The Measurement and Parameterization of Effective Radius of Droplets in Warm Stratocumulus Clouds

Observations from the Meteorological Research Flight's Hercules C-130 aircraft of the microphysical characteristics of warm stratocumulus clouds have been analyzed to investigate the variation of the effective radius of cloud droplets in layer clouds. Results from experiments in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and the sea areas around the British Isles are presented. In situations where entrainment effects are small the (effective radius)3 is found to be a linear function of the (volume-averaged radius)3 in a given cloud and can thus be parameterized with respect to the liquid water content and the droplet number concentration in the cloud. However, the shape of the droplet size spectrum is very dependent on the cloud condensation nuclei (CCN) characteristics below cloud base, and the relationship between effective radius and volume-averaged radius varies between maritime air masses and continental air masses. This study also details comparisons that have been made in stratocumulus between the droplet number concentrations and (a) aerosol concentrations below cloud base in the size range 0.1 to 3.0 μm and (b) CCN supersaturation spectra in the boundary layer. A parameterization relating droplet concentration and aerosol concentration is suggested. The effects of nonadiabatic processes on the parameterization of effective radius are discussed. Drizzle is found to have little effect near cloud top, but in precipitating stratocumulus clouds the parameterization breaks down near cloud base. Comparisons are made between this parameterization of effective radius and others used currently or in the past.