Aerosol and liquid water path relationships in coastal non-drizzling marine stratocumulus over the subtropical southeastern Pacific

Observations of cloud, aerosol, and boundary layer structure were gained from the CIRPAS Twin Otter instrumented aircraft on 18 flights made at 20oS and 72oW during the VAMOS Ocean-Cloud-Atmosphere-Land Study -Regional Experiment (VOCALS-REx). Eight of these flights were under comparable meteorological conditions, although the observed aerosol concentrations in the boundary layer and the liquid water path of the clouds topping the boundary layer varied considerably. On the eight of the flight days, the boundary layer was well mixed, the clouds sampled were non-precipitating, and conditions at the top and the bottom of the mixed layer were similar. The inversion heights observed on these flights ranged from 1000 to 1300 m, potential jumps across a very sharp capping inversion ranged from 13-17 oC, and mixing ratio decreases across the inversion varied from 6-8 g/kg. A strong positive correlation between the LWP, which varied from 15 to 73 gm-2, and the boundary layer CCN, which ranged from 190 to 585 cm-3, was observed. Further analysis of the two highest and the two lowest CCN concentration cases confirms that the conditions at the top of the boundary layer and the turbulent fluxes at the surface cannot explain the observed differences in the LWP. Another independent data set of cloud properties from satellite retrievals combined with a back trajectories analysis demonstrated that the LWP differences observed at the time of the aircraft flights are also prevalent during the night-time hours prior to the aircraft observations made at approximately 09:00 local time. A series of large-eddy simulations were set up to study the impact of different aerosol concentrations on the cloud water content and life time using the latest version of System for Atmospheric Modeling with a two-moment bulk micro-physics scheme. All cases adopted the same initial conditions and forcing from the composited observed data described above, except different initial aerosol and cloud droplet number concentrations. Processes linking the aerosol concentrations and the liquid water path are examined.