92 Modelling theclimate
Cloud radiative forcing
A concept helpful in distinguishing between the two effects of clouds
mentioned in the text is that of cloud radiative forcing. Take the radi-
ation leaving the top of the atmosphere above a cloud; suppose it has
a valueR. Now imagine the cloud to be removed, leaving everything
else the same; suppose the radiation leaving the top of the atmosphere
is nowR′. The differenceR′–Ris the cloud radiative forcing. It can be
separated into solar radiation and thermal radiation. Values of the cloud
radiative forcing deduced from satellite observations are illustrated in
Figure 5.15. On average, it is found that clouds tend slightly to cool the
Earth-atmospheresystem.Figure 5.15The cloud radiative forcing is made up of a solar radiation
and a thermal radiation component, which generally act in opposite
senses, each typically of magnitude between 50 and 100 W.m−^2. The
average net forcing is shown here for the period January to July as a
function of latitude as observed from satellites in two years (1985–6) of
the Earth Radiative Budget Experiment (ERBE) and also as simulated in a
climate model with different schemes of cloud formulation (a simple
threshold relative humidity scheme (RH); a scheme that includes cloud
water as a separate variable (CW); as CW but also with cloud radiative
properties dependent on the cloud water content – thick clouds with a
large water content are more reflective than thin clouds (CWRP)).There
is encouraging agreement between the models’ results and
observations, but also differences that need to be understood. It is
through comparisons of this kind that further elucidation of cloud
radiation feedback will be achieved.