Cirrus Clouds. Part II: Numerical Experiments on the Formation and Maintenance of Cirrus

Abstract

The numerical cirrus cloud model of Starr and Cox is used to investigate the role of various physical processes in the formation and maintenance of cirrus. Effects due to microphysical composition, i.e., crystal habit and size distribution, are found to be quite important in determining the overall cloud water budget. Radiative processes are also shown to affect the organization and bulk properties of the cloud. Substantial differences between simulations of thin cirrus under midday and nighttime conditions are found with the cloud being less dense overall (?20%) but more persistently cellular during the day with all other environmental factors being the same. Cloud-scale interactions and feedbacks between dynamic, thermodynamic and radiative processes and the microphysical composition are significant and strongly modulate the properties of the simulated clouds. A comparison is made between simulations of weakly forced cirrostratus and nonprecipitating altostratus (liquid phase) under comparable environmental conditions. Five times more cloud water is maintained in the altostratus case where the updraft wind speed are greater by a factor of 10. Ale role of the large-wale ascent or descent is also examined. Inferences are drawn from these results with respect to the parameterization of cirrus in large-scale forecast or climate models.