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.