Microphysical properties of stratocumulus clouds during ACE-2

Microphysical measurements performed during 8 flights of the CLOUDYCOLUMN c omponent of ACE-2, with the Meteo-France Merlin-IV, are analyzed in terms o f droplet number concentration and size. The droplet concentration is depen dent upon the aerosol properties within the boundary layer. Its mean value over a flight varies from 55 cm(-3), for the cleanest conditions, to 244 cm (-3), for the most polluted one. For each flight, the variability of the co ncentration, in selected cloud regions that are not affected by mixing with dry air or drizzle scavenging, ranges from 0.5 to 1.5 of the mean value. T he mean volume diameter increases with altitude above cloud base according to the adiabatic cloud model. The frequency distribution of mean droplet vo lume normalized by the adiabatic value, for the selected regions, shows the same dispersion as the distribution of normalized concentration. The value s of droplet concentration versus mean volume diameter are then examined in sub-adiabatic samples to characterize the effects of mixing and drizzle sc avenging. Finally, the ratio of mean volume diameter to effective diameter is analyzed and a simple relationship between these 2 crucial parameters is proposed.