Interactions of radiation, microphysics, and turbulence in the evolution of cirrus clouds

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 new features of the model include a detailed ice microphysical module for the prediction of ice crystal size distributions, a radiation scheme which interacts with the ice crystal size distribution via ice water content (IWC) and a mean effective ice crystal size, the effects of radiation on the diffusional growth of ice crystals, and a second-order closure for turbulence. Simulation results show that initial cloud formation occurs through ice nucleation associated with dynamic and thermodynamic forcings. 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. 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.A three-dimensional (3D) radiative transfer model has also been developed to simulate the transfer of solar and thermal infrared radiation in inhomogeneous cirrus clouds. The model utilizes a diffusion approximation approach for application to inhomogeneous media employing Cartesian coordinates. The extinction coefficient, single-scattering albedo, and asymmetry factor are parameterized in terms of the ice water content and mean effective ice crystal size. We employ the correlated k-distribution method for incorporation of gaseous absorption in multiple scattering atmospheres. Delta-function adjustment is used to account for the strong forward diffraction nature of the phase function of ice particles to enhance computational accuracy. The general second-order partial differential radiative transfer equations with appropriate boundary conditions imposed are solved numerically by using an efficient successive over-relaxation method. Comparisons of the model results with those from plane-parallel (PP) and Monte Carlo models show reasonable agreement for both broadband and monochromatic results. For inhomogeneous cases, upwelling and downwelling fluxes display patterns corresponding to the extinction coefficient field. Cloud inhomogeneity also plays an important role in determining both solar and IR heating rate distributions. The present radiation parameterization is applied to potential cloud configurations generated from GCMs to investigate broken clouds and cloud overlapping effects on the domain-averaged heating rates. Clouds with maximum overlap tend to produce less heating than those with random overlap. Broken clouds show more solar heating, as well as more IR cooling, than a continuous cloud field.