A one-dimensional version of a cloud model with an explicit microphysics scheme is used to simulate a case study of middle and upper level cloud formation and evolution that was observed during the FIRE Arctic Clouds Experiment. In the simulations, the midlevel altostratus cloud is initially liquid phase, then partially freezes, and exists in mixed phase for several hours with a relative equilibrium between the rate of drop production by condensation and their depletion by freezing. The dominant mode of cirrus formation was periodic homogeneous freezing of deliquescent submicron haze particles. These crystal layers form near the tropopause and, subsequently, precipitate into the middle troposphere, causing seeding of the underlying altostratus cloud. Sensitivity tests are conducted varying the initial humidity and nucleation schemes. ¿ 2001 American Geophysical Union