Exploring Aerosol Cloud Interaction Using VOCALS-REx Aircraft Measurements

In situ aircraft measurements during the VAMOS Ocean-Cloud-Atmosphere-Land Study-Regional Experiment (VOCALS-REx) field campaign are employed to study the interaction between aerosol and stratocumulus over the southeast Pacific Ocean, as well as entrainment process near the top of stratocumulus and its possible impacts on aerosol-cloud interaction. The results shows liquid water content (LWC), cloud droplet number concentration (N_d), and cloud depth all exhibit positive correlations with sub-cloud CCN number concentration, but no correlation with above-cloud CCN. A comparison of cloud formation under different aerosol conditions suggests that the increase of LWC is mainly contributed by N_d instead of effective radius of cloud droplets in the polluted case, in which more droplets form with smaller size, while the opposite is true in the clean case. By looking into the influences of dynamical conditions and aerosol microphysical properties on the cloud droplet formation, it is confirmed that cloud droplets are more easily to form under the conditions of large vertical velocity and aerosol size. It is also noted that an increase in aerosol concentration tends to increase both N_d and relative dispersion (ɛ), while an increase in vertical velocity often increases N_d but decreases ɛ. The difference of cloud microphysical properties between entrainment and non-entrainment zones confirms that the entrainment-mixing mechanism is predominantly extreme inhomogeneous in the stratocumulus that capped by a sharp inversion, namely the entrainment reduces N_d and LWC by 28.9 % and 24.8 % on average, respectively, while the size of droplets is relatively unaffected. In entrainment zone, smaller aerosols and drier air entrained from the top induce less cloud droplet with respect to total in-cloud particles (0.56 ± 0.22) than the case in non-entrainment zone (0.73 ± 0.13) by inhibiting aerosol activation and promoting cloud droplets evaporation.