30
H.-J. Bolle
surface pressures which entrain the cooler, wetter, and heavier air from the sea
which warms up rapidly. The air on top of the during summer 22 - 2TC warm sea
can take up large amounts of water vapor but, because of the much higher
temperatures over land, it often needs additional lifts by near coastal mountains or
by mixing with dry convection to move this air up to the condensation level.
Especially warm air moving from south to north over the sea can be loaded up with
large amounts of water vapour. Often this scirocca carries as well desert dust with
it which is released together with the condensing water.
The evaporation processes are determined by the availability of water in the soil,
the pumping through the vegetation and the structure of the relatively thin
Atmospheric Boundary Layer (ABL). The top of the ABL is the altitude at which the
direct influence of the surface on the dynamics of the atmosphere ceases. The ABL
is intimately connected to the surface by the vegetation which protrudes into this
layer thus contributing to the roughness of the surface. Its thickness and its structure
depends on many parameters such as turbulence, dry convection, moisture, and
irradiance. In the case of the Iberian Peninsula ABL height up to 3 km were
experienced, often filled with dust raised from the surface which causes large optical
depths (see section 5.4). Locally it may as well be influenced by lakes, dammed
rivers and small water ponds in which the water of winter rains is stored.
Though there is little rain or dew during summer, the evaporation of about 1
mm/day, as measured during the EFEDA field experiments in June, obviously is
daily handed over through the boundary layer into the free troposphere and carried
away. It results from the transpiration of the remaining matorral vegetation and
evaporation from deeper levels of the soil due to the diurnal heat wave by which a
fraction of the soil heat flux is transformed into latent heat. The exchange processes
between the atmospheric boundary layer and the free troposphere on top of it are of
completely different nature than those at the surface. Since there always is even
during dry summers a small amount of evaporation at the surface this amount of
water must somehow disappear if it is not raining. This is very probably due to the
divergence at the top of the heat low and the variable height of the ABL: During
daytime it is expanding filling its upper parts with water vapour and during
nighttime because ofthe decreasing temperatures it is shrinking leaving some of the
water vapour behind in the free troposphere, where it is transported away with the
large scale wind systems (Fiedler 1994).
In other regions the land-sea circulation leads to cumulus convection over
mountain chains such as the Italian Apennines (Bolle, 1999) but local circulation
systems may also develop into vigorous thunderstorms if the moisture is available
which feeds these systems by its latent heat. As can be seen in satellite imagery,
comparable high temperatures develop in summer in coastal areas where in flat
lands most of the agricultural areas are situated. These areas are in summer only
sparsely vegetated, many fields are bare after the harvest of crops. These hot areas
support the raise of wet air entrained from the sea, they are even spending some
water vapour from irrigated fields though water for permanent and area covering
irrigation becomes rare. Over the mountains ofthe hinterland then convective clouds
and thunderstorms may develop which support the growths of the forests that
H.-J. Bolle
surface pressures which entrain the cooler, wetter, and heavier air from the sea
which warms up rapidly. The air on top of the during summer 22 - 2TC warm sea
can take up large amounts of water vapor but, because of the much higher
temperatures over land, it often needs additional lifts by near coastal mountains or
by mixing with dry convection to move this air up to the condensation level.
Especially warm air moving from south to north over the sea can be loaded up with
large amounts of water vapour. Often this scirocca carries as well desert dust with
it which is released together with the condensing water.
The evaporation processes are determined by the availability of water in the soil,
the pumping through the vegetation and the structure of the relatively thin
Atmospheric Boundary Layer (ABL). The top of the ABL is the altitude at which the
direct influence of the surface on the dynamics of the atmosphere ceases. The ABL
is intimately connected to the surface by the vegetation which protrudes into this
layer thus contributing to the roughness of the surface. Its thickness and its structure
depends on many parameters such as turbulence, dry convection, moisture, and
irradiance. In the case of the Iberian Peninsula ABL height up to 3 km were
experienced, often filled with dust raised from the surface which causes large optical
depths (see section 5.4). Locally it may as well be influenced by lakes, dammed
rivers and small water ponds in which the water of winter rains is stored.
Though there is little rain or dew during summer, the evaporation of about 1
mm/day, as measured during the EFEDA field experiments in June, obviously is
daily handed over through the boundary layer into the free troposphere and carried
away. It results from the transpiration of the remaining matorral vegetation and
evaporation from deeper levels of the soil due to the diurnal heat wave by which a
fraction of the soil heat flux is transformed into latent heat. The exchange processes
between the atmospheric boundary layer and the free troposphere on top of it are of
completely different nature than those at the surface. Since there always is even
during dry summers a small amount of evaporation at the surface this amount of
water must somehow disappear if it is not raining. This is very probably due to the
divergence at the top of the heat low and the variable height of the ABL: During
daytime it is expanding filling its upper parts with water vapour and during
nighttime because ofthe decreasing temperatures it is shrinking leaving some of the
water vapour behind in the free troposphere, where it is transported away with the
large scale wind systems (Fiedler 1994).
In other regions the land-sea circulation leads to cumulus convection over
mountain chains such as the Italian Apennines (Bolle, 1999) but local circulation
systems may also develop into vigorous thunderstorms if the moisture is available
which feeds these systems by its latent heat. As can be seen in satellite imagery,
comparable high temperatures develop in summer in coastal areas where in flat
lands most of the agricultural areas are situated. These areas are in summer only
sparsely vegetated, many fields are bare after the harvest of crops. These hot areas
support the raise of wet air entrained from the sea, they are even spending some
water vapour from irrigated fields though water for permanent and area covering
irrigation becomes rare. Over the mountains ofthe hinterland then convective clouds
and thunderstorms may develop which support the growths of the forests that