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Many facets of the chemical, dynamic, and physical processes occurring in the tropical upper troposphere and tropopause transitional layer are not well understood. Identifying the key processes in this region is essential for progress on issues involving global climate change, stratospheric ozone depletion, and global tropospheric chemistry.

The NASA TC4 (Tropical Composition, Cloud and Climate Coupling) mission will investigate the structure, properties and processes in the tropical Eastern Pacific. A-train satellite observations provide crucial information on the spatial and temporal variations of this region, however, carefully planned TC4 aircraft observations are required, both to validate satellite data and to provide critical observations not available from the satellites. High altitude aircraft will collect tropopause data while the medium altitude aircraft will provide profiles and structure measurements of the tropical upper troposphere and lower stratosphere.

TC4 is sponsored by the NASA Headquarters Atmospheric Composition Focus Area including the Upper Atmospheric Research Program (Michael Kurlyo, Program Manager), the Radiation Science Program (Hal Maring, Program Manager) and the Tropospheric Chemistry Program (Jim Crawford, Program Manager). TC4 is planned for July 2007 in San Jose, Costa Rica.