MISR Science Goals and Objectives
MISR's Study of Clouds
By Roger Davies, MISR Science Team
Why study clouds from space?
More than any other component of the climate system, clouds affect the flow
of energy within Earth's atmosphere, and to its surface. The impact of
clouds on sunlight ranges from the high reflectivity of thunderclouds, which may
allow so little light to reach the surface that it seems the Sun has set
prematurely, to the subtle, iridescent colors of sunlight transmitted through
thin cirrus. The influence of clouds extends to the infrared portion of the
spectrum, where they make an important contribution to Earth's natural
greenhouse. You may have observed the effect of clouds on infrared radiation --
on cloud-free nights, it tends to be cooler than on nights when there is a
persistent low cloud cover.
In addition to their radiative effects, clouds play a key role in Earth's
water cycle. The energy it takes to evaporate water from Earth's surface is
called latent heat. When the vapor re-condenses in the atmosphere to form
clouds, the latent heat is released. This is one of the leading mechanism by
which the surface transfers the surplus energy it receives from sunlight back up
into the atmosphere.
Although clouds must be included in climate studies, they are difficult to
describe mathematically in climate models. A wide variety of cloud properties
must be taken into account: shapes, sizes, vertical and horizontal locations,
lifetimes, numbers of liquid droplets of different sizes, numbers of ice
crystals of different shapes and sizes, and more. The way clouds absorb,
scatter, and emit radiation is influenced by each of these properties. The
effects of clouds on climate are so complicated that the leading climate models
give conflicting answers regarding their impact of climate. There is even
uncertainty as to whether the changes in cloud properties will amplify or
diminish any surface warming that may be caused by increasing atmospheric
greenhouse gases. In fact, the present uncertainty in treating the
radiative effect of clouds in climate models is larger than the entire radiative
consequences expected from a doubling of carbon dioxide.
So how can we get a better grasp on the climatic effects of clouds? The
answer is to make many more observations of cloud properties, along with
measurements of the visible and infrared radiation fields they produce.
Theoretical models can help us choose which properties to measure.
Since the global climate is affected by the long-term average of all
the cloud radiative effects, summaries of the details may be all we really need
to model future change. [One catch, however, is that we must be careful that the
summaries depend on physical relationships between clouds and radiation. If they
are simply empirical relations obtained for the present climate, they may not
apply to a future climate state.] "Making these measurements" is where the
satellites come in.
Satellites can not provide all the answers. But measurements of radiation
from space can play a big role in helping us understand how radiation depends on
cloud properties. They can also help us identify which cloud properties are the
most critical ones to measure. For example, for a long time the reflective
properties of clouds were modeled assuming the clouds could be treated
one-dimensionally, having flat tops and no sides. We now know from analyzing
lots of global satellite observations that cloud sides and non-flat cloud tops
are too important to be neglected.
The beauty of satellite-based measurements is that they offer the only
practical way of making cloud measurements over the entire global. A single
satellite instrument orbiting Earth can provide global coverage in a few
days, and modern computers can help us dig the important information out of the
vast amounts of data produced.
What will MISR tell us about clouds?
In general, clouds don't reflect solar radiation equally in all directions.
So if you have only a measurement of reflectivity from a single direction, it is
difficult to deduce the total amount of light reflected by the cloud (that is,
its "albedo").
We expect MISR to add significantly to our knowledge of clouds and solar
radiation in several ways, but its most important contributions will be to
provide more accurate estimates of cloud albedo. MISR's nine cameras span much
of the range of angles over which cloud reflectivity varies. [But all possible
angles are not covered even with MISR, so care will be taken in treating the
missing directions.] The albedos retrieved by MISR are expected to be ten times
more accurate than those obtained from similar measurements with only a single
camera looking straight down.
For climate studies, cloud albedo must include all wavelengths in the solar
spectrum, whereas MISR measures reflectivity in only four spectral bands.
Fortunately, most of the directional effects across the solar spectrum are
captured by the MISR bands. So we will use data from another instrument on the
EOS-AM spacecraft, CERES,
to help fill in the gaps. CERES will measure reflectivity from only a single direction,
but will cover the entire solar spectrum.
We also plan to study the physical structure of clouds themselves with MISR.
Most people are familiar with the visual effects of stereo, since binocular
vision plays a role in our own perception of depth. With just two cameras, MISR
can also measure distances by the same principle. Using all nine cameras, it
will be possible to get even more information about cloud structure. MISR will
retrieve cloud heights routinely, and for special case studies, will derive
details about cloud shape, cloud thickness, and the roughness of cloud tops.
Visit the Radiation,
Clouds and Climate Laboratory web site to learn more about how MISR will
help with research on cloud properties.
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