A numerical study of the evolution of nocturnal cirrus by a two-dimensional model with explicit microphysics
Abstract
A two-dimensional numerical model with a resolution of 100 m is developed to simulate the evolution of cirrus. The main feature of this model is the use of a zero-and- first-moment-conserving bin scheme to simulate the evolution of ice crystal distribution. Diffusional growth of ice crystals and haze droplets, sedimentation, homogeneous nucleation of haze droplets, and aggregation of ice crystals are explicitly calculated; deposition nucleation is parameterized. This model also contains an interactive radiative transfer module. This study focuses on the evolution of geometrically and optically thin, high cirrus in nighttime conditions. It is shown that the radiatively driven mixed layer can be maintained in clouds composed of small ice crystals. In contrast, the radiative destabilization weakens and eventually ceases in cirrus composed of large ice crystals; such type of cirrus anvils is debris of convective hot towers and descends with time due to sedimentation. The effects of shapes on the evolution of cirrus are also explored. Sensitivity tests are done using the same mass distribution of ice crystals when changing the prescribed shapes. It is found that in the long run, the change of sedimentation rate due to using different shapes dominates the evolution of cirrus anvils. In addition, the generation of new ice crystals by homogeneous nucleation of haze particles, which only takes place in highly ice supersaturated regions, is sensitive to the assumption of ice crystal shape. In contrast, aggregation does not greatly affect the simulation results.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1997
- Bibcode:
- 1997PhDT........72L
- Keywords:
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- Physics: Atmospheric Science