The Airborne Tropical TRopopause EXperiment (ATTREX): Measurements of Clouds and Supersaturation near the Tropical Tropopause

Speaker: Eric Jensen, NASA Ames

When: Wednesday, August 29, 2012, 3:30 p.m. Mountain Time
Location: Room 2A305, DSRC (NOAA Building), 325 Broadway, Boulder
Directions: Refer to More Information under our Seminar Schedule

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The NASA Airborne Tropical TRopopause EXperiment (ATTREX) is a series of airborne campaigns focused on understanding physical processes in the Tropical Tropopause Layer (TTL) and their role in atmospheric chemistry and climate. A particular ATTREX emphasis is understanding the dehydration of air as it passes through the cold tropical tropopause region. ATTREX is using the high-altitude, long-duration NASA Global Hawk Unmanned Aircraft System to make in situ and remote-sensing measurements spanning the Pacific.

The first ATTREX flights were conducted out of Dryden Flight Research Center during the fall of 2011. Eleven of the twelve ATTREX instruments were integrated on the Global Hawk, including instruments measuring water vapor, clouds, meteorological conditions, a wide range of tracers, and chemical compounds. Three long flights were conducted to the tropics on 29-30 October, 5-6 November, and 9-10 November. Preliminary results will be presented. In particular, we will focus on measurements of TTL cirrus and supersaturation. The fall ATTREX measurements provide clear evidence for two distinct classes of thin cirrus in the TTL: (1) layers with low ice concentrations and low extinctions that have little apparent effect on ice supersaturation, and (2) thin layers (embedded in the deeper, low-ice concentration clouds) with much higher ice concentrations that effectively deplete any vapor in excess saturation. We hypothesize that the low ice concentration clouds are produced by a background population of heterogeneous ice nuclei with concentrations less than 100/L, and the thin, high ice concentration layers are produced when wave motions drive the supersaturation above the threshold for homogeneous freezing of an abundant population of aqueous aerosols. The ATTREX measurements, along with past high-altitude aircraft measurements, indicate that low-ice concentration TTL cirrus occur frequently in various tropical regions. These findings have implications for our understanding of the regulation of stratospheric humidity by TTL cirrus. The predominance of the low-ice concentration cirrus near the tropical tropopause may permit considerably more water to pass through the cold trap into the stratosphere than is typically assumed in models.