Observations of the width of cloud droplet spectra in stratocumulus

The width of the cloud droplet size distribution (spectrum) is a parameter of significant importance. It impacts the development of drizzle through the collision-coalescence and effects optical properties of warm clouds, in particular the effective radius. Because of relatively coarse spatial resolution of large-scale models of weather and climate, processes affecting the cloud droplet spectrum in such models have to be parameterized. Typically, such models are capable in estimating the cloud droplet number concentration, for instance through the parameterization of the cloud-base nucleation. The width of the cloud droplet spectrum, required to represent drizzle formation and the effective radius, can be subsequently parameterized as a function of droplet number concentration. Several such relationships have been proposed in the past. This paper will present theoretical arguments and analysis of cloud observations from the Second Aerosol Characterization Experiment (ACE2) to further investigate this issue. In contrast to previous studies, the emphasis here is to distinguish between nearly adiabatic drizzle-free cloud volumes and volumes affected by entrainment and drizzle, and to use the local droplet concentration provided by the Fast FSSP. The observed range of droplet concentrations for various flights is between 20 (very clean maritime) and 400 (polluted) droplets per cubic cm. As in previous studies, the nondimensional spectral width, the standard deviation divided by the mean, is used in the analysis. The analysis shows that for each flight (i.e., for approximately constant aerosol loading) the width of the droplet spectrum in nearly adiabatic volumes decreases with the increasing number concentration, in agreement with theoretical argument relating the width of the spectrum to the strength of the updraft. For different flights (i.e., for different aerosol loadings), the pattern is not as clear, with the nondimensional width tending to increase with the droplet concentration. This is primarily due to the decrease of the mean radius with the increasing droplet concentration. The diluted cloudy volumes in ACE2 are currently under investigation. We hope to relate the width of the cloud droplet spectrum to the mean droplet radius and the cloud dilution, and compare the width to theoretical predictions based on the recently-completed direct numerical simulations of cloud-clear-air interfacial mixing. These results and their implications will be discussed at the conference.