because a substantial amount
of the net radiation can be converted into latent heat which is exported from the
region.
In early autumn the situation may change insofar as now the sea remains
relatively warm in comparison to the atmosphere and the land. At this time of the
year convection may start already over sea and strong latent heat fluxes lead to
thunderstorms that enter the land. Though the surface temperature of large land
areas at this time of the year tend towards sea surface temperatures the contrast
between mountainous areas and plains is still considerable. This locally leads to
complex valley-mountain circulation systems and even catabatic winds known as
"Bora" at the eastern Adriatic coast.
Topography also gives rise to small scale climate anomalies such as dry lowland
"islands" surrounded by elevated areas with higher rainfall (Guzzi, 1981). The
opposite is experienced if in a dry environment ground water reaches the surface e.g.
in depressions and creates the well known "oasis effect" with reduced temperatures
and higher air humidity.
4.5 Experimental Strategies to Deal with the Hierarchy of Scales
The discrepancy between the wide range of scales involved in the land surface -
atmosphere interactions and the limited possibilities to measure the quantities which
govern the land-surface processes and the surface - atmosphere interaction enforces
to rationalize the number of stations which have to be distributed over an area
compatible with the resolution of global climate models. Ideally the measurements
should be carried out for time periods the individual processes need to go through
at least one complete cycle. This would be of the order of days if the diurnal wave
of the energy fluxes is considered, of the order of a year if the annual hydrological
or vegetation cycles are investigated, of the order of a decennium if the climate
variability is taken into account, and of the order of hundred years, if the life cycle
of a forest is under observation. It is clear that the long time scales can only be
covered by operational observation systems with in most cases spatially scattered
measuring sites.
EFEDA (see Appendix 1) was an experiment designed to study the shorter end
of the time scales and specifically the transition from the wet to the dry period
during the month of June in the desertification threatened area of Castilla - La
Mancha in Spain. To bridge nevertheless over to the longer time scales, some
measurements were prolonged to three months, some groups revisited the area after
three years and satellite data have been evaluated for ten years. Furthermore it could
be relied upon data of the meteorological and hydrological networks.
The goal was to investigate simultaneously all interacting processes from local
31
remained there.
The situation is different in areas where enough water is available like in the
large irrigation systems which partly use fossil water resources from underneath of
the Sahara. Here the surface temperature is kept low because a substantial amount
of the net radiation can be converted into latent heat which is exported from the
region.
In early autumn the situation may change insofar as now the sea remains
relatively warm in comparison to the atmosphere and the land. At this time of the
year convection may start already over sea and strong latent heat fluxes lead to
thunderstorms that enter the land. Though the surface temperature of large land
areas at this time of the year tend towards sea surface temperatures the contrast
between mountainous areas and plains is still considerable. This locally leads to
complex valley-mountain circulation systems and even catabatic winds known as
"Bora" at the eastern Adriatic coast.
Topography also gives rise to small scale climate anomalies such as dry lowland
"islands" surrounded by elevated areas with higher rainfall (Guzzi, 1981). The
opposite is experienced if in a dry environment ground water reaches the surface e.g.
in depressions and creates the well known "oasis effect" with reduced temperatures
and higher air humidity.
4.5 Experimental Strategies to Deal with the Hierarchy of Scales
The discrepancy between the wide range of scales involved in the land surface -
atmosphere interactions and the limited possibilities to measure the quantities which
govern the land-surface processes and the surface - atmosphere interaction enforces
to rationalize the number of stations which have to be distributed over an area
compatible with the resolution of global climate models. Ideally the measurements
should be carried out for time periods the individual processes need to go through
at least one complete cycle. This would be of the order of days if the diurnal wave
of the energy fluxes is considered, of the order of a year if the annual hydrological
or vegetation cycles are investigated, of the order of a decennium if the climate
variability is taken into account, and of the order of hundred years, if the life cycle
of a forest is under observation. It is clear that the long time scales can only be
covered by operational observation systems with in most cases spatially scattered
measuring sites.
EFEDA (see Appendix 1) was an experiment designed to study the shorter end
of the time scales and specifically the transition from the wet to the dry period
during the month of June in the desertification threatened area of Castilla - La
Mancha in Spain. To bridge nevertheless over to the longer time scales, some
measurements were prolonged to three months, some groups revisited the area after
three years and satellite data have been evaluated for ten years. Furthermore it could
be relied upon data of the meteorological and hydrological networks.
The goal was to investigate simultaneously all interacting processes from local