Interdependence of Radiation and Microphysics in Cirrus Clouds

Abstract

The important microphysical relationships determining the radiative properties and growth of ice crystals in stratiform cirrus clouds are investigated. A horizontally infinite cloud layer is modeled in the midlatitude upper troposphere. Optical properties of spheres of equal surface area are assumed to represent the scattering characteristics of nonspherical crystals, while the delta-Eddington approximation is used to solve the radiative transfer equations. Classical expressions for ice particle growth and sublimation are coupled to those for radiative energy exchange in order to follow ice particle evolution within the cloud. The radiative properties of the clouds influence the balance among the cloud physical processes within the cloud. In the top 5 percent of optically thin clouds, the ice particle energy balance is essentially between latent and heat diffusion. In the case of clouds with large optical depths, the energy balance is between latent heat and radiation, i.e., radiative cooling enhances particle growth by vapor deposition. In the lower 5 percent of optically thin or thick clouds, latent heat and radiation are balanced by the diffusion of heat from the particle to the environment. Here, upwelling radiation enhances particle sublimation at cloud base. Environmental ice saturation ratio is the primary factor determining the energy balance during growth of ice crystals. When the ice saturation ratio is ?1, crystal growth rates are small, and radiative heating/cooling exercises a strong influence. However, for ice saturation ratios more than a percentage above or below unity, radiative influences on growth rates of crystals with lengths less than 200 ?m are negligible. We have followed the one-dimensional temporal evolution of 1-km thick cirrus cloud layers subsiding in still air. Crystals at cloud top grow larger with time while those at cloud base sublimate as the cloud settles into dry air, with the vertical fall distance greater for larger initial crystal lengths. The temporal evolution of the cloud microphysical characteristics results in modification of the radiation fields, both within the cloud and at the cloud boundaries.