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

Abstract

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 summarized 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. The solar heating tends to be concentrated near the cloud top in both unstable cases, but with quite different IR heating profiles. The IR heating warms the warm cloud near the base and cools it near the top, whereas the cold cloud is radiatively warmed throughout, mostly because it is optically thinner than the warm cloud. In the stable cases, homogeneous nucleation (the homogeneous freezing of haze solution droplets, as parameterized in Part I of this paper) does not occur for the parameterization of heterogeneous nucleation used, leaving the number concentration of ice much smaller than in the unstable cases. However, the individual crystals can grow larger, due to less competition for water vapor, so that they fall out of the initial saturated layer faster than in the unstable cases. The sedimentation rate is slower in the cold stable case than in the warm stable one, again due to slower growth rates at cold temperatures. In neither stable case does the induced diabatic heating destabilize the cloud layer. In this numerical study, it is found that cirrus cloud becomes much less persistent in the simulation without any radiative processes. For warm cirrus, the growth is slightly more vigorous in the daytime than at night, while the opposite is true for cold cirrus. It is found that latent heating plays two opposite roles in the evolution of cirrus. First, it augments the initial perturbation in the growing stage of the cloud, but then tends to stabilize the cloud layer, limiting the development of cirrus at later stages of the cloud evolution. Ice crystal habit has great impact on the evolution of cirrus, especially in an unstable atmosphere. The radiative heating rates are also found to be very sensitive to the type of ice crystal in the cloud. Finally, the role of ice aggregates in the evolution of cirrus is also explored. It is found that the aggregates reduce the volume absorption coefficient, thus decreasing the optical depth and tending to reduce the radiative destabilization.