152
R. Stuhlmann
cloudbase altitude, and, with suitable complementary measurements, is a promising method to
provide estimates of water and ice mass.
7.1 Introduction
Within the next century, the regional distribution and magnitude of global environmental
change will have a profound societal impact. This makes it urgently necessary to provide unequivocal answers to questions about theses effects. Up to now, there is still a lack of knowledge
of the independent processes that affect our climate system. One problem, which still has to be
worked on, is how good are the projections of future climate change caused by anthropogenic
alterations of atmospheric compositions. Increasing greenhouse gases in the Earth's atmosphere
will perturb the Earth's radiation balance, forcing future climate change. Current general circulation models predict a warming of the atmosphere in the range form 1.7° to 5.4°C for a
doubling of carbon dioxide (Houghton et al., 1990). The large variaton in predicted temperature changes comes from differences in treatment of climate feedback mechanisms, which can
either amplify of moderate the warming of the atmosphere depending on the model (Cess et
al., 1989; 1990). A critical factor here is the strength and sign of the cloud-radiation feedback,
since it plays a important role in determining the magnitude and geographical distribution of
climate changes that result from natural or human forcing of climate change.
First investigations by a one-dimensional model which predict the temperature profile from
a thermodynamic balance (Manabe and Strickler, 1964) showed that the global mean surface
temperature is very close to the observed value of 288 K when clouds were included in the
model calculations. Without clouds the computed temperature was about 13°C higher. About
20 years ago, several researchers started to investigate the role of cloud radiative effects within
the climate system by using satellite observations to estimate the effect of clouds on the energy
balance at the top of the atmosphere (e.g.: Cess, 1976; Cess et al., 1982; Ellis, 1978; Hartmann
and Short, 1980; Ohring and Clapp, 1980; Ohring et al., 1981). Since then, the quality and
detail of the observational data sets for cloud cover and Earth Radiation Budget (ERB) have
greatly improved. Within this lecture the effect of clouds on the radiative heating of the Earth
surface-atmosphere system will be discussed.
7.2 Radiation Budget TOA
Starting with the NASA Nimbus/ERB mission (Jacobowitz et al., 1984) and the more recent
Earth Radiation Budget Experiment (ERBE) (Barkstrom and Smith, 1986) a climatology of
the ERB with continually improving measurement accuracy was established. To obtain the
radiation fluxes at the top of the atmosphere (TOA) from the ERBE satellites a sophisticated
inversion method was applied (Smith et al., 1986) to account for the spectral sensitivity of the
instruments, the angular variation of the radiances depending on surface type and amount of
clouds present in the observed scene. In addition, a procedure was applied to obtain proper
averages of the fluxes from temporally and spatially sparse samples (Brooks and Minnis, 1984).
These data sets are used by the scientific community to determine the energy exchange between
space and the Earth surface-atmosphere system.
The radiative heating of the Earth surface-atmosphere system is determined by its absorption
of shortwave (0.2-4.0 /-lm) radiation emerging from the sun, which partly penetrates the system
and partly is reflected back to space. This heating is compensated by a cooling determined
by the exchange of longwave radiation (4-100 /-lm) with space, which is emitted by the Earth
R. Stuhlmann
cloudbase altitude, and, with suitable complementary measurements, is a promising method to
provide estimates of water and ice mass.
7.1 Introduction
Within the next century, the regional distribution and magnitude of global environmental
change will have a profound societal impact. This makes it urgently necessary to provide unequivocal answers to questions about theses effects. Up to now, there is still a lack of knowledge
of the independent processes that affect our climate system. One problem, which still has to be
worked on, is how good are the projections of future climate change caused by anthropogenic
alterations of atmospheric compositions. Increasing greenhouse gases in the Earth's atmosphere
will perturb the Earth's radiation balance, forcing future climate change. Current general circulation models predict a warming of the atmosphere in the range form 1.7° to 5.4°C for a
doubling of carbon dioxide (Houghton et al., 1990). The large variaton in predicted temperature changes comes from differences in treatment of climate feedback mechanisms, which can
either amplify of moderate the warming of the atmosphere depending on the model (Cess et
al., 1989; 1990). A critical factor here is the strength and sign of the cloud-radiation feedback,
since it plays a important role in determining the magnitude and geographical distribution of
climate changes that result from natural or human forcing of climate change.
First investigations by a one-dimensional model which predict the temperature profile from
a thermodynamic balance (Manabe and Strickler, 1964) showed that the global mean surface
temperature is very close to the observed value of 288 K when clouds were included in the
model calculations. Without clouds the computed temperature was about 13°C higher. About
20 years ago, several researchers started to investigate the role of cloud radiative effects within
the climate system by using satellite observations to estimate the effect of clouds on the energy
balance at the top of the atmosphere (e.g.: Cess, 1976; Cess et al., 1982; Ellis, 1978; Hartmann
and Short, 1980; Ohring and Clapp, 1980; Ohring et al., 1981). Since then, the quality and
detail of the observational data sets for cloud cover and Earth Radiation Budget (ERB) have
greatly improved. Within this lecture the effect of clouds on the radiative heating of the Earth
surface-atmosphere system will be discussed.
7.2 Radiation Budget TOA
Starting with the NASA Nimbus/ERB mission (Jacobowitz et al., 1984) and the more recent
Earth Radiation Budget Experiment (ERBE) (Barkstrom and Smith, 1986) a climatology of
the ERB with continually improving measurement accuracy was established. To obtain the
radiation fluxes at the top of the atmosphere (TOA) from the ERBE satellites a sophisticated
inversion method was applied (Smith et al., 1986) to account for the spectral sensitivity of the
instruments, the angular variation of the radiances depending on surface type and amount of
clouds present in the observed scene. In addition, a procedure was applied to obtain proper
averages of the fluxes from temporally and spatially sparse samples (Brooks and Minnis, 1984).
These data sets are used by the scientific community to determine the energy exchange between
space and the Earth surface-atmosphere system.
The radiative heating of the Earth surface-atmosphere system is determined by its absorption
of shortwave (0.2-4.0 /-lm) radiation emerging from the sun, which partly penetrates the system
and partly is reflected back to space. This heating is compensated by a cooling determined
by the exchange of longwave radiation (4-100 /-lm) with space, which is emitted by the Earth