CEDAR: Polar Mesospheric Clouds Using the Whole Atmosphere Community Climate Model 3

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This project will combine global modeling with ground-based and satellite observations of polar mesospheric clouds (PMCs) to investigate formation, growth, and dissipation mechanisms of the clouds. PMCs occur at a threshold temperature and water vapor pressure and grow exponentially with reduced temperatures, providing highly sensitive indicators of mesospheric conditions. Anthropogenic inputs, including water vapor from space shuttle exhaust and long-term increases in atmospheric methane and carbon dioxide, are suggested causes for the recent appearance of these clouds (observed every summer since 1885, but never prior) and are predicted to produce long-term changes in their brightness and occurrence. Whether such long-term changes have yet been observed is a question currently debated, due to large natural variability. Natural influences on PMC occurrence and brightness include the solar cycle, volcanic eruptions, and dynamical perturbations. The goals of this proposal are to incorporate PMC and sulfate aerosol microphysics into a global chemistry climate model extending through the mesosphere and to investigate the influences of volcanic perturbations and space shuttle exhaust on PMC formation. Attainment of these goals will facilitate future investigations of the separate and coupled effects of anthropogenic influences, solar variability, volcanic sulfate aerosol, and dynamics. The work will utilize the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model 3 (WACCM3), with substantial focus on validating mesospheric dynamics and chemistry in the model via comparisons with ground-based and satellite data. The project is a continuation of current collaborative efforts to incorporate microphysics for a number of aerosol types, including sulfate aerosol and Polar Stratospheric Clouds, into the WACCM3 model. Of particular interest is investigation of a recent hypothesis on the existence of a mesospheric sulfate aerosol layer. This new source of nuclei for PMCs explains the dramatic decrease in PMC occurrence following the 1991 eruption of Mt. Pinatubo. WACCM3 with microphysics will be run with and without inclusion of volcanic input to better understand the influence of each on PMC variability. This will be the first time PMCs have been modeled microphysically in three dimensions with interactive dynamics and chemistry, enabling further study of natural and anthropogenic influences on the clouds. The broader impacts of the project include the networks and partnerships it will foster, the broad dissemination of results, public outreach, and benefits to society. The project will integrate research and education by offering graduate students opportunities to study the upper atmosphere. This work will be part of a recently instituted program in which a number of scientists and students are learning the WACCM model so that it can be applied to the various scientific issues, in collaboration with scientists at NCAR. Incorporation of PMC microphysics into WACCM3 will provide a new tool to the upper atmospheric research community, enabling scientists to more accurately test theories about PMC formation and evolution, interpret observations, and understand mesospheric change and coupling to lower altitudes. Websites will be developed to describe current understanding regarding PMCs and global change, aimed at both students and the general public.