Energy and Water Cycles in the Climate System ...
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atmosphere and ground. In the future, also the emission in some microwave bands will be
added to obtain information on the water and ice content of clouds, which amounts to only
a few percent of the total water content of the atmosphere. The horizontal resolution will
enhanced from the present 250 km to about 30 km to show more regional detail (ISCCP-II).
The liquid water contained in clouds can be estimated from measurements of the emission in
the microwave region. But they often contain larger errors due to the structure of cloud fields
and the relatively wide footprint of about 30 to 50 km of such measurements, causing in many
cases" beam-filling" problems.
All these passive satellite measurements failed so far to provide sufficiently accurate informations on the often very thin cirrus and its characteristics, in particular when it is located on
top of lower cloud fields. Further, no information is directly obtained from them on the lower
cloud boundaries and thus cloud thicknesses. Therefore, the space agencies are now investigating the feasibility for new satellites with backscattering and cloud profiling radar (working in
the millimeter range of the spectrum) onboard. Once such data can be included into analysis
schemes, the lower cloud boundary altitudes can be estimated within uncertainty ranges of
about 300 m, which corresponds to a temperature uncertainty of 3 to 4 K, or in the downward longwave radiation of 5 Wm- 2 (at a lower cloud boundary temperature of O°C). The
measurements from the recent Shuttle mission with a lidar on board (LITE) has demonstrated
that spaceborne backscattering lidar may even be able to identify the upper boundary of the
planetary boundary layer, when moderately thick clouds are above.
1.5 Radiation Budget Components
(see also contributions by Del Genio, Rossow, Stuhlmann)
It is the meridionally nonuniform distribution of solar radiative energy, which forces all circulation processes within our climate system. Its average radiative power of 343 Wm- 2 (i.e.
one-fourth of the solar constant) shows some small variations of up to 1 to 2% due to dynamical
processes within the Sun's photosphere and interior, of which the so-called II-year cycle might
be strong enough to effect processes within our climate system. Statistical analyses show some
evidence in various connections (e.g.: Currie, 1995). The amplitude of the annual variation of
insolation is due to the ellipticity of the earth's orbit around the Sun and amounts to 7% or
about 23 Wm- 2 between December and June.
This incident radiation, as schematically shown in Fig. 1.7, is only partly reflected back to space
by clouds, the atmosphere and ground (in total about 30%, of which one half is solely due to
clouds), while an amount of about 20% of it heats directly the atmosphere and clouds (see also
the radiation flux divergence figure in Section 1.3 of this paper), and the remainder penetrates,
however then spectrally considerably modified to ground. Fig. 1.7 relates these partitions to
those of the terrestrial heat radiation and the exchange of sensible and latent heat between the
atmosphere and ground, where the latter two amount to about 5% and 24%, respectively, of
the solar radiation incident at the top of the atmosphere.
1.5.1 Radiation budget at top of the atmosphere
Regional distributions of all radiation budget components at the top of the atmosphere and at
ground had been estimated in the pre-satellite age from the at that time known climatologies
of clouds and their properties and the temperatures and other quantities as well (e.g.: London,
1957). Since the first meteorological satellites were launched in the sixties of this century, estimates of the radiation budget components have been made with different degrees of complexity
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atmosphere and ground. In the future, also the emission in some microwave bands will be
added to obtain information on the water and ice content of clouds, which amounts to only
a few percent of the total water content of the atmosphere. The horizontal resolution will
enhanced from the present 250 km to about 30 km to show more regional detail (ISCCP-II).
The liquid water contained in clouds can be estimated from measurements of the emission in
the microwave region. But they often contain larger errors due to the structure of cloud fields
and the relatively wide footprint of about 30 to 50 km of such measurements, causing in many
cases" beam-filling" problems.
All these passive satellite measurements failed so far to provide sufficiently accurate informations on the often very thin cirrus and its characteristics, in particular when it is located on
top of lower cloud fields. Further, no information is directly obtained from them on the lower
cloud boundaries and thus cloud thicknesses. Therefore, the space agencies are now investigating the feasibility for new satellites with backscattering and cloud profiling radar (working in
the millimeter range of the spectrum) onboard. Once such data can be included into analysis
schemes, the lower cloud boundary altitudes can be estimated within uncertainty ranges of
about 300 m, which corresponds to a temperature uncertainty of 3 to 4 K, or in the downward longwave radiation of 5 Wm- 2 (at a lower cloud boundary temperature of O°C). The
measurements from the recent Shuttle mission with a lidar on board (LITE) has demonstrated
that spaceborne backscattering lidar may even be able to identify the upper boundary of the
planetary boundary layer, when moderately thick clouds are above.
1.5 Radiation Budget Components
(see also contributions by Del Genio, Rossow, Stuhlmann)
It is the meridionally nonuniform distribution of solar radiative energy, which forces all circulation processes within our climate system. Its average radiative power of 343 Wm- 2 (i.e.
one-fourth of the solar constant) shows some small variations of up to 1 to 2% due to dynamical
processes within the Sun's photosphere and interior, of which the so-called II-year cycle might
be strong enough to effect processes within our climate system. Statistical analyses show some
evidence in various connections (e.g.: Currie, 1995). The amplitude of the annual variation of
insolation is due to the ellipticity of the earth's orbit around the Sun and amounts to 7% or
about 23 Wm- 2 between December and June.
This incident radiation, as schematically shown in Fig. 1.7, is only partly reflected back to space
by clouds, the atmosphere and ground (in total about 30%, of which one half is solely due to
clouds), while an amount of about 20% of it heats directly the atmosphere and clouds (see also
the radiation flux divergence figure in Section 1.3 of this paper), and the remainder penetrates,
however then spectrally considerably modified to ground. Fig. 1.7 relates these partitions to
those of the terrestrial heat radiation and the exchange of sensible and latent heat between the
atmosphere and ground, where the latter two amount to about 5% and 24%, respectively, of
the solar radiation incident at the top of the atmosphere.
1.5.1 Radiation budget at top of the atmosphere
Regional distributions of all radiation budget components at the top of the atmosphere and at
ground had been estimated in the pre-satellite age from the at that time known climatologies
of clouds and their properties and the temperatures and other quantities as well (e.g.: London,
1957). Since the first meteorological satellites were launched in the sixties of this century, estimates of the radiation budget components have been made with different degrees of complexity