Climate, Climate Variability and Impacts in the Mediterranean Area: An Overview
57
vegetated areas where the solar irradiance is smaller but its absorbed fraction higher.
Furthermore the vegetation, as long as water is available, keeps down the
temperature and infrared emission because of the evaporation, Bare soils with their
low emissivity heat up at daytime to high thermodynamic temperatures at which
infrared emission compensates the absorbed solar and atmospheric radiation minus
the mostly sensible heat fluxes and cools strongly at night into the mostly cloudless
sky. These extreme temperature contrasts reduced the chance for survival of
vegetation to species that withstand the high noon and low night temperatures with
little water. In these sparsely vegetated areas the net radiation energy is nearly
completely converted into sensibly heat. In the more vegetated areas of the North the
temperature wave is dampened though the amount of absorbed radiation energy is
not much less than in the South because the here smaller solar irradiance is
compensated by the lower albedo. The surface furthermore does not need to
compensate for the absorbed solar energy by infrared emission and sensible heat
fluxes alone, the evaporation of water in addition cools the surface. Because of the
high emissivity the surface emits more infrared energy than the desert at equal
thermodynamic temperature. If the vegetation for a short time vanishes (e.g. after
harvest or in dry summers) the soil is not immediately changing its composition and
colour and the albedo might only be little affected by the change at the surface. This
is because vegetation has a very low albedo in the visible part of the spectrum but a
high albedo at near infrared wavelengths while the soil may have a medium high
albedo both in the visible and the near infrared and thus nearly exhibits the same
broadband albedo as the vegetation. This property, of course, varies regionally.
By making use of its spectral reflection properties the state of the vegetation can
be traced by remote sensing techniques. There are many possible combinations of
these spectral reflectances which lead to so-called "vegetation indices". The most
simple combinations are ratio of visible to near infrared reflectances or the
difference of these two reflectances normalized to their sum. The analysis of time
series of these data provides information about the length and phase of the
vegetation period, the intensity of the "greenness" as well as the interannual
variability. As an example the 10 years time series for the Iberian Peninsula of
Koslowsky (1999) is reproduced in Fig. 13. It clearly indicates the variable pattern
of the dry areas. An interesting result is, that whatever happens during the early
vegetation period (which is governed by the winter rain) it does not much affect the
summer picture of which only one representative scene is reproduced in Fig. ISa.
This reflects the large water deficit in summer which cannot be overcome by strong
winter precipitation: The winter- and springtime precipitation is used up in summer
mainly because of the high net radiation. If this picture is combined with the
effective surface temperature (Fig. 14 and ISb) the impact of vegetation on surface
temperature can be visualized: Vegetation covered areas remain cooler due to
evaporation.
After a forest fire the albedo for a short time is reduced which means more solar
energy can be absorbed and a black "hot spot" is visualized. In most cases vegetation
reappears soon but it may be of a different type than prior to the fire because the
survival of the seeds depends on the species and the intensity of the fire. An
57
vegetated areas where the solar irradiance is smaller but its absorbed fraction higher.
Furthermore the vegetation, as long as water is available, keeps down the
temperature and infrared emission because of the evaporation, Bare soils with their
low emissivity heat up at daytime to high thermodynamic temperatures at which
infrared emission compensates the absorbed solar and atmospheric radiation minus
the mostly sensible heat fluxes and cools strongly at night into the mostly cloudless
sky. These extreme temperature contrasts reduced the chance for survival of
vegetation to species that withstand the high noon and low night temperatures with
little water. In these sparsely vegetated areas the net radiation energy is nearly
completely converted into sensibly heat. In the more vegetated areas of the North the
temperature wave is dampened though the amount of absorbed radiation energy is
not much less than in the South because the here smaller solar irradiance is
compensated by the lower albedo. The surface furthermore does not need to
compensate for the absorbed solar energy by infrared emission and sensible heat
fluxes alone, the evaporation of water in addition cools the surface. Because of the
high emissivity the surface emits more infrared energy than the desert at equal
thermodynamic temperature. If the vegetation for a short time vanishes (e.g. after
harvest or in dry summers) the soil is not immediately changing its composition and
colour and the albedo might only be little affected by the change at the surface. This
is because vegetation has a very low albedo in the visible part of the spectrum but a
high albedo at near infrared wavelengths while the soil may have a medium high
albedo both in the visible and the near infrared and thus nearly exhibits the same
broadband albedo as the vegetation. This property, of course, varies regionally.
By making use of its spectral reflection properties the state of the vegetation can
be traced by remote sensing techniques. There are many possible combinations of
these spectral reflectances which lead to so-called "vegetation indices". The most
simple combinations are ratio of visible to near infrared reflectances or the
difference of these two reflectances normalized to their sum. The analysis of time
series of these data provides information about the length and phase of the
vegetation period, the intensity of the "greenness" as well as the interannual
variability. As an example the 10 years time series for the Iberian Peninsula of
Koslowsky (1999) is reproduced in Fig. 13. It clearly indicates the variable pattern
of the dry areas. An interesting result is, that whatever happens during the early
vegetation period (which is governed by the winter rain) it does not much affect the
summer picture of which only one representative scene is reproduced in Fig. ISa.
This reflects the large water deficit in summer which cannot be overcome by strong
winter precipitation: The winter- and springtime precipitation is used up in summer
mainly because of the high net radiation. If this picture is combined with the
effective surface temperature (Fig. 14 and ISb) the impact of vegetation on surface
temperature can be visualized: Vegetation covered areas remain cooler due to
evaporation.
After a forest fire the albedo for a short time is reduced which means more solar
energy can be absorbed and a black "hot spot" is visualized. In most cases vegetation
reappears soon but it may be of a different type than prior to the fire because the
survival of the seeds depends on the species and the intensity of the fire. An