December 27, 2009
By Dr. Michael Reynolds
The report below was co-written by Dr. Lin Chambers, Director of the NASA CERES S’COOL Project. For more information on the S’COOL project see the links listed below.
Clouds. Every person living on Earth has seen them. We all know that they bring us rain, snow, shadows, interesting shapes. However, clouds play a crucial role in the Earth climate system. Since we have had the vantage point of looking at Earth from space, we know that about half of the planet is always cloud-covered – in beautiful, shifting patterns. Besides bringing weather, these clouds play an important role in the Earth’s energy budget. They affect the amount of sunlight that reaches the Earth’s surface, and the amount of heat that escapes back to space.
Albedo. Sunlight is reflected by white (or light colored) materials and is absorbed by dark colored materials. Clouds reflect sunlight back to space and in so doing they help cool our climate. Without clouds, the temperature of the Earth would increase substantially as more of the Sun’s energy would reach the surface. The fraction of sunlight reflected back to space is called albedo. Both clouds and snow contribute to albedo but clouds are the main contributor. Today about 28% of the total energy from the sun is reflected to space.
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| Fig. 1. A photograph taken at sunset as Ocean Watch makes its way south to the doldrums (described in the science report on the Brazil Current). The convergence creates towering Cumulus clouds that precipitate and grow to the top of the atmosphere. This photo shows the complexity of cloud observations. We see a towering Cumulonimbus anvil cloud that grows from near the surface up to several thousand meters. Winds at altitude sweep the top of the cloud away as it grows to make the anvil shape. Along the horizon are low Cumulus clouds. Above the Cumulus is a mixture of mid-level, Alto Stratus, and high, Cirrus clouds. Each cloud type has a different effect on the global energy budget so a measure of cloud fraction for each type is a critical. |
High clouds and low clouds. As scientists have delved into the details of studying clouds, they have learned that the picture is not simple. Depending on what type, or level of clouds is present, clouds can actually have opposite effects on energy flows. High, thin clouds (i.e., the wispy-looking cirrus) actually have a warming effect on the Earth: they let in sunlight but do not allow heat to escape. Low, thick clouds (i.e., fluffy-looking cumulus or featureless stratus) have a cooling effect: they reflect the Sun’s light back to space, but also allow heat to escape from their relatively warm cloud tops. Currently scientists are delving even deeper into the details of clouds, and how they may respond to changes in the environment. This requires lots of information, both from space and from the ground.
Cloud observations from space. Our understanding of the physics of clouds has been highlighted as a key uncertainty in the reports of the Intergovernmental Panel on Climate Change (IPCC). Understanding this part of the Earth system is one of NASA’s highest priority scientific research areas. NASA studies clouds from space with a special instrument called the Clouds and the Earth’s Radiant Energy System (CERES). There are CERES instruments currently on three Earth observing spacecraft: TRMM (launched in 1997), Terra (launched in December 1999), and Aqua (launched in 2002). The instruments on Terra and Aqua are still operating at this time, with Terra holding a 10th anniversary celebration in December. Both Terra and Aqua overfly essentially the entire Earth every day. Using measurements obtained by the CERES instrument, scientists seek to gain a better understanding of the role of clouds and the energy cycle in global climate change. CERES provides high accuracy measurements of reflected sunlight and heat energy leaving the Earth. The team then uses other instruments on Terra and Aqua to identify the cloud conditions associated with those measurements. Ground observations of clouds are an important component of the CERES program. Observations made from Earth provide “ground truth” validation of the satellite measurements. Quality assurance grows with more ground truth observations.
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| Fig. 2. An example of a cloud image from above the Arctic Circle. The far north is dominated by vast shields of low Cumulus Stratus, and also challenges the satellite with bright snow and ice often covering the surface. In this situation the ground observer cannot see higher cloud layers, but can trivially distinguish clouds from snow or sea ice. Observations from space and the ground provide complementary information. |
S’COOL. The Students’ Cloud Observations On-Line (S’COOL) project began in 1997 to collect surface observer reports of cloud conditions from widely distributed K-12 schools for comparison to the space-based measurements. S’COOL is administered from the Langley Research Center in Hampton, Virginia. S’COOL observations are used as a source of “ground truth” to assist in the validation of the cloud identification from space.
Ocean Watch Observations. The voyage of Ocean Watch is adding substantially to the S’COOL database by contributing observations from interesting places around the American continents where students don’t normally go. Ocean Watch is reporting observations through the S’COOL Rover component, launched for the 12th anniversary of the project in early 2009. In contrast to the main S’COOL Project, which collects cloud observation reports from fixed sites (i.e., schools), the Rover aspect allows people or groups anywhere in the world to obtain satellite overpass schedules and submit reports. With a moving platform such as Ocean Watch – and one with limited connectivity to the internet at that! — predicting the satellite overpass times is somewhat of a challenge, but the crew has worked out a system and done quite well. A large number of their observation reports have been matched to satellite data, which requires observing within +/- 15 minutes of the time that the satellite passes over the sailboat position. This tight time-matching is required because clouds can change substantially in short time periods, and we want to be sure that we are comparing ‘apples to apples’ and getting as much information as possible to compare to the satellite.
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| Fig 3. Screenshot from S’COOL Rover website showing the comparison between the observation report from the Ocean Watch crew and the corresponding satellite retrieval of cloud information for that time and location. This comparison corresponds to the satellite image in Fig. 2. This and other Ocean Watch reports are accessible from the S’COOL Rover database (see link below). |
Results from the S’COOL program. Since the S’COOL Project began, here are some of the things we’ve learned:
1) The satellite is often not able to detect small amounts of thin cirrus cloud, which are easily seen from the ground. This is because the satellite has limited spatial resolution from its location ~700 km above the surface, and also because of the challenge of detecting thin clouds against the variable background of the Earth surface. An observer on the surface, in contrast, is looking for clouds against a fairly uniform background: the blue sky.
2) Sometimes clouds at one level are obscured by clouds at other layers. For instance, ground observers provide important complementary information about the lower layers of clouds, while the satellite instruments can view the uppermost layer. Newer, remote sensing instruments, such as CALIPSO or CloudSat, now provide vertical profiles of cloud layers all the way to the surface. These instruments are considered ‘active’ sensors, as they create their own source of illumination using lasers or radar, but they can only profile a thin strip of clouds immediately under the satellite track.
3) There are indications that the satellite algorithms are still sometimes confused by the presence of snow or ice on the surface, despite the best efforts of the CERES team to correctly separate clouds from snow. Since both are cold, bright surfaces, this is a big challenge for satellite observations. It is no challenge at all to a student standing on snowy ground to see even small wisps of cloud in the sky. We continue to seek data from snowy or icy parts of the world to better understand when and where this is or is not a problem.
4) Taken overall, the cloud coverage reported by students and other observers is in good agreement with the cloud coverage measured from the satellite instruments.
Conclusion. Studies continue on these and other aspects of comparisons between surface and satellite-based observations. We plan a focused study on the complete set of Ocean Watch reports to determine whether we can learn any new things from a number of previously unobserved areas along the route of the Around the Americas voyage. The entire database of reports since January 1997 is also accessible on the S’COOL website for anyone who might be interested. These observations are one important piece in understanding the puzzle that clouds represent.
Links:
S’COOL home page http://scool.larc.nasa.gov
S’COOL Rover Database http://scool.larc.nasa.gov/en_view_rover.html
CERES home page http://science.larc.nasa.gov/ceres
Aqua Satellite http://terra.nasa.gov
Terra Satellite http://aqua.nasa.gov
CALIPSO http://www-calipso.larc.nasa.gov/
CloudSat http://cloudsat.atmos.colostate.edu/
This is an amazing story!
Thanks for the great images!