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Effects of all-sky heating rates, nucleation mechanisms, convection, and waves on water vapor at the tropical tropopause

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Wednesday, 26 January 2011
Effects of all-sky heating rates, nucleation mechanisms, convection, and waves on water vapor at the tropical tropopause
Washington State Convention Center
Leonhard Pfister, NASA/Ames Research Center, Moffett Field, CA; and E. Jensen

Recent work has shown that clouds as observed by the CLOUDSAT and CALIOP satellite instruments have a significant effect on radiative heating rates in the Tropical Tropopause Layer (TTL). When clouds are included, positive heating rates are found as low as 13 km in boreal winter, about 1.5 km lower than the level derived from clear-sky calculations. Recent work has also shown that nucleation of ice crystals at cold tropical tropopause temperatures is probably controlled by heterogeneous mechanisms, rather than the homogeneous nucleation previously thought. CLOUDSAT and CALIPSO have also enhanced our capabilities in ascertaining the altitude of convective cloud tops, which should clearly affect TTL water. Mesoscale and tropical waves (not typically well represented in global analyses) can also affect water by creating anomalously cold temperatures that enhance nucleation. The purpose of this paper is to evaluate the effects of these physical processes on the water distribution using a full microphysical, trajectory-based model. We examine cases from both boreal summer and boreal winter and compare with satellite observations. The key finding is that nucleation mechanisms and heating rates have substantial impacts on the water distribution, while convective injection (due to temperature control) has lower impacts. Wave motions tend to dehydrate at the highest altitudes; at lower altitudes, wave dehydration is counteracted by evaporation of greater numbers of falling ice crystals. Some puzzles remain, including excessive moisture at the bottom of the TTL associated with ascent in the 13-14 km region.