Aerosol and Droplet Dependent Contact Freezing: Parameterisation Development and Case Study

Abstract

parameterisation for contact freezing is presented, which combines theoretical expressions for determining the collision efficiency with experimentally determined freezing efficiency results. The parameterisation 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 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 parameterisation is used in a high resolution, semi?idealised 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.