Interactions of Radiation, Microphysics, and Turbulence in the Evolution of Cirrus Clouds

Abstract

A two-dimensional cirrus cloud model has been developed to investigate the interaction and feedback of radiation, ice microphysics, and turbulence-scale turbulence, and their influence on the evolution of cirrus clouds. The model is designed for the study of cloud-scale processes with a 100-m grid spacing. The authors have incorporated a numerical scheme for the prediction of ice crystal size distributions based on calculations of nucleation, diffusional growth, advection, gravitational sedimentation, and turbulent mixing. The radiative effect on the diffusional growth of an individual ice crystal is also taken into account in the model. The model includes an advanced interactive radiative transfer scheme that employs the δ-four-stream approximation for radiative transfer, the correlated k-distribution method for nongray gaseous absorption, and the scattering and absorption properties of hexagonal ice crystals. This radiation scheme is driven by ice water content and mean effective ice crystal size that represents the ice crystal size distribution. To study the effects of entrainment and mixing on the cloud, a second-order turbulence closure has been developed and incorporated into the model. Simulation results show that initial cloud formation occurs through ice nucleation associated with dynamic and thermodynamic forcings. Radiation becomes important for cloud evolution once a sufficient amount of ice water is generated. Radiative processes enhance both the growth of ice crystals at the cloud top by radiative cooling and the sublimation of ice crystals in the lower region by radiative heating. The simulated ice crystal size distributions depend strongly on the radiation fields. In addition, the radiation effect on individual ice crystals through diffusional growth is shown to be significant. Turbulence begins to play a substantial role in cloud evolution during the maintenance and dissipation period of the cirrus cloud life cycle. The inclusion of turbulence tends to generate more intermediate-to-large ice crystals, especially in the middle and lower parts of the cloud. Incorporation of the second-order closure scheme enhances instability below the initial cloud layer and brings more moisture to the region above the cloud, relative to the use of the traditional eddy mixing theory.