Drizzle formation in marine stratocumulus clouds—experimental and modeling studies

This paper describes a study of microphysical processes in drizzling marine stratocumulus using aircraft observations and a simple cloud model. The overall goal is to further the understanding of precipitation formation processes that influence the indirect effects of atmospheric aerosols on climate.

Data presented in this paper are in-situ, airborne measurements coming from second Aerosol Characterization Experiment (ACE2) and Dynamics and Chemistry of Marine Stratocumulus Experiment (DYCOMS-II). In both experiments analysis of cloud droplet and drizzle drop spectra have been based on data collected by two spectrometers (Fast Forward Scattering Spectrometer Probe and One-dimensional Array Probe Particle Measuring Systems). During ACE2 characteristic cloud droplets concentration (CDNC) was observed from 55 up to 250 cm-3 and cloud depth from 150 to 260 m. In DYCOMS-II generally stratocumulus was thicker (from 290 to 500m) and more homogeneous, where CDNC varied from 125 up to 205. Maximum drizzle concentration of these two experiments was 1.35 cm-3. This large data set reveals good opportunity to study drizzle and cloud evolution in marine boundary layer of middle latitudes. As anticipated, the observed amount of drizzle was positively correlated with the stratocumulus depth and negatively correlated with CDNC.

To investigate on non-linearity of drizzle formation and to separate the impact of the CDNC from the cloud depth effect, a simple two-dimensional cloud model with prescribed idealized flow pattern and detailed microphysics was applied in a series of simulations. The use of cloud droplet spectra divided into bins allowed to include all major microphysical processes such as droplet nucleation, growth by condensation and collision-coalescence. Collision-coalescence process is treated as a stochastic process with a numerical solution obtained by a new mass conserved flux method. In performed simulations, the characteristics of cloud condensation nuclei, the cloud depth and vertical velocity were systematically varied. Model results (mixing ratios and concentrations of cloud droplets and drizzling drops, optical thickness and liquid water path) compared favorably with the few observed cases and suggest scaling relationships that can be used in parameterizations of indirect aerosol effect on marine stratocumulus.