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TC4 Science Overview

The tropical tropopause transition layer (TTL) is one of the most mysterious regions of the atmosphere. Only a handful of research aircraft are able to reach the layer, which extends from about 12 km altitude to a few kilometers above the tropopause (the coldest point in the lower atmosphere), which is located at about 16-17 km altitude. Satellites have opened a new window into the TTL but they have difficulty observing it because there are often high clouds in the layer, which obscure the view, and because it has less mass than the underlying regions of the atmosphere. However, the TTL is of critical importance to the Earth's climate and atmospheric chemistry. The TTL is the gateway to the stratosphere. Deep convection sometimes penetrates the TTL to reach the stratosphere, while gentle upward motions within the TTL may also loft materials across the tropical tropopause. Hence the chemistry of the stratosphere may be affected in a significant way by processes that alter the transport across the TTL and by the chemicals in the TTL. Changes in water vapor in the stratosphere and upper troposphere can play an important role in modulating the climate since water is the most powerful greenhouse gas in the atmosphere. The TTL is the main dehydration region for air entering the stratosphere, and it is also an important reservoir for moisture lofted by tropical convection. Understanding how water behaves in the TTL is one key to better understanding the greenhouse effect, and global climate change. The TTL also contains cirrus clouds. One type of cirrus forms from anvils, the flattened tops of tropical cumulus clouds. Tropical cumulus pump vast quantities of air in just a few minutes from near the tropical surface to the TTL where they spill the air out into their anvils. The TTL also contains cirrus clouds that may be formed in situ. Many of these are so thin that they cannot be seen with the naked eye and so are called sub-visible cirrus. These clouds are easily seen by satellite however, and are now known to cover a large fraction of the tropics. Finally, the transport to and evolution of trace gases such as ozone in the TTL play an important role in determining what gases are carried into the stratosphere and upper troposphere globally. In order to learn more about the mysterious TTL NASA has designed the Tropical Composition, Clouds and Climate Coupling Experiment, TC4. Three NASA research aircraft will complement a suite of NASA Earth observing satellites during July and August of 2007 as part of TC4. There will also be ground based observing facilities and balloon launches. Participants will come from multiple NASA centers, NOAA, NCAR, numerous universities, and Costa Rica. more >>