PI: Elisabeth Moyer (University of Chicago) co-PIs: Thomas Ackerman (University of Washington) Peter Blossey (University of Washington) Stephan Fueglistaler (Princeton University) Zhiming Kuang (Harvard University)
The tropics are blanketed with a ubiquitous layer of thin, high-altitude ice clouds (cirrus) that occupies one of the coldest and least-understood parts of the atmosphere, the "tropical tropopause layer" or TTL. As air ascends through the TTL, it is effectively freeze-dried by temperatures as low as -75 Celsius (-100 Fahrenheit), and leaves its water vapor behind as minute ice crystals that settle out slowly over timescales of days to weeks. Because the TTL lies over twice as high as most aircraft can fly, it can be studied only by remote-sensing instruments on satellites or by a handful of specialized U.S. and European research aircraft. In the last decade, our limited understanding of the TTL has been upended by new satellite measurements that suggest its ice crystal layer is significantly denser than had been previously thought, and with a stronger effect on the Earth's radiation budget. One potential explanation is that fast-rising tropical thunderstorms carry water directly to the TTL, influencing altitudes higher than had been expected. This PIRE program will bring together an international team for coordinated studies that will advance knowledge of this important natural phenomenon. The international partners are France, Germany and Switzerland. This award is co-funded by the Climate and Large-Scale Dynamics program.
The PIRE research team will integrate efforts to address a series of questions: How does deep convection affect the TTL, in terms of moisture and energy budgets? How do internal dynamics within cirrus affect their characteristics and persistence? What factors determine where, when, and how cirrus forms? And how do cirrus in turn affect large-scale circulations? These questions require separate but linked modeling exercises addressing deep convection, TTL cirrus, and the global circulation, as the problems differ substantially in scale. We will generate the first tropics-wide cirrus simulation that combines explicit treatment of the interactions among microphysics, radiation and dynamics with boundary conditions that realistically represent the TTL, and will use it to evaluate both controls on cirrus properties and resulting consequences for global atmospheric dynamics. Our partnership with European collaborators is especially important for integrating observations, and European partners will lead in developing observation-based climatologies of TTL cirrus characteristics. The project is designed to also leverage off new observations: the high-altitude StratoClim aircraft campaign that will sample cirrus over the Asian Monsoon in summer 2017, and laboratory studies of cirrus formation conducted at the AIDA Aeorsol and Cloud Chamber in Karlsruhe, Germany.