Physical processes in polar stratospheric ice clouds

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Abstract
A one‐dimensional model of cloud microphysics has been used to simulate the formation and evolution of polar stratospheric ice clouds. The model results are in general agreement with many of the observed properties of these clouds, including their optical properties, impact on water vapor, and particle size. It is found that the clouds must undergo preferential nucleation upon the preexisting aerosols, just as tropospheric cirrus clouds do. Therefore there is an energy barrier between stratospheric nitric acid particles and ice particles, implying that nitric acid does not form a continuous set of solutions between the trihydrate and ice. The Kelvin barrier is not significant in controlling the rate of formation of ice particles, except in air parcels that have cooled very slowly. We find that the cloud properties are very sensitive to the rate at which the air parcels cool. In wave clouds, with cooling rates of hundreds of degrees per day, most of the existing aerosols nucleate and become ice particles. Such clouds have particles with sizes of the order of a few microns, and optical depths of the order of unity, and are probably not efficient at removing materials from the stratosphere. In clouds that form with cooling rates of a few degrees per day or less, only a small fraction of the aerosols become cloud particles. In such clouds the particle radius is larger than 10 μm, the optical depths are low, and the water vapor is efficiently removed. These clouds are probably formed in air parcels as they circulate around the Antarctic vortex. Seasonal simulations show that the lowest water vapor mixing ratio is determined by the lowest temperature reached and that the time when clouds disappear is controlled by the time when temperatures begin to rise above the minimum values. Hence clouds occur in the early winter at temperatures that are higher than those at which clouds occur in the late winter. The rate of decline of cloud altitude is not an indication of the fall speed of individual particles nor of vertical air motion, as had been previously suggested. The altitude of the clouds declines during the winter because the temperatures in the Antarctic increase earlier at the higher altitudes. The ice clouds are not able to remove a significant amount of nitric acid through physical processes such as coagulation with, or nucleation upon, nitric acid aerosols. Such removal may occur through other processes, not included in our simulations, such as vapor phase transfer. A considerable amount of further work could be done to improve upon our simulations. Improvements would include a treatment of the three‐dimensional structure of wave clouds, a more complete treatment of the interactions between clouds and atmospheric motions on the seasonal time scale, and a treatment of the nitric acid vapor phase interactions with ice particles. Laboratory studies of the vapor pressures of water and nitric acid above impure ice crystals are needed. In addition, laboratory investigations of the ice‐nucleating properties of nitric acid crystals would be useful. Direct observations of the sizes and concentrations of the particles in clouds formed over a time period of a few days are not available and are important to obtain, since these dominate the sedimentation removal process.