24
H.-J. Bolle
4 The Complexity of the Mediterranean System:
Interactions at Various Scales
4.1 Scales of Interaction
Different disciplines use different names and definitions for the scales which they
are addressing. Therefore here some neutral terms are used. It is clear what the
global scale means and "large or macro scale" addresses something of the order of
a continent. The "synoptic scale" (also addressed as 'meso ex', 4 * 10 4 - 4 * 10 6
km 2 ) is that of travelling eddies, of the order of 10 6 km 2 . "Mesoscale" covers an area
of the order of 10 4 - 10 5 km 2 (sometimes "meso P" is used for 4 * 10 2 - 4 * 10 4
km 2 ). The "regional scale" is that of a province or a small to medium sized
watershed (10 3 - 10 4 km 2 ) and "local" covers about 1- 10 3 km 2 (almost equivalent to
"meso y", 4 - 400 km 2 ). The scale below I km 2 also sometimes is addressed as
'micro'.
The vertical extent of the layer in which interactions between the atmosphere and
the ground is perceptible has different and variable dimensions. The surface extends
its influence on the structure of and processes in the atmosphere up to the top of the
atmospheric boundary layer which ranges between a few hundred metres to about
3 km, depending on the roughness and temperature of the surface. The lower
boundary is determined by hydrological and thermal processes in the soils and
aquifers as well as the structure and vegetation of the surface. The diurnal heat wave
penetrates the soil to 30 - 40 cm depth. Evaporation occurs in the upper centimetres
of the soil but plants can draw water from much deeper layers. To understand the
interaction between the locally highly variable sub-surface hydrological regime and
the atmospheric system the processes of heat and water transfers in soils are as
important as the evaporation processes at the surface and the transport processes just
above the surface.
4.2 Large Scale Processes
The specific situation of the European -African -Near East Medi terrane an is that land
masses with highly structured coastlines surround nearly completely the
Mediterranean Sea at a latitude where the general atmospheric circulation regime
has a large seasonal variability. This variability is caused by the shift of the wets
wind belt (the Westerlies) between a more southward position during winter into a
more northward position during summer. This brings the Mediterranean basin under
the influence of the Westerlies during winter and the descending branch of the
Hadleyl circulation during summer. The influence of the Westerlies respectively the
2 The Hadley circulation consists of rising wet air near the equator that descents as very dry air around
(continued ... )
H.-J. Bolle
4 The Complexity of the Mediterranean System:
Interactions at Various Scales
4.1 Scales of Interaction
Different disciplines use different names and definitions for the scales which they
are addressing. Therefore here some neutral terms are used. It is clear what the
global scale means and "large or macro scale" addresses something of the order of
a continent. The "synoptic scale" (also addressed as 'meso ex', 4 * 10 4 - 4 * 10 6
km 2 ) is that of travelling eddies, of the order of 10 6 km 2 . "Mesoscale" covers an area
of the order of 10 4 - 10 5 km 2 (sometimes "meso P" is used for 4 * 10 2 - 4 * 10 4
km 2 ). The "regional scale" is that of a province or a small to medium sized
watershed (10 3 - 10 4 km 2 ) and "local" covers about 1- 10 3 km 2 (almost equivalent to
"meso y", 4 - 400 km 2 ). The scale below I km 2 also sometimes is addressed as
'micro'.
The vertical extent of the layer in which interactions between the atmosphere and
the ground is perceptible has different and variable dimensions. The surface extends
its influence on the structure of and processes in the atmosphere up to the top of the
atmospheric boundary layer which ranges between a few hundred metres to about
3 km, depending on the roughness and temperature of the surface. The lower
boundary is determined by hydrological and thermal processes in the soils and
aquifers as well as the structure and vegetation of the surface. The diurnal heat wave
penetrates the soil to 30 - 40 cm depth. Evaporation occurs in the upper centimetres
of the soil but plants can draw water from much deeper layers. To understand the
interaction between the locally highly variable sub-surface hydrological regime and
the atmospheric system the processes of heat and water transfers in soils are as
important as the evaporation processes at the surface and the transport processes just
above the surface.
4.2 Large Scale Processes
The specific situation of the European -African -Near East Medi terrane an is that land
masses with highly structured coastlines surround nearly completely the
Mediterranean Sea at a latitude where the general atmospheric circulation regime
has a large seasonal variability. This variability is caused by the shift of the wets
wind belt (the Westerlies) between a more southward position during winter into a
more northward position during summer. This brings the Mediterranean basin under
the influence of the Westerlies during winter and the descending branch of the
Hadleyl circulation during summer. The influence of the Westerlies respectively the
2 The Hadley circulation consists of rising wet air near the equator that descents as very dry air around
(continued ... )