The Role of Entrainment and Mixing in Altering the Relationship Between Aerosol Concentration and Cloud Drop Number Concentration

Two basic methods have been used to relate the change in droplet number concentration to aerosol number concentration. The first method uses empirically derived relationships based on either observed aerosol number concentrations and droplet concentrations or observed aerosol sulfate or droplet mass concentrations and droplet number concentrations. This method was followed by Jones et al. (1994) and Boucher and Lohmann (1995). Jones et al. (1994) linked aerosol mass and droplet number concentration, while Boucher and Lohmann (1995) linked sulfate mass to droplet concentrations. The second method for relating aerosol number concentrations and droplet number concentrations uses a mechanistic model that simulates the relationship between aerosol particles and droplets determined at cloud base based on a model of the microphysics of cloud formation. In this method the change in droplet number concentration is associated with a given change in the aerosol concentration, size distribution, aerosol composition, and updraft velocity. While the empirical method is based on observations and may therefore avoid possible inaccuracies associated with the parameterization of microphysics processes in large-scale models, current models must still assume a relationship between aerosol mass and aerosol number to apply the Jones et al. parameterization. Also, the parameterization of Boucher and Lohmann may be inaccurate because the use of sulfate from cloud water measurements to predict Nd is uncertain because of variations in the cloud scavenging efficiencies for aerosol sulfate and because the production of sulfate in cloud water would obscure the relationship between sulfate mass and Nd. The second mechanistic method, has been developed in our work, but still requires some assumptions to determine the aerosol size distribution and chemical composition. Furthermore, the method does not account for mixing processes within the cloud, which may alter the relationship between aerosol number concentration and droplet concentration.