THE RELATIONSHIP BETWEEN CLOUD DROPLET AND AEROSOL NUMBER CONCENTRATIONS FOR CLIMATE MODELS

Abstract

Aerosols have an effect on cloud droplet size and concentration, and thus affect the radiative properties of clouds. The purpose of this study is to develop parameterized equations between droplet number concentration (Nd) and total aerosol number concentration (Na) for use in numerical weather prediction models and global climate models. In situ observations of droplet and aerosol number concentrations collected during four field projects are used. Variables N d and ;Na for stratus and stratocumulus clouds are averaged over 10-km lengths to represent scales close to those used in these models. TheNd value is added to the interstitial aerosol number concentration (Ni) to obtain in cloud Na. The relationships generated are compared with current parameterizations. The results suggest that parameterized equations obtained from three field projects, representing clouds formed over land, are similar. The fourth one, representing clouds over ocean, is found similar to the other three for lower concentration ofNa, but different for higher values ofNa. The value ofNd in clouds generally increases withNa. This suggests that the relationship betweenNd andNa may be universal. However, local variability can be very important.