28
H.-J. Bolle
internal dynamics which to a large degree is determined by the temperature
gradients between the sea and the surrounding more or less vegetated land surfaces
as well as their large topographic and ecological manifold. The topographic effects
gain importance and interact with the much smoother large scale pressure systems
in the Mediterranean area. Mesoscale phenomena now prevail, which result from
topography, specifically the land-sea distribution. Because of its mountains and land
cover differences local to regional phenomena develop like the "pumping" on top of
elevated hot surfaces which create thermal lows in the high reaching Atmospheric
Boundary Layer (ABL), mountain induced wind systems or topographically induced
precipitation pattern. During summer semi-permanent high pressure systems with
sinking motion of the dry upper tropospheric air impede moist warm boundary layer
air to reach the condensation level.
A marked difference exists between topographically highly structured areas like
the central Mediterranean and the Aegis and the large nearly quadratic landmass of
the Iberian Peninsula or the long stretched North African coast. The circulation
systems over Italy and Greece are strongly modified by coastlines, mountains and
hills. The zonally stretched North African coast is positioned parallel to the
boundary between the subtropics and the mid latitudes. During winter north of the
coastline dominate the Westerlies and south of it in the lower troposphere - the
atmospheric boundary layer - north-easterly winds which are directed towards the
Inter Tropical Convergence Zone (ITCZ) at 10 'N. During summer high pressure
dominates in the North of the coastline and the ITCZ is positioned near 20
c
North.
Easterly winds prevail throughout the troposphere over the Sahara. The subtropical
jetstream develops at the position of the maximum temperature gradient near 12 km
height at 30 ON in winter and 35-40 eN in summer. Further South, near the ITCZ,
two easterly jetstreams develop, the African Easterly Jet at 3 km height and the
Tropical Easterly Jet at 12 km height. The eastward directed winds north of the
Sahara and the westward directed winds south of the Sahel lead to a depression over
North-western Africa.
Water shortage, wild fires and tilling generate treeless or sparsely vegetated areas
which in summertime heat up under the intensive insolation. This causes dry
convection and "heat lows" in the lower atmosphere which entrain the surrounding
air like a chimney (see e.g. Smith, 1986). Nevertheless in most cases no clouds
develop for two reasons. Firstly the air is quickly warming so that the relative
humidity even of entrained primarily wet maritime air becomes low and secondly
because the sinking motion of the overlying high pressure system prevents the air
to reach its condensation level. The situation is different if there are forested
mountains in the hinterland which push the sea breeze upward and even feed it with
additional water evaporated from the woods. In these cases at the front of the sea
breeze clouds develop over the mountains which can lead to rain and even to strong
summertime thunderstorms. In contrast the coastal zones, where agriculture is
concentrated, remain dry because here the land is heated up and dry convection
dominates.
Over the Iberian Peninsula often a low pressure system develops during summer
in the boundary layer (Gaertner et aI., 1993). The centre of this thermal low lies over
H.-J. Bolle
internal dynamics which to a large degree is determined by the temperature
gradients between the sea and the surrounding more or less vegetated land surfaces
as well as their large topographic and ecological manifold. The topographic effects
gain importance and interact with the much smoother large scale pressure systems
in the Mediterranean area. Mesoscale phenomena now prevail, which result from
topography, specifically the land-sea distribution. Because of its mountains and land
cover differences local to regional phenomena develop like the "pumping" on top of
elevated hot surfaces which create thermal lows in the high reaching Atmospheric
Boundary Layer (ABL), mountain induced wind systems or topographically induced
precipitation pattern. During summer semi-permanent high pressure systems with
sinking motion of the dry upper tropospheric air impede moist warm boundary layer
air to reach the condensation level.
A marked difference exists between topographically highly structured areas like
the central Mediterranean and the Aegis and the large nearly quadratic landmass of
the Iberian Peninsula or the long stretched North African coast. The circulation
systems over Italy and Greece are strongly modified by coastlines, mountains and
hills. The zonally stretched North African coast is positioned parallel to the
boundary between the subtropics and the mid latitudes. During winter north of the
coastline dominate the Westerlies and south of it in the lower troposphere - the
atmospheric boundary layer - north-easterly winds which are directed towards the
Inter Tropical Convergence Zone (ITCZ) at 10 'N. During summer high pressure
dominates in the North of the coastline and the ITCZ is positioned near 20
c
North.
Easterly winds prevail throughout the troposphere over the Sahara. The subtropical
jetstream develops at the position of the maximum temperature gradient near 12 km
height at 30 ON in winter and 35-40 eN in summer. Further South, near the ITCZ,
two easterly jetstreams develop, the African Easterly Jet at 3 km height and the
Tropical Easterly Jet at 12 km height. The eastward directed winds north of the
Sahara and the westward directed winds south of the Sahel lead to a depression over
North-western Africa.
Water shortage, wild fires and tilling generate treeless or sparsely vegetated areas
which in summertime heat up under the intensive insolation. This causes dry
convection and "heat lows" in the lower atmosphere which entrain the surrounding
air like a chimney (see e.g. Smith, 1986). Nevertheless in most cases no clouds
develop for two reasons. Firstly the air is quickly warming so that the relative
humidity even of entrained primarily wet maritime air becomes low and secondly
because the sinking motion of the overlying high pressure system prevents the air
to reach its condensation level. The situation is different if there are forested
mountains in the hinterland which push the sea breeze upward and even feed it with
additional water evaporated from the woods. In these cases at the front of the sea
breeze clouds develop over the mountains which can lead to rain and even to strong
summertime thunderstorms. In contrast the coastal zones, where agriculture is
concentrated, remain dry because here the land is heated up and dry convection
dominates.
Over the Iberian Peninsula often a low pressure system develops during summer
in the boundary layer (Gaertner et aI., 1993). The centre of this thermal low lies over