Cloud Droplet Size Distribution Width and Aerosol Relationships in Warm-Phase Clouds

Cloud properties and microphysical process rates are highly dependent on the droplet size distribution characteristics such as the mean droplet size, total number concentration, and, in the case of many microphysics parameterizations, the assumed size distribution function and its width. It has been suggested that the width of the size distribution should depend on the aerosol number concentration or droplet number concentration. In order to investigate this potential relationship between the size distribution width and number concentration, we have run simulations of shallow cumulus and stratocumulus clouds with a bin microphysics scheme. Different relationships are found depending on how the simulation output is analyzed which has important implications for how the relationship should be parameterized in bulk microphysics schemes. We also run simulations with a bulk microphysics scheme with different proposed relationships between the size distribution width and droplet concentration to investigate whether such relationships are important for modeling aerosol-cloud interactions in these shallow clouds. Changes to cloud properties such as cloud fraction, cloud depth, and cloud albedo are discussed.