The PI will study the phenomenon of mesospheric clouds, their atmospheric environment, and their role in middle atmospheric global change. The PI will combine (1) data analysis of existing data gathered by spacecraft, rockets and ground-based radar; and (2) theoretical modeling of the microphysics of mesospheric ice particle evolution, and of the atmospheric environment, including coupled dynamics, radiation, and photochemistry of the mesosphere and lower thermosphere. The research will increase understanding of physical processes occurring in the upper mesosphere, an atmospheric region which is still poorly understood because of its complexity and inaccessibility.***