Aerosol- and Droplet-Dependent Contact Freezing: Parameterization Development and Case Study

Abstract

Abstract A parameterization for contact freezing is presented that combines theoretical expressions for determining the collision efficiency with experimentally determined freezing efficiency results. The parameterization has dependencies on aerosol and cloud droplet physical properties, including electric charges, as well as ambient temperature and humidity. The highest freezing rate is obtained at large aerosol and large cloud droplet sizes, and at cold temperatures and low relative humidities, with typical dust aerosol and droplet properties. The number concentration of ice nucleating particles (INPs) in the contact freezing mode are generally lower than those in the immersion freezing or deposition nucleation mode; however, under certain conditions contact INP concentrations can exceed those of the other modes. The new parameterization is used in a high-resolution, semi-idealized simulation of a deep convective cloud, and a number of sensitivity studies are performed. Results indicate the greatest sensitivity is to the best-fit function to laboratory data. The simulations show that droplet properties and ambient relative humidity contribute significantly to contact freezing.