A Numerical Study of Cirrus Clouds. Part II: Effects of Ambient Temperature, Stability, Radiation, Ice Microphysics, and Microdynamics on Cirrus Evolution

Using the numerical cirrus cloud model developed in Part I of this study, the development of cirrus is examined for four different environmental regimes: warm unstable, cold unstable, warm stable, and cold stable. Despite similar initial saturation conditions, the clouds in the warm and cold cases develop very differently. As sum-marized below, these four simulations suggest that homogeneous freezing of haze particles and cold temperatures are two major reasons for persistent cirrus clouds. The numerical model is also used here to investigate the roles of several other physical factors in cirrus evolution for a selected environmental regime. These additional factors are radiation, ice crystal habit, latent heat, and ice crystal aggregation. In the warm unstable case, ice particles quickly grow large enough to fall out of the initially supersaturated cloud generation region into the subsaturated lower region. But in the cold unstable case, due to slow growth of cold temperatures, sedimentation is greatly postponed. Thus the cold cirrus tends to persist longer than the warm cirrus. In the unstable cases, diabatic heating tends to slightly destabilize the upper part of the cloud, whose circulation thus is most persistent near the cloud top. The circulation transports water vapor upward to enable long-lasting new ice particle generation