Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
29
the southern central part of the Estremadura. It can appear from April to September
with a maximum of occurrence in July and August (above 50% of the days). Portella
and Castro (1996) summarise its characteristics in the following way: (i) During the
early morning hours winds run parallel to the coast and turn to the interior as the
low intensifies moving up pollutants through river valleys and mountain passes. (ii)
In the Mediterranean zone stronger surface pressure gradients almost coincide with
the orographic line which divide the Atlantic and Mediterranean basins. (iii) A zone
with strong surface convergence in the NE of the peninsula coincides with the
maximum of the occurrence of summer storms. (iv) The vertical structure ofthe low
shows strong thermal influence below the 850 hPa level, convergent flow up to the
700 hPa level, and strong divergence just above. In the 500 hPa level geostrophic
winds prevail. Fiedler and Adrian (see Fiedler et al. 1996 and Bolle 1998) computed
the progression of the sea-land circulation front during one day on the basis of
EFEDA data and demonstrated that the Mediterranean sea-land circulation front just
reached in the evening the area of Albacete but did not go beyond.
The vertical convection over the Iberian Peninsula is supported by the higher
temperatures of non-forested land surfaces and the contrast between the surface
temperature of the surrounding sea and the land. Maximum surface temperatures
range between 52 and 60'C over bare soils and 49 - 56 C over a fallow field,
depending on the wind speed. Over a fallow field with short herbs three to four
degree lower temperatures were measured as compared to the bare soil, and 10 'C
lower temperatures over alfalfa. This demonstrates the effect of even sparse
vegetation in this hot environment (RESRAPS Final Report 1994). Measurements
at leaves of grapevine resulted in maximum temperatures of 35 - 38C while
temperatures of nearby bare soil reached 60C. The nighttime temperatures were
very close (Giordani et at. 1996). The maximum temperature differences between
the surface and the air at 2m height are in the average 26C over bare soil and 22
"C over fallow land. The high temperatures develop because of the deforestation in
many places, the low matorral vegetation, harvested crops, fallow land and nonirrigated agriculture which create a desert-like situation during the summer months
as reflected in the vegetation index derived from satellite data (compare section 6.3).
In the EFEDA experimental area near the centre of the heat low during the
intensive measuring periods of June 1991 and July 1994 the noon values of the
albedo over fallow land were 27±1 % and despite this high albedo the maximal
values of the net radiation fluxes were 500 to 525 Wm- 2 of which about 300 Wm- 2
were returned to the atmosphere as sensible heat flux and approximately 75 Wm- 1
as latent heat flux. The residuum of the fluxes measured at the top of the surface
suggest a soil heat flux of 125 - 150 Wm- 2 at noon.
The COMPARE/PYREX exercise has emphasized the fact that the orography
representation still not is sufficiently close to reality for accurately forecasting an
orographic flow at the mesoscale (Georgelin et aI., 2000). It is mainly the land-sea
circulation that becomes responsible for the exchange of dry and humid air between
land and sea. Already during spring large thermal contrasts build up during daytime
between sea and land causing warm air to rise in coastal zones. This generates low
29
the southern central part of the Estremadura. It can appear from April to September
with a maximum of occurrence in July and August (above 50% of the days). Portella
and Castro (1996) summarise its characteristics in the following way: (i) During the
early morning hours winds run parallel to the coast and turn to the interior as the
low intensifies moving up pollutants through river valleys and mountain passes. (ii)
In the Mediterranean zone stronger surface pressure gradients almost coincide with
the orographic line which divide the Atlantic and Mediterranean basins. (iii) A zone
with strong surface convergence in the NE of the peninsula coincides with the
maximum of the occurrence of summer storms. (iv) The vertical structure ofthe low
shows strong thermal influence below the 850 hPa level, convergent flow up to the
700 hPa level, and strong divergence just above. In the 500 hPa level geostrophic
winds prevail. Fiedler and Adrian (see Fiedler et al. 1996 and Bolle 1998) computed
the progression of the sea-land circulation front during one day on the basis of
EFEDA data and demonstrated that the Mediterranean sea-land circulation front just
reached in the evening the area of Albacete but did not go beyond.
The vertical convection over the Iberian Peninsula is supported by the higher
temperatures of non-forested land surfaces and the contrast between the surface
temperature of the surrounding sea and the land. Maximum surface temperatures
range between 52 and 60'C over bare soils and 49 - 56 C over a fallow field,
depending on the wind speed. Over a fallow field with short herbs three to four
degree lower temperatures were measured as compared to the bare soil, and 10 'C
lower temperatures over alfalfa. This demonstrates the effect of even sparse
vegetation in this hot environment (RESRAPS Final Report 1994). Measurements
at leaves of grapevine resulted in maximum temperatures of 35 - 38C while
temperatures of nearby bare soil reached 60C. The nighttime temperatures were
very close (Giordani et at. 1996). The maximum temperature differences between
the surface and the air at 2m height are in the average 26C over bare soil and 22
"C over fallow land. The high temperatures develop because of the deforestation in
many places, the low matorral vegetation, harvested crops, fallow land and nonirrigated agriculture which create a desert-like situation during the summer months
as reflected in the vegetation index derived from satellite data (compare section 6.3).
In the EFEDA experimental area near the centre of the heat low during the
intensive measuring periods of June 1991 and July 1994 the noon values of the
albedo over fallow land were 27±1 % and despite this high albedo the maximal
values of the net radiation fluxes were 500 to 525 Wm- 2 of which about 300 Wm- 2
were returned to the atmosphere as sensible heat flux and approximately 75 Wm- 1
as latent heat flux. The residuum of the fluxes measured at the top of the surface
suggest a soil heat flux of 125 - 150 Wm- 2 at noon.
The COMPARE/PYREX exercise has emphasized the fact that the orography
representation still not is sufficiently close to reality for accurately forecasting an
orographic flow at the mesoscale (Georgelin et aI., 2000). It is mainly the land-sea
circulation that becomes responsible for the exchange of dry and humid air between
land and sea. Already during spring large thermal contrasts build up during daytime
between sea and land causing warm air to rise in coastal zones. This generates low