Modeling with explicit spectral water and ice microphysics of a two-layer cloud system of altostratus and cirrus observed during the FIRE Arctic Clouds Experiment

A one-dimensional version of a cloud model with an explicit microphysics sc heme is used to simulate a case study of middle and upper level cloud forma tion and evolution that was observed during the FIRE Arctic Clouds Experime nt. 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 condensa tion 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, precipit ate into the middle troposphere, causing seeding of the underlying altostra tus cloud. Sensitivity tests are conducted varying the initial humidity and nucleation schemes.