Scale Effects on Averaging of Cloud Droplet and Aerosol Number Concentrations: Observations and Models

The purpose of this study, using in situ observations from five field projects, is to analyze cloud boundaries and averaging scales related to droplet number concentration (Nd) and total aerosol number concentration (Na), and to discuss parameterizations of these variables for use in numerical weather prediction and global climate models. Here Nd and Na for stratus and stratocumulus clouds are averaged over various lengths from 1 km up to 35 km. The relationships between these variables for 1-s and 200-s data are compared with current parameterizations. Comparisons between Nd and Na show that Na plays an important role for activating cloud droplets. The variability in Nd from 1-s data is estimated to have a standard deviation of about ±150 cm−3. Median values of Nd representing scales from about 0.1 km up to approximately 35 km are found to be dependent on scale and the presence of clear patches in the clouds. The average values of Nd are also dependent on the lower concentration threshold used to define the cloud boundaries. It is concluded that scale effects, including clear air regions, should be considered when developing parameterization schemes used for modeling studies of cloud systems.