The effective radius in large-scale models: impact of aerosols and coalescence

The effective radius (re) is an important parameter used in large-scale models to calculate the radiative fluxes in conjunction with clouds. Methods for its retrieval in water clouds have been developed in earlier studies. The effective radius depends not only on the amount of cloud water but also on the cloud droplet number concentration and the shape of the droplet distribution. The droplet number concentration is linked to the aerosol number concentration in a complex way, and possible relationships between aerosols and droplets are presented. Promising results are obtained with a temperature-dependent formulation of the dynamic nucleation. Climatological aerosol number concentrations have proved to yield reasonable results, although improvements might certainly be achieved by employing aerosol data that account for the variability in space and time. The suggested parameterizations for the aerosol-droplet relation and re are compared against observations made in marine and continental clouds. Comparisons against other parameterizations are also presented. Coalescence changes the shape of the droplet spectrum and reduces the droplet number concentration. At the same time, the cloud droplets become larger and, thus, re increases. An attempt is made to include the effects of coalescence in the parameterization of re. The effective radius in clouds with drizzle is found to be almost twice as large compared to non-drizzling clouds, in agreement with observations.