Parcel model for aerosol activation

Description

Activation of aerosol particles to form cloud drops depends both on the aerosol itself (size distribution of critical supersaturation) and on the environment (thermodynamic properties of pre-cloud air, updraft velocity, entrainment rate and thermodynamic properties of entrained air) which, together, govern the maximum supersaturation experienced by the parcel. This maximum supersaturation, together with the kinetics of drop growth, determine the number of cloud droplets formed. A simple, transparent box model of activation and growth of aerosol particles in an air parcel has been developed to examine the number concentration of cloud droplets formed as a function of pre-existing aerosol properties, updraft velocity, entrainment rate, and atmospheric thermodynamic properties. This model can be used to examine such issues as the competition between updraft velocity and number concentration in governing cloud drop number concentration, the feedback between aerosol properties and maximum supersaturation, the role of the relative dispersion of aerosol size distribution on the number and size distribution of the resulting cloud drops, and the rate of mixing of entrained air versus the rate of diffusive mass transport and droplet growth. It is anticipated that a simple transparent model such as this will be highly informative both as input to modeling cloud formation and growth in more complex, parameterized models, and in guiding the conduct and interpretation of measurement of cloud formation processes and the controlling variables.