Parameterization and Impact of Ice initiation Processes Relevant to Numerical Model Simulations of Cirrus Clouds

Abstract

An effort to improve descriptions of ice initiation processes of relevance to cirrus clouds for use in regional-scale numerical cloud models with bulk microphysical schemes is described. This is approached by deriving practical parameterizations of the process of ice initiation by homogeneous freezing of cloud and haze (CCN) particles in the atmosphere. The homogeneous freezing formulations may be used with generalized distributions of cloud water and CCN (pure ammonium sulfate assumed). Numerical cloud model sensitivity experiments were made using a microphysical parcel model and a mososcale cloud model to investigate the impact of the homogeneous freezing process and heterogeneous ice nucleation processes on the formation and makeup of cirrus clouds. These studies point out the critical nature of assumptions made regarding the abundance and character of heterogeneous ice nuclei (IN) present in the upper troposphere. Conclusions regarding the sources of ice crystals in cirrus clouds and the potential impact of human activities on these populations must await further measurements of CCN and particularly IN in upper-tropospheric and lower-stratospheric regions.