7.2.4 Radiative Processes
7.2.4.1 Radiative processes in the troposphere
Radiative processes constitute the ultimate source and sink of energy in the
climate system. They are generally well known, and particularly the clear-sky
long-wave transfer, including the absorption properties of most greenhouse gases.
Their treatment in atmospheric general circulation models relies on several
approximations: the fluxes are computed as averaged quantities over a few spectral
intervals, the propagation is limited to the vertical upwelling or downwelling
directions, and the role of sub-grid scale features of the clouds or aerosols
is essentially neglected. Although this methodology is believed to have only
a marginal impact on the accuracy of computed long-wave fluxes, the analysis
of satellite, ground-based or aircraft measurements (Cess et al., 1995; Pilewskie
and Valero, 1995; Ramanathan et al., 1995) has generated a concern that the
radiative algorithms used in climate models could significantly underestimate
the atmospheric shortwave absorption. Similar results have been obtained as
part of the ARM/ARESE experiment (Valero et al., 1997; Zender et al., 1997).
The excess, or “anomalous” observed absorption may reach typical values
of about 30 to 40 Wm-2. Its very existence, however, remains controversial and
is at odds with other investigations. Li et al. (1995) analysed large global
data sets from satellite (ERBE) and ground observations (GEBA). They did not
find the anomalous absorption except for a few tropical sites over a short period
of time. The exception appears to be induced by enhanced absorption due to biomass
burning aerosols (Li, 1998). Limited accuracy of the measurements (Imre et al.,
1996) and the methodology of analysis (Arking et al., 1996; Stephens, 1996;
Barker and Li, 1997) have been raised as possible contributing factors to the
finding of the anomalous absorption.
A comparison of the ARM-ARESE measurements with a state of the art radiation
code, which uses measured atmospheric quantities as an input, and relies on
the same physical assumptions as used in climate models, indicated that the
anomalous absorption is larger for cloudy than for clear conditions, and also
tends to be larger for visible rather than for near-infrared fluxes (Zender
et al., 1997). These two characteristics are consistent with the results of
a comparison between the output of the CCM3 general circulation model and observations
from Nimbus 7 (Collins, 1998). However, Li et al. (1999) analysed all the data
sets collected during the ARM/ARESE experiment made by space-borne, air-borne
and ground-based instruments and did not find any significant absorption anomaly.
They traced the source of controversial findings to be associated with inconsistent
measurements made by some air-borne radiometers as used in the studies of Valero
et al. (1997) and Zender et al. (1997) with those from all other instruments.
However, other studies (Cess et al., 1999; Pope and Valero, 2000; Valero et
al., 2000), that do find a significant absorption anomaly, have further demonstrated
that the ARESE data do indeed satisfy a number of consistency tests as well
as being in agreement with measurements made by other instruments. Meanwhile,
evidence of enhanced cloud absorption has been found from measurements of the
MGO aircraft laboratory (Kondratyev et al., 1998). Inclusion of anomalous absorption
has been found to improve the representation of atmospheric tides (Braswell
and Lindzen, 1998). There finally exists evidence of an effect associated with
clear-sky conditions – in the presence of aerosols: models tend to overpredict
clear-sky diffusion to the surface (Kato et al., 1997).
The three dimensional nature of the solar radiation diffusion by cloud (Breon,
1992; Cahalan et al., 1994; Li and Moreau, 1996) is unaccounted for by present
climate models and may play some role in anomalous absorption. In a recent study
using a Monte-Carlo approach to simulate three-dimensional radiative transfer,
O’Hirok and Gautier (1998a; 1998b) show that cloud inhomogeneities can
increase both the near-infrared gaseous absorption and cloud droplet absorption
in morphologically complex cloud fields. The enhancement is caused when photons
diffused from cloud edges more easily reach water-rich low levels of the atmosphere
and when photons entering the sides of clouds become trapped within the cloud
cores. The amplitude of those effects remains limited to an average range of
6 to 15 Wm-2, depending on the solar angle. The inhomogeneities significantly
affect the vertical distribution of the heating, though, with potential consequences
on cloud development. Incorporating the effect of cloud inhomogeneities in radiative
algorithms may become a necessity, and recent efforts have been made along that
path (Oreopoulos and Barker, 1999).
If anomalous absorption turns out to be real, it is an effect that will need
to be incorporated into radiation schemes. Evaluation of its importance is hampered
by lack of knowledge of a physical mechanism responsible for the absorption,
and hence lack of a physical basis for any parametrization. Modelling studies
by Kiehl et al. (1995) have demonstrated the sensitivity of the simulated climate
to changes in the atmospheric absorption. As a radiative forcing, anomalous
absorption is fundamentally different from water vapour or CO2 in that it does
not significantly alter the Earth’s net radiation budget. Instead, it shifts
some of the deposition of solar energy from the ground to the atmosphere (Li
et al., 1997), with implications for the hydrological cycle and vertical temperature
profile of the atmosphere. Anomalous absorption may not, however, appreciably
affect climate sensitivity (Cess et al., 1996).
Model validation is also affected. Many of the data which are used to tune
or validate model parametrizations, such as the Liquid Water Path (LWP) or the
droplet equivalent radius, are obtained from space measurements by the inversion
of radiative algorithms, which ignore cloud inhomogeneities and anomalous absorption.
This gives a strong importance to satellite instruments such as POLDER which
provide measurements of the same scene at a variety of viewing angles, and provides
a good test of the plane-parallel hypothesis in retrievals of cloud quantities.
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